What is the tensile stress area of M8×1.25?

36.61 mm² (0.05674 in²). It is computed from the thread's own geometry with the ISO 898-1 formula As = (π ÷ 4) × (d − 0.938194 × P)², and checked against published values. Multiply it by the strength on your fastener's spec sheet to get a load.

How do you calculate tensile stress area?

For ISO metric threads, As = (π ÷ 4) × (d − 0.938194 × P)² in mm², where d is the basic major diameter and P the pitch. For Unified inch threads, At = 0.7854 × (D − 0.9743 ÷ n)² in in², where n is threads per inch. Both compute the area of a circle at the mean of the pitch and minor diameters — the section a fastener actually fails across.

Why does this calculator ask for the material strength instead of listing grades?

Because we could not find a free, verifiable source for SAE J429 and ISO 898-1 proof strengths. Both standards tabulate those values rather than deriving them, both are paywalled, and the free federal handbooks that cover fastener design do not reproduce them. Shipping unverifiable numbers would break the one thing this reference promises, so the strength is a user input taken from your fastener's own documentation.

Can I work out a bolt's strength from its property class?

Partly. An ISO metric property class X.Y has a nominal tensile strength of X × 100 MPa and a nominal yield of 0.X·Y times that — so 8.8 is 800 MPa nominal tensile, 640 MPa nominal yield. Proof stress, which is the figure most bolt calculations actually want, is separately tabulated and is not derivable from the designation; it also differs above and below 16 mm for class 8.8. Take it from the spec sheet.

Does this calculator give bolt torque?

No, deliberately. A torque figure embeds two judgements — the friction coefficient K and the fraction of proof load you are targeting — and the same bolt takes very different torques dry, lubricated or plated. Publishing one number as 'the' torque is how parts get broken. This calculator stops at the load, exposes every input, and shows the formula.

Does this cover metric and inch threads?

Yes — 68 metric threads and 82 inch threads, 150 in total, each with a stress area in both mm² and in². Taper pipe threads have no tensile stress area in this dataset and are not offered.

Calculator · threads

Thread Tensile Stress Area Calculator — Area × Your Strength

https://shopmath.org/calculators/thread-tensile · data verified August 1, 2026

The tensile stress area of M8×1.25 is 36.61 mm² (0.05674in²), computed from the thread’s own geometry and checked against published tables. Multiply it by the strength on your fastener’s spec sheet and you have its capacity. We do not fill that strength in for you — there is no free verified source for fastener proof strengths, and this site does not ship numbers it cannot trace.

Threads: 150
Inch: 82
Metric: 68
Strength: Your input
Verified: August 1, 2026

Thread Tensile Stress Area Calculator

The stress area is computed from the thread's own geometry and appears as soon as you pick a thread. The strength is yours to supply — the reason for that is the next section, and it is not an oversight.

Thread tensile calculator

Solve for

Thread

150 inch and metric threads with a verified stress area.

Fastener strengthMPa

Enter your fastener’s proof or tensile strength from its spec sheet. This site does not ship material strength data — see the note below for why.

Unit

Applied loadkN

The tensile load on one fastener.

Unit

Tensile stress area · M8×1.25

Strength needed

36.61

mm² tensile stress area — the section a fastener breaks across

Stress area: 0.05674in²
Stress area: 36.61mm²

The stress area above is computed from the geometry of M8×1.25 and checked against published tables. To turn it into a load, enter the proof or tensile strength printed on your fastener’s spec sheet — this site deliberately ships no material strength table. Full M8×1.25 spec page →

The formula, with your numbers in it

ISO 898-1

```
As  =  (π ÷ 4) × ( d − 0.938194 × P )²        [mm²]
```
```
As  =  (π ÷ 4) × ( 8 − 0.938194 × 1.250 )²
```
= 0.05674 in² · 36.61 mm²
The tensile stress area is the area of a circle at the mean of the pitch and minor diameters — the section a fastener actually breaks across, which is smaller than the major diameter suggests and larger than the minor diameter would give. This value comes from the database, where it was checked against published tables to better than 0.25%.

Why This Calculator Has No Material Strength Dropdown

What we could not verify, we did not ship

Every competing bolt calculator offers a dropdown — Grade 5, Grade 8, class 8.8, class 10.9 — and fills in a proof strength for you. We went looking for a source for those numbers that could be verified and published: SAE J429 and ISO 898-1 both tabulate them rather than deriving them, both are paywalled, and neither has a free primary equivalent. MIL-HDBK-5J, which is free and enormous, does not carry them either.

