ISO 37
Rubber Tensile Stress-Strain Properties

Written by Chris Thomas, Universal Grip Company · Reviewed by Frank Bertini · Updated October 24, 2026

ISO 37 is the international standard test method for the tensile stress-strain properties of vulcanized and thermoplastic rubber. A dumbbell or a ring is stretched to rupture at a fixed speed while force and extension are recorded, giving tensile strength, elongation at break, the stress at a given elongation (the rubber trade's "modulus") and, for some compounds, a yield point. It is the first property in every rubber specification worldwide, the reference method behind ISO product standards for gloves, condoms, hoses, seals and belts, and the international counterpart of ASTM D412 and DIN 53504. It sits beside ISO 34-1 tear strength in the basic rubber lab.

This guide covers the current edition, ISO 37:2024, and what changed since 2011; the full die table for Types 1, 1A, 2, 3 and 4 with tolerances, test lengths, thickness and speed in one place; the Type A and B rings with their pulley rigs and speeds; the conditioning timeline from ISO 23529; the force and extensometer classes; the step-by-step procedure; every formula with a worked example; the precision data from the standard's interlaboratory programme; the differences from ASTM D412 and the DIN 53504 S2 speed trap; and the grips, dies, extensometers and machines Universal Grip supplies for the test.

In this guide: What ISO 37 Measures · Editions: 2011, 2017 and the Current ISO 37:2024 · Dumbbell Test Pieces: Types 1, 1A, 2, 3 and 4 · Ring Test Pieces: Types A and B · Machine, Grips and Extensometer Requirements · Conditioning and Number of Test Pieces · Step-by-Step Procedure · Calculations and a Worked Example · Common Errors and Their Size · ISO 37 vs ASTM D412 and DIN 53504 · Equipment for ISO 37: Grips, Dies, Extensometers and Machines · FAQ

What ISO 37 Measures

ISO 37 applies to vulcanized rubber and to thermoplastic rubbers (TPE, TPV, TPU) in the form of dumbbells cut from sheet or product, and rings cut from sheet or tube. The properties are defined in Clause 3 of the standard, and the vocabulary matters because it differs from the plastics and metals methods:

• Tensile strength (TS): the maximum tensile stress recorded in stretching the test piece to rupture, in MPa, on the original cross-section of the test length. TSb is the stress at break, which for most rubbers is the same value.

• Elongation at break (Eb): the strain at rupture as a percentage of the test length, the ISO term for the distance between the two reference marks on the narrow portion (what ASTM calls the bench-mark or gauge length).

• Stress at a given elongation (Se): the stress at 100 %, 200 %, 300 % elongation and so on, written S100, S200, S300. It is not an elastic modulus and nothing in the standard calls it one; it is the single best indicator of cure state and filler loading and the number most compound specifications set limits on.

• Elongation at a given stress (Es) and, for thermoplastic rubbers and some compounds that show one, stress and elongation at yield (Sy, Ey).

Rubber stretches to several hundred percent, thins as it stretches and has a stress-strain curve that is non-linear from the first percent, so the plastics conventions of ISO 527 (0.05 to 0.25 % modulus, yield by local maximum) do not apply. That is why ISO 37 has its own dumbbells, its own speeds, its own extensometer classes and its own definitions. Our elastomers testing page gives the lab overview; this guide covers the standard itself.

Editions: 2011, 2017 and the Current ISO 37:2024

ISO 37:2024 is the seventh edition, published in May 2024, and replaces ISO 37:2017. Most equipment pages still cite 2011 or even 2005, which matters because the apparatus requirement changed in between.

• 2011 to 2017: the force-measurement requirement was tightened from ISO 5893 Class 2 to Class 1 (± 1 % of reading instead of ± 2 %); the die cutter angle was specified at 30 to 35°; the ring pulley dimensions were corrected; and the interlaboratory precision results were added as Annex B.

• 2017 to 2024: a normative calibration schedule was added as Annex D, which lists what must be calibrated and how often for the test to be compliant; the carbon-black designations in the precision annex were corrected; symbols were aligned with the ISO drafting rules. The dies, speeds and formulas did not change.

A laboratory running ISO 37 therefore now needs a Class 1 force channel and a documented calibration schedule.

