Every other tool on this site checks a bolt you already have. This one works the other way: enter the working load, the safety factor, and the grade, and the calculator walks the standard size ladder — M3 to M48, #10 to 1-1/2″ — until the required tensile stress area is met, then shows the neighboring sizes with their utilizations so the selection is transparent, not oracular.
Load, safety factor, grade, series.
Sizes without a standard fine series fall back to coarse automatically.
Recommended: M10 — 64.7% utilized
Required area 37.5 mm² · M10 provides 58.0 mm² (18.56 kN at SF = 2.5).
Tension acts on the tensile stress area As = 0.7854·(d − 0.9382·p)²; shear (threads excluded) acts on the full shank area with allowable 0.62·Fu/SF per the AISC convention.
| Size | Area (mm²) | Capacity | Utilization |
|---|---|---|---|
| M6 | 20.1 | 6.44 kN | 186.4% |
| M8 | 36.6 | 11.71 kN | 102.4% |
| M10picked | 58.0 | 18.56 kN | 64.7% |
| M12 | 84.3 | 26.97 kN | 44.5% |
| M14 | 115.4 | 36.94 kN | 32.5% |
Bolt sizing is a five-step chain from a load to a part number. Every step is visible in the results panel above — nothing is hidden behind a lookup table you can’t audit.
The force the bolt actually carries in service — axial tension through the joint, or shear across the shank. When in doubt, use the worst credible load, not the average, because the safety factor assumes an honest starting number.
Multiply by SF to convert a working load into a design load. The factor is not arbitrary decoration: it is the explicit, reviewable acknowledgement that loads scatter and materials vary, and choosing it is an engineering decision, not a calculator setting.
Divide the design load by the allowable stress — the grade's ultimate strength (or 0.62·Fu for shear with threads excluded) divided by the same SF. The output is a required tensile stress area in mm², the quantity that actually connects load to geometry.
Standard sizes are the constraint that makes the problem interesting: the requirement almost never lands exactly on a stocked part, so the calculator picks the smallest standard size whose area meets it and shows how much margin that rounding just bought you.
A diameter is not a design. Take the recommendation to the torque calculator to set the preload, to the length calculator for the stack-up, and — if the bolt lands in a tapped hole of a softer material — to the thread engagement check before the drawing is released.
Most sizing errors trace to grabbing the wrong area for the check at hand. A bolt has four candidate cross-sections, and each exists because a different failure surface lives there.
| Area | Definition | Where it acts | Relative size |
|---|---|---|---|
| Nominal | The label diameter, e.g. 10 mm | Nowhere directly — it identifies the size | 100% (reference) |
| Shank | Full circle at nominal diameter | Bearing checks; shear with threads excluded | 100% |
| Tensile stress As | Effective threaded area from pitch & minor dia. | Axial tension through the threads; preload | ≈78–88% of shank |
| Root (minor) | Circle at the thread root | Fatigue crack initiation site, not used directly | ≈60–72% of shank |
The tensile stress area deserves the emphasis it gets throughout this site: it is deliberately larger than the root circle — because the thread’s helical geometry spreads load across many turns — and deliberately smaller than the shank — because the threaded portion is what breaks in a standard tensile test. The formula As = 0.7854·(d − 0.9382·p)² reconciles both facts in one line, and every capacity number on this page runs through it.
The intuition says a finer thread means more material and therefore more strength — and the formula agrees, but only barely. At M10, the coarse series (1.5 mm pitch) gives As ≈ 58.0 mm²; the fine series (1.25 mm) gives As ≈ 61.2 mm², a 5.5% gain. At 1/2 inch, UNF's 20 TPI over UNC's 13 TPI buys about 7%. The reason the gain is so small: the tensile stress area is anchored mostly by the pitch diameter, which moves only fractions of a millimeter between series, while the nominal diameter — the term that dominates the formula — doesn't move at all.
What fine threads genuinely buy is elsewhere. The larger root radius and smaller grain disturbance improve fatigue behavior. The shallower helix angle holds preload more steadily under vibration. On thin-walled tapped parts, the deeper engagement per turn and the greater number of load-sharing turns make fine series the default — which is why the thread engagement page treats the series as a first-class input.
Assembly speed matters, the parts tolerate handling wear, the joint is not fatigue-critical, and maximum capacity per size comes from stepping up the nominal diameter rather than shaving the pitch. Coarse also runs cheaper in high-volume tapping.
The joint sees vibration, wall thickness limits the engagement depth, preload must be set precisely in a short span of rotation, or the extra few percent of stress area happens to rescue a marginal size without a redesign.
A bracket carries 12 kN of static tension. The design load is 12 × 2.5 = 30 kN; class 8.8 allows 800/2.5 = 320 MPa; the required tensile stress area is therefore 30,000/320 ≈ 93.8 mm². The coarse-series ladder gives M10 (58.0 mm²) — no — M12 (84.3 mm²) — still short — M14 (115 mm²) — first size over the line. The recommendation lands on M14 at roughly 82% utilization, with M12 shown right below it failing at 111% so the near-miss is visible rather than guessed at.
Two honest readings of that result. First, the safety factor is doing its job: at SF 2.5 the “obvious” M12 — the same nominal as the load in kN — is insufficient, and only the arithmetic reveals it. Second, the recommendation is sensitive to the grade: switch to class 10.9 (1040 MPa) and the requirement drops to 72.1 mm², which M12 satisfies with room to spare. Material strength and diameter trade against each other, and the neighbor table exists precisely to make that trade visible before a purchase order is cut.
| Size | Pitch / TPI | As (mm²) | Proof load (kN) | Ultimate (kN) |
|---|---|---|---|---|
| M3 | 0.50 | 5.0 | 2.4 | 4.0 |
| M4 | 0.70 | 8.8 | 4.2 | 7.0 |
| M5 | 0.80 | 14.2 | 6.8 | 11.3 |
| M6 | 1.00 | 20.1 | 9.7 | 16.1 |
| M8 | 1.25 | 36.6 | 17.6 | 29.3 |
| M10 | 1.50 | 58.0 | 27.8 | 46.4 |
| M12 | 1.75 | 84.3 | 40.4 | 67.4 |
| M14 | 2.00 | 115.4 | 55.4 | 92.4 |
| M16 | 2.00 | 156.7 | 75.2 | 125.3 |
| M20 | 2.50 | 244.8 | 117.5 | 195.8 |
| M24 | 3.00 | 352.5 | 169.2 | 282.0 |
| M30 | 3.50 | 560.6 | 269.1 | 448.5 |
| M36 | 4.00 | 816.7 | 392.0 | 653.4 |
| M42 | 4.50 | 1,120.9 | 538.0 | 896.7 |
| M48 | 5.00 | 1,473.1 | 707.1 | 1,178.5 |
Proof load at 75% of the grade’s minimum yield, ultimate at minimum tensile — Class 8.8 for the metric ladder, SAE Grade 5 for the imperial one. Re-select the grade in the calculator to see any other grade’s capacities per size.
This bolt size calculator is the front door of the toolset: once it names a diameter, the bolt torque calculator on the homepage sets the wrench, the bolt length calculator sizes the shank, and the shear calculator verifies the side load. The full set lives on the all calculators page.
How the selection runs, which area each check uses, and how the answer changes with grade, series, and safety factor.
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