A number that cannot be traced is exactly the thing this site was built to stop reproducing. So the proof-strength table is not here, and it will not appear until there is a source worth citing next to it. The strength field is empty on purpose, and the honest place to fill it from is the documentation for the fastener actually in your hand.

What the calculator does not know, your supplier does. A fastener certificate, a manufacturer’s catalogue, or the standard the fastener is made to will all state a proof stress and a minimum tensile strength for that class in that diameter range — and the diameter range matters, which is another reason a single dropdown value would be misleading. ISO 898-1 class 8.8, for instance, has different proof stresses above and below 16 mm.

Two things you can compute yourself from the designation, and which are genuinely derivable: an ISO metric property class X.Y has a nominal tensile strength of X × 100 MPa and a nominal yield of 0.X·Y × that value — so 8.8 is 800 MPa nominal tensile and 640 MPa nominal yield. Proof stress is a separate tabulated figure and is not derivable from the designation, which is the whole difficulty.

A Worked Tensile Stress Area Example — M8×1.25

The area half of the calculation, done in full, with nothing assumed about the material.

M8×1.25 has a basic major diameter of 0.3150 in (8.000 mm) and a pitch of 1.250 mm. The ISO 898-1 stress area formula takes the mean of the pitch and minor diameters and returns the area of a circle at that diameter:

As = (π ÷ 4) × (8.000 − 0.938194 × 1.250)² = 36.61 mm²

That is 0.05674 in². It is the section a fastener actually breaks across — smaller than the major diameter suggests, because the thread has removed material, and larger than the minor diameter alone would give, because the threads still carry load.

From there the rest is one multiplication. If the fastener’s spec sheet says 640 MPa, the capacity is 36.61 mm² × 640 MPa = 23.43 kN. If it says something else, the answer moves — which is exactly why the field is yours to fill in. mm² × MPa = N, so no conversion constant enters the metric calculation at all.

Tensile Stress Areas for Common Thread Sizes

The area column is verified data. The capacity column is worked at one stated strength purely to show the arithmetic — it is not a claim about any particular fastener.

Tensile stress area for common inch and metric threads
Property	Tensile stress area		Property
Thread	in²	mm²	kN at 640 MPa
1/4"-20 UNC	0.03182	20.53	13.1
3/8"-16 UNC	0.07749	49.99	32.0
1/2"-13 UNC	0.14190	91.55	58.6
M6×1	0.03119	20.12	12.9
M8×1.25	0.05674	36.61	23.4
M10×1.5	0.08989	57.99	37.1
M12×1.75	0.13061	84.27	53.9
M16×2	0.24284	156.67	100.3

The kN column is 640 MPa × the stress area and nothing more. 640 MPa is the nominal yield of ISO property class 8.8, stated here as an example value so the column has a defined meaning — it is not the proof stress of your fastener and should not be used as one. Use the calculator with the figure from your own spec sheet.

Sources and Method for the Thread Tensile Calculator

Inch thread stress areascomputed

NBS Handbook H28 (1969) Part I §§6-7 formula system (public domain, 17 U.S.C. 105)

Verified against threads-unified: 1064 checks passed across 3 harness(es)

Computed from the standard's formulas, then checked against published tables.

Metric thread stress areascomputed+compiled

ISO 68-1 basic profile computed; ISO 261/262 diameter-pitch pairs and 6H class limits compiled

Verified against threads-metric: 215 checks passed across 3 harness(es)

Part computed from the standard's formulas, part compiled from agreeing public sources.

Formula

Tensile stress area — Unified inch threads

At  =  0.7854 × ( D − 0.9743 ÷ n )²          [in²]

  D = basic major diameter (in)
  n = threads per inch

ASME B1.1 / NBS H28 form. Reproduces published values for 1/4-20 through 1-8 to better than 0.15%.

Tensile stress area — ISO metric threads

As  =  (π ÷ 4) × ( d − 0.938194 × P )²        [mm²]

  d = basic major diameter (mm)
  P = pitch (mm)

ISO 898-1 form; 0.938194·P is the offset that puts the diameter at the mean of the pitch and minor diameters. Reproduces published values for M3 through M20 to better than 0.25%.