Dumbbell Test Pieces: Types 1, 1A, 2, 3 and 4

Five dumbbell dies are defined in Table 2 of the standard; the die dimensions below are in millimetres and the letters follow the standard's Figure 1: A overall length, B width of the ends, C length of the narrow portion, D width of the narrow portion, E small transition radius and F large transition radius. Test length (Table 1), thickness and speed are added so that everything needed to run each type is in one place.

• Type 1: A ≥ 115, B 25 ± 1, C 33 ± 2, D 6.2 ± 0.2, E 14 ± 1, F 25 ± 2. Test length 25 ± 0.5, thickness 2.0 ± 0.2, speed 500 mm/min. The classic large dumbbell and the closest to ASTM D412 Die C.

• Type 1A: A ≥ 100, B 25 ± 1, C 21 ± 1, D 5 ± 0.1, E 11 ± 1, F 25 ± 2. Test length 20 ± 0.5, thickness 2.0 ± 0.2, speed 500 mm/min. Introduced because its shape gives the fewest breaks outside the test length and the lowest variance of all five (Annex C); the preferred type when the choice is free.

• Type 2: A ≥ 75, B 12.5 ± 1, C 25 ± 1, D 4 ± 0.1, E 8 ± 0.5, F 12.5 ± 1. Test length 20 ± 0.5, thickness 2.0 ± 0.2, speed 500 mm/min. The mid-size die, equivalent to DIN 53504 S2 and the one most German data refer to.

• Type 3: A ≥ 50, B 8.5 ± 0.5, C 16 ± 1, D 4 ± 0.1, E 7.5 ± 0.5, F 10 ± 0.5. Test length 10 ± 0.5, thickness 2.0 ± 0.2, speed 200 mm/min. For small products and limited material.

• Type 4: A ≥ 35, B 6 ± 0.5, C 12 ± 0.5, D 2 ± 0.1, E 3 ± 0.1, F 3 ± 0.1. Test length 10 ± 0.5, thickness 1.0 ± 0.1, speed 200 mm/min. The miniature die for gloves, condoms, thin extrusions and O-ring cross-sections.

Thickness. The standard thickness is 2.0 ± 0.2 mm for Types 1, 1A, 2 and 3 and 1.0 ± 0.1 mm for Type 4. Non-standard thicknesses may be used up to 3.0 mm (Types 1 and 1A), 2.5 mm (Types 2 and 3) and 2.0 mm (Type 4) and must be reported, because strength and elongation both change with thickness. Thickness is measured at the centre and both ends of the test length with a dead-weight gauge per ISO 23529, the median is used and no reading may differ from the median by more than 2 %. The width of the narrow portion is taken as the distance between the die's cutting edges, measured to 0.05 mm, not measured on the rubber.

Dies. Dies must be sharp, with the cutting edges of the narrow portion parallel within 0.05 mm and an edge angle of 30 to 35°; a dull or chipped die leaves a ragged edge that starts the tear early and lowers both strength and elongation. Cut with a single stroke in a die press on a cutting board, with the sheet grain direction recorded; test pieces are normally cut with the grain along the test length and the direction reported. Our hardened-steel ISO 37 dies are made to the Table 2 tolerances and can be resharpened; the D412 guide explains why hardened dies outlast ribbon-steel ones.

Ring Test Pieces: Types A and B

Rings are used for tube, for products from which a dumbbell cannot be cut, and for automated testing where a ring on two pulleys needs no gripping. The standard notes that rings give a lower tensile strength than dumbbells, give a similar elongation at break when it is based on the initial internal circumference, and cannot show grain effects.

• Type A (normal ring): internal diameter 44.6 ± 0.2 mm, radial width and axial thickness 4 ± 0.2 mm (external diameter 52.6 mm). Width must be within 0.2 mm of the median around the ring and thickness within 2 %. Tested on the standard pulley rig with pulleys of 25.5 mm diameter and an initial centre distance of 30 mm (an alternative rig uses 22.3 mm pulleys at 35 mm); speed 500 mm/min.

• Type B (miniature ring): internal diameter 8 ± 0.1 mm, section 1 ± 0.1 mm (external diameter 10 mm), used only when there is not enough material for Type A. Pulleys 4.50 mm diameter at 5.5 mm centre distance; speed 100 mm/min.

