Fasteners
Bolt Preload Torque Calculator
Preload torque for metric bolts M3–M64: stress area and proof load per ISO 898-1, preload by joint type, torque via the K-factor.
Parameters
Results
- Tensile stress area(A_s)
- 57.99 mm2
- Proof load(F_proof)
- 33634 N
- Recommended preload(F_p)
- 25225 N
- Preload utilisation(F_p/F_proof)
- 0.75
- Preload torque(T)
- 50.5 Nm
Design checks
- [PASS] K = 0.2 within the usual 0.10–0.30 bandtorque_coefficient_range
- [PASS] M10×1.5 is an ISO 724 coarse or common fine pitchpitch_series
Formulas
A_s = 0.7854·(d − 0.9382·P)²= 57.9896
Expression: 0.7854 * (nominalDiameter - 0.9382 * pitch)^2
0.7854 — π/4
0.9382 — ISO 898-1 stress-area constant (2·H/... simplification of the basic triangle)
F_proof = A_s · σ_sp= 33634.0
Expression: tensileStressArea * proofStress
F_p = factor · F_proof (0.75 reusable / 0.9 permanent)= 25225.5
Expression: preloadFactor * proofLoad
F_p / F_proof — how close the preload runs to the proof load= 0.750000
Expression: preloadForce / proofLoad
T = K · F_p · d (N·mm — displayed as N·m)= 50451.0
Expression: torqueCoefficient * preloadForce * nominalDiameter
Results are for preliminary design. Verify before manufacturing.
Export
STEP export runs entirely in your browser; the geometry kernel downloads once on first export.
How the bolt preload torque calculator works
The calculator starts from the ISO 898-1 tensile stress area A_s = 0.7854·(d − 0.9382·P)², which accounts for the thread helix rather than simply using the nominal diameter. The proof load F_proof = A_s·σ_sp uses the ISO 898-1 proof stresses: 580 MPa for class 8.8, 830 MPa for 10.9 and 970 MPa for 12.9. The recommended preload is a share of that proof load — 0.75·F_proof for joints that will be reassembled, 0.9·F_proof for permanent connections — and the tightening torque follows the classic torque-preload relation T = K·F_p·d, where K is the torque (nut) coefficient that bundles thread and head friction. K ≈ 0.2 for dry steel surfaces and 0.10–0.15 for lubricated ones; because roughly 90% of the input torque is spent overcoming friction, a small change in K moves the achieved preload far more than a change in the torque itself.
The utilisation output F_p/F_proof shows how close the preload runs to the proof load. Two advisory checks run on every change: the torque coefficient stays inside the usual 0.10–0.30 band, and the thread callout (diameter × pitch) matches an ISO 724 coarse pitch or a common fine pitch — a pitch outside the known series does not stop the calculation (custom threads are legitimate) but marks the result for a second look, since the stress-area approximation assumes a standard profile. Preload scatter in practice is dominated by friction, so for torque-critical joints consider tightening by angle or by yield instead.
Working on the structure around the joint? Try the beam deflection calculator.