Bolt torque calculator
An estimate of tightening torque and clamp load for a bolt, from its size, grade and how well lubricated it is. Use the fastener maker’s figure where one is given.
Important: torque is only a rough way to set bolt tension: friction varies so much that the real clamp load can be ±25% or worse. Every result here is an estimate EST. For safety-critical joints (lifting, pressure, vehicle, structural), use the figure from the manufacturer or designer, and use a calibrated torque wrench.
How it works, guides and notes
How it works
- Tensile stress area As = π/4 × (d − 0.9382 P)².
- Proof load = As × proof stress (ISO 898-1: 8.8 = 580 MPa up to M16 and 600 MPa above, 10.9 = 830 MPa, 12.9 = 970 MPa; SAE J429: grade 5 = 85 ksi, grade 8 = 120 ksi).
- Preload F = proof load × the target percentage (75% is a common choice for reusable joints).
- Torque T ≈ K × F × d, where K is the nut factor for the thread condition.
Sources: ISO 898-1 property classes; SAE J429 grades; the short-form torque–tension relation T = K F d from standard machine design texts. Values are for reference only.
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