Bolt Torque Calculator

Bolt torque calculator: the short answer

This bolt torque calculator applies the standard short-form torque-tension formula: torque (ft-lb) = K × D × F ÷ 12, where K is the nut factor, D is the nominal bolt diameter in inches, and F is the target clamp force in pounds. The ÷ 12 converts inch-pounds to foot-pounds. With the defaults, a 3/4 in bolt at K = 0.2 and 20,000 lb of clamp force needs 250 ft-lb.

You choose the clamp force you want. The calculator tells you how hard to turn the wrench to get there.

What each part of the formula does

• D, diameter. A fatter bolt has a larger thread radius, so the same clamp force takes more torque to reach.
• F, clamp force. Double the preload you want and you double the torque.
• K, nut factor. This one number lumps together friction under the nut face, friction in the threads, and the thread geometry. It is measured by testing, not derived.

Because the formula is a straight multiplication, every input scales the answer in proportion. Change K from 0.2 to 0.15 and the torque drops by a quarter.

Worked examples

1. The default: 3/4 in bolt, K = 0.2, 20,000 lb

• K × D × F = 0.2 × 0.75 × 20,000 = 3,000 in-lb
• ÷ 12 = 250 ft-lb

2. A 1/2 in bolt, 9,000 lb clamp force

• Plain steel, K = 0.2: 0.2 × 0.5 × 9,000 ÷ 12 = 75 ft-lb
• Lubricated, K = 0.15: 0.15 × 0.5 × 9,000 ÷ 12 = 56 ft-lb

Example 2 shows why lubrication matters. If you oil the threads and still tighten to the dry figure of 75 ft-lb, the clamp force rises to about 12,000 lb, a third more than planned, which can overstretch a bolt.

Reference torque, Grade 5 and Grade 8 coarse thread

Clamp force here is 75% of proof load, with K = 0.2. Proof strength is 85,000 psi for Grade 5 and 120,000 psi for Grade 8.

Size	Stress area (in²)	Grade 5 clamp	Grade 5 torque	Grade 8 clamp	Grade 8 torque
1/4-20	0.0318	2,030 lb	8 ft-lb	2,860 lb	12 ft-lb
3/8-16	0.0775	4,940 lb	31 ft-lb	6,980 lb	44 ft-lb
1/2-13	0.1419	9,050 lb	75 ft-lb	12,770 lb	106 ft-lb
5/8-11	0.226	14,410 lb	150 ft-lb	20,340 lb	212 ft-lb
3/4-10	0.334	21,290 lb	266 ft-lb	30,060 lb	376 ft-lb
1-8	0.606	38,630 lb	644 ft-lb	54,540 lb	909 ft-lb

These are calculated planning values. Published charts differ slightly because they assume different K values and preload targets.

How to pick your inputs

1. Measure the diameter. Use the nominal size, such as 0.5 for a 1/2 in bolt, not the thread root.
2. Decide the clamp force. For general steel joints, 75% of proof load is a common rule. Gaskets, soft materials, and wood joints need much less.
3. Match K to the threads. Clean, oiled, plated, or rusty: pick the value that matches what you will actually tighten.
4. Run the calculator and set your torque wrench to the result.

Common mistakes

• Using a dry torque on lubricated bolts. This is the classic way to snap a bolt. Lubricant lowers K, so the same torque gives far more stretch.
• Confusing ft-lb and in-lb. A wrench set to 250 in-lb delivers only about 21 ft-lb. Check the scale.
• Re-using torque-to-yield bolts. Some engine and structural bolts are designed to stretch permanently. They should not be reused.
• Relying on torque for slip-critical steel. Structural steel connections are usually tightened by turn-of-nut or tension indicators. Check the specification for the job.

To check whether a bolt can carry the joint’s working load once tightened, use the bolt shear calculator. For flange layouts, see the bolt circle calculator.

This is a planning estimate using standard mechanical formulas. For structural, pressure, or safety-critical joints, follow the fastener or equipment manufacturer’s spec and confirm with a licensed engineer.