• Pulley rig: one pulley free, the other driven at 10 to 15 r/min so the ring circulates and the strain is evened out around it; the pulleys must turn freely so that no friction force is added to the reading.

Ring strain is calculated from the pulley centre distance and the pulley diameter: elongation at break uses the initial internal circumference π × d, while stress at a given elongation uses the mean circumference π × (d + W). Mixing the two gives an S100 that is several percent off. We supply ring spindle fixtures with the Table 3 pulley dimensions, and the same fixtures run ASTM D1414 O-ring tests.

Machine, Grips and Extensometer Requirements

• Force: ISO 5893 Class 1, ± 1 % of reading, which in practice means an ISO 7500-1 class 1 (or class 0.5) load cell used within its calibrated range. A Type 4 piece of a soft compound breaks at a few tens of newtons, so a 100 N or 500 N cell is needed; a 5 kN cell is not class 1 at 30 N. See our calibration page.

• Speed: the machine must provide 100, 200 and 500 mm/min, each held within ± 10 % under load, and must be able to apply the small prestress of about 0.1 MPa that the procedure allows.

• Extensometer: ISO 5893 Class D for Types 1, 1A and 2 and Class E for Types 3 and 4. The classes are defined by range and gauge: Class D covers 1,200 % elongation on a 20 mm test length and Class E 1,200 % on a 10 mm test length, both with a maximum error of ± 2 %. A contact clip-on extensometer is not suitable for rubber because its mass and knife edges mark and distort the piece; a long-travel extensometer with light contact arms or a non-contact video or laser extensometer tracking two reference marks is the normal choice. Crosshead travel is not an ISO 37 elongation: the shoulders and the grips stretch too.

• Grips: must hold the end tabs symmetrically so that the pull is along the axis of the piece, must not slip as the rubber thins, and must not cut it. Self-tightening designs are preferred: pneumatic grips with serrated or rubber-faced jaws and constant air pressure, or eccentric roller grips that tighten as the force rises. Screw vise grips work for stiff TPEs but need re-tightening on soft compounds.

Conditioning and Number of Test Pieces

ISO 37 refers to ISO 23529 for the standard laboratory temperature, 23 ± 2 °C (or 27 ± 2 °C in tropical countries); humidity control is not required for ordinary rubbers. The timing rules are the ones most often broken:

• At least 16 hours must elapse between vulcanization and testing.

• Non-product tests (laboratory-mixed and cured sheets) are completed within 4 weeks of vulcanization; product tests preferably within 3 months of manufacture, and in any case within 2 months of receipt.

• At least 3 hours at the standard laboratory temperature immediately before testing, for pieces cut from a sheet already at that temperature.

• Latex rubber pieces are conditioned for at least 96 hours with humidity control.

• Buffed pieces (thickness reduced by grinding) are tested between 16 and 72 hours after buffing.

The minimum number of test pieces is three, and the standard notes that five give a lower uncertainty; most product specifications require five. The result reported is the median of the individual values for each property, not the mean, which is why an odd number of pieces is convenient. Pieces that break outside the test length (in the shoulder or at the grip) are discarded and replaced.

Step-by-Step Procedure

• Verify the machine: Class 1 force in the range to be used, speed within ± 10 %, extensometer Class D or E, dies within Table 2, calibration schedule per Annex D.

• Cut the dumbbells (or rings) from conditioned sheet with a sharp die in one stroke; record the grain direction; check for edge defects and bubbles.

• Measure thickness at the centre and both ends of the test length; take the median; reject the piece if any reading is more than 2 % from the median. Take the width from the die.

• Mark the test length on the narrow portion (25 mm Type 1; 20 mm Types 1A and 2; 10 mm Types 3 and 4) if a non-contact extensometer is used.

• Insert the end tabs symmetrically in the grips, centred and with the narrow portion vertical; zero the force; apply a prestress of about 0.1 MPa if needed to straighten the piece and zero the extensometer.

• Run at 500 mm/min (Types 1, 1A, 2; Type A rings) or 200 mm/min (Types 3, 4) or 100 mm/min (Type B rings); record force and test-length extension continuously to rupture.

