Bolt Torque Calculator

Formula

• Tightening torque: T = K × F × d. T in N·m, F in N, d in metres (d_mm ÷ 1000), K dimensionless.
• Reverse clamp load: F = T / (K × d). Reported here in kN.
• Proof load (metric coarse): F_proof ≈ As × Sp. As in mm², Sp in MPa → F_proof in N; divide by 1000 for kN.
• Grade mode preload: F = (% of proof / 100) × F_proof. Then T = K × F × d.
• ft·lb conversion: T_ft·lb = T_N·m × 0.737562.
• K from lubrication preset, or Custom K when override > 0 (lubrication is folded into K — do not also apply a separate lube % on top).

How it works

Pick torque-from-clamp, clamp-from-torque, or suggest-from-grade. K comes from lubrication or a custom override. Grade mode uses ISO coarse tensile stress area and proof stress. Confirm final torque with the fastener supplier chart for critical joints.

Pick Input mode: Torque from clamp load (default), Clamp load from torque, or Suggest from grade (% proof). Choose a metric size (or Custom diameter), lubrication, optional Custom K, and property class (used for the % of proof result). Mode 0 needs clamp load in kN; mode 1 needs applied torque in N·m; mode 2 also needs % of proof and a metric size for As (switching into grade mode from Custom jumps to M12). CALCULATE fills Results and updates the live 3D K-factor sketch (Play/Pause on the diagram). Editing a field clears Results. RESET restores M12 oiled 25 kN defaults.

One K, not K plus a second lubrication percent

Default CALCULATE path: M12, K = 0.15, F = 25 kN → T = 0.15 × 25,000 × 0.012 = 45 N·m.

Switch lubrication to Dry steel (K = 0.20) on the same clamp load to see 60 N·m.

Reverse mode: clamp load from a fixed wrench setting

Reverse example: T = 45 N·m, K = 0.15, d = 0.012 m → F = 25 kN.

Use this when a drawing already lists a torque and you need an estimated preload for a joint check.

Grade mode uses proof load, not ultimate tensile as the target

Grade path: proof = As × Sp, then F = (%/100) × proof, then T = K × F × d.

The pad warns when estimated preload exceeds 90% of proof — prefer ≤75–80% for reusable joints.

K-method scatter and when to leave this pad

Worked example

Default Torque from clamp load: M12, lightly oiled (K = 0.15), target clamp load 25 kN. Reproduce on CALCULATE.

1. Leave Input mode on Torque from clamp load. Size M12, lubrication Lightly oiled, Custom K = 0, clamp load 25 kN.
2. CALCULATE. F = 25,000 N, d = 0.012 m, K = 0.15 → T = 0.15 × 25,000 × 0.012 = 45 N·m (≈ 33.2 ft·lb).
3. Dry check: set lubrication to Dry steel (K = 0.20), same F → T = 60 N·m.
4. Reverse example: mode Clamp load from torque, 45 N·m, K = 0.15 → F = 25 kN.
5. Grade example: Suggest from grade, M12, 8.8, 75% proof, K = 0.15 → proof ≈ 50.6 kN, F ≈ 37.9 kN, T ≈ 68.3 N·m.

Result: Default: 45 N·m. Dry same clamp: 60 N·m. Reverse 45 N·m → 25 kN. Grade M12 8.8 @ 75% oiled ≈ 68.3 N·m.

When to use

• Setting a first-pass torque wrench value from target clamp load
• Comparing dry vs oiled vs coated K before you open a supplier chart
• Estimating preload implied by an existing torque callout
• Suggesting torque from metric size, property class, and % of proof
• Quick screening when a full VDI 2230 analysis is not yet justified

Limitations

• K-factor method often has ±25–30% preload scatter in real joints.
• Single K folds lubrication in; not a separate material-K × lube-% product.
• Not a VDI 2230 detailed friction model (no pitch / μ_t / μ_b / D_km).
• Does not check thread stripping, bearing stress, gasket relaxation, or joint separation.
• Grade As values are ISO metric coarse on this pad; fine pitch and inch grades need supplier tables.
• Not a substitute for the fastener manufacturer’s assembly chart on critical joints.

FAQ

What is the K factor in bolt torque?

K is an empirical nut factor that lumps thread friction and under-head friction into one number. Typical dry steel is about 0.20; lightly oiled about 0.15. On this pad, lubrication presets set K directly.

What formula does this calculator use?

T = K × F × d with F in newtons and d in metres. Reverse mode uses F = T / (K × d). Grade mode sets F from % of proof × As × Sp, then applies the same torque formula.

Why is my torque different from other online calculators?

Some sites multiply by (1 − lubrication%/100) on top of a material K. This pad’s oiled/moly presets already include lubrication in K. Matching numbers means using the same K convention, not stacking both.

How accurate is bolt torque from the K method?

Often only within about ±25–30% on preload because friction varies. Use it for planning and comparison; verify critical joints with supplier charts, stretch, or ultrasonic tension.

What percent of proof load should I use?

Many reusable-joint charts aim near 70–75% of proof. This pad defaults to 75% in grade mode and warns above 90%. Follow the fastener standard or drawing note for your joint.

When should I not use this calculator?

Safety-critical joints, high dynamic load, gasketed pressure flanges under code, and any joint that requires VDI 2230 or a manufacturer torque-tension curve.

Where do I check if the bolt can take my axial load?

Use the bolt strength calculator for As, proof/yield/ultimate loads, and FoS vs applied tension. This pad is for wrench torque and preload from K.