Capacity, safety factor and allowable load

capacity        =  As × S
safety factor   =  capacity ÷ applied load
allowable load  =  capacity ÷ target safety factor

  S = the strength YOU entered, from the fastener's own documentation

mm² × MPa = N exactly, so the metric result carries no conversion constant. The pound-force figure is that same number converted at 1 lbf = 4.4482216152605 N.

Assumptions

NO MATERIAL STRENGTH DATA IS SHIPPED. SAE J429 and ISO 898-1 proof strengths are tabulated rather than derivable and have no free verified source, so the strength is a required user input. Take it from your fastener's spec sheet or certificate.
The stress area is computed from BASIC thread dimensions — the theoretical values before any tolerance class is applied.
This is a static tensile calculation on the thread's stress area. It says nothing about fatigue, shock or cyclic loading, all of which govern more joints than static tension does.
It also says nothing about thread engagement LENGTH. A fastener at full tensile capacity in a shallow tapped hole strips the hole instead of breaking the bolt, and that is a different calculation.
Preload, torque and the friction coefficient that connects them are deliberately absent. A torque figure embeds a judgement about K and a target preload fraction, and this site does not publish judgement dressed as measurement.
Taper pipe threads have no tensile stress area in this dataset and are not offered.
Inch thread stress areas scope: Le = 1D for UNC/UNF, 9P for UNEF — determined empirically against H28 Table 2.21, resolving data-sourcing.md open item 5. EXCLUDED: form (roll) tap drills, class 1A/1B limits, class 1AR allowances, constant-pitch UN series.
Metric thread stress areas scope: EXCLUDED: ISO 965-1 tolerance-grade formulas (not located — data-sourcing.md open item 2), 6g external limits (single-sourced), form tap drills, M68-M100 (verified demand cliff at M64).

Data last verified

August 1, 2026

How this data is built and checked →

How to Cite This Thread Tensile Calculator

Citation

ShopMath. "Thread Tensile Stress Area Calculator — Area × Your Strength." ShopMath, verified August 1, 2026, https://shopmath.org/calculators/thread-tensile

BibTeX

@misc{shopmath-calculators-thread-tensile,
  title        = {Thread Tensile Stress Area Calculator — Area × Your Strength},
  author       = {{ShopMath}},
  year         = {2026},
  howpublished = {\url{https://shopmath.org/calculators/thread-tensile}},
  note         = {Data verified 2026-08-01}
}

Permanent URL

https://shopmath.org/calculators/thread-tensile

Common Questions About Thread Tensile Stress Area

What is the tensile stress area of M8×1.25?: 36.61 mm² (0.05674 in²). It is computed from the thread's own geometry with the ISO 898-1 formula As = (π ÷ 4) × (d − 0.938194 × P)², and checked against published values. Multiply it by the strength on your fastener's spec sheet to get a load.
How do you calculate tensile stress area?: For ISO metric threads, As = (π ÷ 4) × (d − 0.938194 × P)² in mm², where d is the basic major diameter and P the pitch. For Unified inch threads, At = 0.7854 × (D − 0.9743 ÷ n)² in in², where n is threads per inch. Both compute the area of a circle at the mean of the pitch and minor diameters — the section a fastener actually fails across.
Why does this calculator ask for the material strength instead of listing grades?: Because we could not find a free, verifiable source for SAE J429 and ISO 898-1 proof strengths. Both standards tabulate those values rather than deriving them, both are paywalled, and the free federal handbooks that cover fastener design do not reproduce them. Shipping unverifiable numbers would break the one thing this reference promises, so the strength is a user input taken from your fastener's own documentation.
Can I work out a bolt's strength from its property class?: Partly. An ISO metric property class X.Y has a nominal tensile strength of X × 100 MPa and a nominal yield of 0.X·Y times that — so 8.8 is 800 MPa nominal tensile, 640 MPa nominal yield. Proof stress, which is the figure most bolt calculations actually want, is separately tabulated and is not derivable from the designation; it also differs above and below 16 mm for class 8.8. Take it from the spec sheet.
Does this calculator give bolt torque?: No, deliberately. A torque figure embeds two judgements — the friction coefficient K and the fraction of proof load you are targeting — and the same bolt takes very different torques dry, lubricated or plated. Publishing one number as 'the' torque is how parts get broken. This calculator stops at the load, exposes every input, and shows the formula.
Does this cover metric and inch threads?: Yes — 68 metric threads and 82 inch threads, 150 in total, each with a stress area in both mm² and in². Taper pipe threads have no tensile stress area in this dataset and are not offered.