• Discard any piece that breaks outside the narrow portion; test at least three (preferably five) valid pieces.

• Calculate TS, Eb and the stresses at the required elongations for each piece; report the medians, the thickness, the type, the grain direction and the temperature.

Calculations and a Worked Example

All stresses use the original cross-section of the narrow portion, W × t, with W the die width and t the median thickness; force in newtons and dimensions in millimetres give megapascals directly.

• Tensile strength: TS = Fm ÷ (W × t); stress at break TSb = Fb ÷ (W × t).

• Elongation at break: Eb = 100 × (Lb − L0) ÷ L0, where L0 is the test length and Lb the test length at break.

• Stress at a given elongation: Se = Fe ÷ (W × t), with Fe the force at the elongation of interest (100 %, 200 %, 300 %).

• Elongation at a given stress: Es = 100 × (Ls − L0) ÷ L0.

• Yield: Sy = Fy ÷ (W × t) and Ey = 100 × (Ly − L0) ÷ L0, only for compounds that show a yield point.

• Rings: the cross-section is 2 × W × t because two legs carry the load; Eb is based on the initial internal circumference π × d and Se on the mean circumference π × (d + W), using the pulley centre distance and pulley diameter to compute the stretched circumference.

Worked example, Type 1 dumbbell. Die width W = 6.2 mm, median thickness t = 2.02 mm, so the cross-section is 6.2 × 2.02 = 12.52 mm². Test length L0 = 25 mm. The force at 100 % elongation (test length 50 mm) is 24.0 N, at 300 % (100 mm) it is 78.0 N, and the maximum force, at rupture, is 410 N with the test length at 171 mm. Then S100 = 24.0 ÷ 12.52 = 1.92 MPa; S300 = 78.0 ÷ 12.52 = 6.23 MPa; TS = 410 ÷ 12.52 = 32.7 MPa; Eb = 100 × (171 − 25) ÷ 25 = 100 × 146 ÷ 25 = 584 %. Repeat for each piece and report the medians.

Precision from Annex B. The 2024 edition reports interlaboratory results that tell you what agreement to expect. For a natural rubber compound on Type 1A pieces: TSb mean 34.9 MPa with repeatability r 0.67 MPa and reproducibility R 2.63 MPa; Eb 687 % with r 29.9 and R 57.8 percentage points; S100 1.89 MPa with r 0.07 and R 0.28 MPa; S200 4.58 MPa with r 0.38 and R 0.70 MPa. In words: one lab should repeat strength within about 2 % and elongation within 4 %, while two labs may differ by 7 to 8 %.

Common Errors and Their Size

• 3 mm sheet called standard. Several equipment pages give Die C or Type 1 as 3 mm thick; that is the upper limit for non-standard pieces, not the 2.0 ± 0.2 mm standard, and a 3 mm piece gives a different strength and elongation from a 2 mm one. Report any non-standard thickness.

• Elongation from crosshead travel. The shoulders, the tabs inside the grips and the grips themselves all stretch; a crosshead-based elongation on a Type 1 piece reads 20 to 40 % high and is not an ISO 37 result. Use a Class D or E extensometer or marked reference lines.

• Dull dies. A chipped edge is the single most common cause of low, scattered elongation; resharpen or replace, and never cut two layers at once.

• Mean instead of median. ISO 37 reports the median; the mean of five pieces including one shoulder break can be 10 % low.

• Wrong speed by type. Types 3 and 4 run at 200 mm/min, not 500; Type B rings at 100 mm/min. DIN 53504 S2 pieces run at 200 mm/min by default, so a DIN S2 result and an ISO Type 2 result on the same compound differ by a few percent in strength and must not be compared.

ISO 37 vs ASTM D412 and DIN 53504

The three rubber tensile standards are similar in principle and a well-run lab can use one set of equipment for all of them, but they are not equivalent and results should be labelled with the method used.

• Dumbbells: ISO 37 Types 1, 1A, 2, 3, 4; ASTM D412 Dies A to F; DIN 53504 S1, S1A, S2, S3, S3A. ISO Type 1 (6.2 mm neck, 25 mm test length, 115 mm long) is close to D412 Die C (6 mm neck, 25 mm bench marks, 115 mm long) but the neck widths and radii differ. ISO Type 1A has no D412 twin. ISO Type 2 = DIN S2; ISO Type 1 = DIN S1; ISO Type 1A = DIN S1A; ISO Type 4 = DIN S3.

