---
type: "calculator"
name: "Thread Tensile Stress Area Calculator"
standard: "ASME B1.1 / NBS H28 and ISO 898-1 stress-area forms"
verified: "August 1, 2026"
url: https://shopmath.org/calculators/thread-tensile
---

# Thread Tensile Stress Area Calculator

The tensile stress area of M8×1.25 is 36.61 mm² (0.05674 in²), computed from the thread's own geometry. Multiply it by the strength off your fastener's certificate to get its capacity — this site publishes no material strength figures, because none could be verified from a free source.

## Inputs

| Input | Range | Notes |
| --- | --- | --- |
| Thread | any thread in the database carrying a tensile stress area | Taper pipe threads have no tensile stress area in this dataset and are not offered. |
| Fastener strength | required user input, in MPa or ksi | No material strength data is shipped. Take the figure from the fastener's own spec sheet or certificate. |
| Applied load or target safety factor | solve for either | Give the load to get the safety factor, or the safety factor to get the allowable load. |

## Worked example — stress area of M8×1.25

- **Thread**: [M8×1.25](https://shopmath.org/threads/metric-coarse/m8x1.25.md)
- **Basic major diameter**: 8.000 mm
- **Pitch**: 1.250 mm
- **Tensile stress area**: 36.61 mm²
- **Same value in inch units**: 0.05674 in²
- **Recomputed from geometry**: 36.61 mm²

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

The capacity step is deliberately left to you: `capacity = As × S`, where S is the strength from the fastener's own documentation. mm² × MPa = N exactly, so no conversion constant enters the metric result.

## Stress areas for common sizes

| Thread | Area (in²) | Area (mm²) |
| --- | --- | --- |
| [1/4"-20 UNC](https://shopmath.org/threads/unc/1-4-20.md) | 0.03182 | 20.53 |
| [3/8"-16 UNC](https://shopmath.org/threads/unc/3-8-16.md) | 0.07749 | 49.99 |
| [1/2"-13 UNC](https://shopmath.org/threads/unc/1-2-13.md) | 0.14190 | 91.55 |
| [M6×1](https://shopmath.org/threads/metric-coarse/m6x1.md) | 0.03119 | 20.12 |
| [M8×1.25](https://shopmath.org/threads/metric-coarse/m8x1.25.md) | 0.05674 | 36.61 |
| [M10×1.5](https://shopmath.org/threads/metric-coarse/m10x1.5.md) | 0.08989 | 57.99 |

Every one of the 150 threads carrying a stress area has the figure on its own page.

## Sources and Method for the Thread Tensile Stress Area Calculator

### Inch thread stress areas

- **Source**: NBS Handbook H28 (1969) Part I §§6-7 formula system (public domain, 17 U.S.C. 105)
- **Method**: computed
- **Verified against**: threads-unified: 1064 checks passed across 3 harness(es)

### Metric thread stress areas

- **Source**: ISO 68-1 basic profile computed; ISO 261/262 diameter-pitch pairs and 6H class limits compiled
- **Method**: computed+compiled
- **Verified against**: threads-metric: 215 checks passed across 3 harness(es)

### Formulas

**Tensile stress area — Unified inch threads**

```text
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**

```text
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**

```text
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

## Related pages

- Threads: [Screw thread reference](https://shopmath.org/threads.md)
- Tapping the hole: [Tap drill calculator](https://shopmath.org/calculators/tap-drill.md)
- All calculators: [Machining calculators](https://shopmath.org/calculators.md)

---

*Source: [ShopMath](https://shopmath.org/calculators/thread-tensile) — data verified August 1, 2026. Reference data; verify against the governing standard before manufacture.*