• Thickness: both ISO 37 and D412 use 2.0 ± 0.2 mm as standard; D412 permits 1.3 to 3.3 mm, ISO up to 3.0 mm for Types 1 and 1A.

• Speed: ISO 500 mm/min for Types 1, 1A, 2 and 200 mm/min for Types 3, 4; D412 500 ± 50 mm/min for all dies; DIN 53504 S2 200 mm/min by default, 500 only by agreement. This is the trap when German data are compared with ISO or ASTM data.

• Rings: ISO Type A (44.6 mm ID, 4 × 4 mm) and Type B (8 mm ID, 1 × 1 mm) at 500 and 100 mm/min; D412 Type 1 and Type 2 rings defined by internal circumference (50 and 100 mm).

• Tensile set: D412 includes a tensile-set procedure (hold at a stated elongation, release, measure the residual extension); ISO 37 does not, and ISO 2285 covers set separately.

A TPE specified as a rubber is tested to ISO 37 at 500 mm/min; the same material specified as a plastic goes to ISO 527-2 at 50 mm/min, and the strengths differ. The ASTM D412 guide covers the ASTM side at the same depth, and the ISO 34-1 guide covers tear tests run on the same machine.

Equipment for ISO 37: Grips, Dies, Extensometers and Machines

Capacity first. A Type 1 piece of a 30 MPa compound at 12.4 mm² breaks at about 370 N; a 1A piece (10 mm²) at 300 N; a Type 2 (8 mm²) at 240 N; a Type 4 (2 mm²) at 60 N; a Type A ring (2 × 16 mm²) of the same compound at about 1 kN. A 1 to 5 kN frame with the right load cell covers everything in the standard, and the travel matters more than the force: a Type 1 piece at 700 % elongation needs about 300 mm of crosshead travel plus the grip opening.

• Grips. Pneumatic grips with serrated or rubber-faced jaws at 500 N to 2.5 kN hold every dumbbell type at a constant, adjustable pressure and open with a foot pedal for fast throughput; self-tightening eccentric roller grips grip harder as the force rises and are the choice for soft, high-elongation compounds that slip from fixed jaws; vise grips suit stiff TPEs. Jaw faces should be at least as wide as the 25 mm tab of Types 1 and 1A. Our guide to choosing tensile grips compares them.

• Ring fixtures. Spindle fixtures with free-running pulleys to the Table 3 dimensions for Type A and Type B rings, with a driven spindle at 10 to 15 r/min where the standard rig is required; the same spindles run ASTM D1414 O-rings. See accessories.

• Dies and cutting. Hardened-steel dies for Types 1, 1A, 2, 3 and 4 to the Table 2 tolerances, an arbor or pneumatic die press and a cutting board; we also supply the DIN 53504 and ASTM D412 die sets so that one press serves all three standards.

• Extensometers. Long-travel (XL) extensometers with light contact arms that follow the piece to 1,000 % and beyond, meeting Class D on 20 mm and Class E on 10 mm test lengths; video extensometers that track two ink marks with no contact at all, the right answer for thin Type 4 pieces, for gloves and for latex; the gauge-length marking tool and the sample-marking guide go with them.

• Machines. A Mark-10 tensile tester for QC labs running a few pieces a day; a 2.5 kN or 5 kN benchtop UTM with 1,000 mm or more of travel, Class 1 load cells from 100 N upward and software that reports the median and S100/S300 automatically for a full rubber laboratory. Both run ISO 37, ISO 34-1, ASTM D412 and D624 with a grip change.

ISO 37 FAQ

What is ISO 37?

ISO 37 is the international standard for the tensile stress-strain properties of vulcanized and thermoplastic rubber. A dumbbell (Types 1, 1A, 2, 3 or 4) or a ring (Type A or B) is stretched to rupture at 500, 200 or 100 mm/min to give tensile strength, elongation at break and the stress at a given elongation. The current edition is ISO 37:2024.

What are the ISO 37 dumbbell dimensions?

Narrow-portion widths are 6.2 ± 0.2 mm (Type 1), 5 ± 0.1 (1A), 4 ± 0.1 (2 and 3) and 2 ± 0.1 (4); overall lengths at least 115, 100, 75, 50 and 35 mm; test lengths 25, 20, 20, 10 and 10 mm. Standard thickness is 2.0 ± 0.2 mm, or 1.0 ± 0.1 mm for Type 4.

What is the test speed in ISO 37?

500 ± 50 mm/min for Types 1, 1A and 2 dumbbells and for Type A rings; 200 ± 20 mm/min for Types 3 and 4; 100 ± 10 mm/min for Type B rings. DIN 53504 S2 pieces run at 200 mm/min by default, so DIN and ISO Type 2 results are not directly comparable.

Which ISO 37 dumbbell type should I use?

Type 1A when the choice is free: the standard's Annex C shows it gives the fewest breaks outside the test length and the lowest variance. Type 1 is the traditional large piece closest to ASTM D412 Die C; Type 2 matches DIN 53504 S2; Types 3 and 4 are for small products, gloves, condoms and limited material.

How thick must ISO 37 test pieces be?

2.0 ± 0.2 mm for Types 1, 1A, 2 and 3 and 1.0 ± 0.1 mm for Type 4. Non-standard thicknesses up to 3.0 mm (Types 1, 1A), 2.5 mm (2, 3) or 2.0 mm (4) may be tested but must be reported, because strength and elongation change with thickness.

How many test pieces does ISO 37 require?

A minimum of three; the standard notes that five give lower uncertainty and most specifications ask for five. The result for each property is the median, and pieces that break outside the narrow portion are discarded and replaced.

What extensometer does ISO 37 require?

ISO 5893 Class D for Types 1, 1A and 2 (1,200 % on a 20 mm test length) and Class E for Types 3 and 4 (1,200 % on 10 mm), each within ± 2 %. In practice that means a long-travel extensometer with light contact arms or a non-contact video extensometer reading two marked reference lines; crosshead travel is not acceptable.

What is the conditioning for ISO 37?

Per ISO 23529: at least 16 hours between vulcanization and testing, at least 3 hours at 23 ± 2 °C immediately before the test, laboratory sheets tested within 4 weeks of cure and products within 3 months, latex conditioned 96 hours with humidity control, buffed pieces tested 16 to 72 hours after buffing.

How is tensile strength calculated in ISO 37?

TS = Fm ÷ (W × t), the maximum force divided by the die width times the median thickness of the narrow portion, in MPa. Elongation at break is Eb = 100 × (Lb − L0) ÷ L0 on the test length; stress at 100 % elongation is S100 = F100 ÷ (W × t). For a Type 1 piece 6.2 × 2.02 mm breaking at 410 N, TS = 32.7 MPa.

What is the difference between ISO 37 and ASTM D412?

Both stretch 2.0 mm thick dumbbells at 500 mm/min, but ISO 37 uses Types 1, 1A, 2, 3 and 4 (Types 3 and 4 at 200 mm/min) while D412 uses Dies A to F; the neck widths, radii and ring specimens differ; D412 adds a tensile-set procedure that ISO 37 does not have; and ISO 37 requires an ISO 5893 Class 1 force channel. They are similar but not equivalent.

What changed in ISO 37:2024?

The 2024 edition added a normative calibration schedule (Annex D) and corrected the precision annex; the dies, speeds and formulas are unchanged. The bigger technical change was in 2017, when the force-measurement requirement moved from Class 2 to Class 1 (± 1 %).

What grips and machine are used for ISO 37?

Pneumatic grips with serrated or rubber faces, or self-tightening roller grips for soft high-elongation compounds, on a 1 to 5 kN machine with a Class 1 load cell sized to the force (a Type 1 piece of a 30 MPa compound breaks at about 370 N, a Type 4 piece at about 60 N) and at least 300 mm of travel for 700 % elongation on a Type 1 piece.

Need pneumatic or roller grips, cutting dies, a long-travel extensometer or a complete tensile system for ISO 37? Contact Universal Grip with your dumbbell type, expected elongation and machine connection and we will quote the right setup.