Bolt Torque Calculator
Find the right tightening torque for a bolt. Pick the bolt size and grade, your preload target, and the thread condition — get the clamp load and the torque in ft-lb, in-lb and N·m, the proof load and tensile stress area, plus by-size and by-condition comparison tables and a downloadable PDF.
| Size | Clamp load | Torque (ft·lb) | Torque (N·m) |
|---|---|---|---|
| Pick a bolt to compare | |||
| Condition | Nut factor | Torque (in·lb) | Torque (ft·lb) |
|---|---|---|---|
| Pick a bolt to compare | |||
How to use the bolt torque calculator
Four quick steps turn a bolt size, grade and condition into the tightening torque — in ft·lb, in·lb and N·m — with the clamp load, proof load and tensile stress area, plus by-size and by-condition tables and a downloadable report.
Pick size & grade
The UNC bolt size and grade (Grade 2, 5, 8 or socket cap).
Set the preload
The clamp load target as a percent of proof — 60%, 75% or 90%.
Choose the condition
The thread condition (nut factor K) — dry, zinc, oiled or waxed.
Read results & save a PDF
Get the torque in three units and the clamp load — then download a report.
The formulas behind the torque
Torque is the nut factor times the diameter times the clamp load. The clamp load is a share of the proof load, which comes from the grade and the thread’s stress area.
▶ Worked example
A 1/2"-13 Grade 5 bolt (proof strength 85,000 psi, stress area 0.1419 in²) tightened to 75% of proof, dry (K 0.20).
Proof load is 85,000 × 0.1419 ≈ 12,062 lb, so the clamp load is 0.75 × 12,062 ≈ 9,046 lb.
T = 0.20 × 0.5 × 9,046 ≈ 905 in·lb = 75.4 ft·lb (102.2 N·m)
A lower nut factor — say waxed threads at K 0.10 — would halve the torque for the same clamp load.
Bolt torque quick reference
The diameter and tensile stress area of each UNC size, the proof strength of each grade, and the torque by size for a common Grade 5 joint.
| Size | Diameter (in) | Stress area (in²) |
|---|---|---|
| #10-24 | 0.190 | 0.0175 |
| 1/4″-20 | 0.250 | 0.0318 |
| 5/16″-18 | 0.3125 | 0.0524 |
| 3/8″-16 | 0.375 | 0.0775 |
| 7/16″-14 | 0.4375 | 0.1063 |
| 1/2″-13 | 0.500 | 0.1419 |
| 5/8″-11 | 0.625 | 0.226 |
| 3/4″-10 | 0.750 | 0.334 |
| 7/8″-9 | 0.875 | 0.462 |
| 1″-8 | 1.000 | 0.606 |
| Grade | Proof strength (psi) |
|---|---|
| SAE Grade 2 | 55,000 |
| SAE Grade 5 | 85,000 |
| SAE Grade 8 | 120,000 |
| Alloy socket cap (A574) | 140,000 |
| Size | Clamp load (lb) | Torque (ft·lb) | Torque (N·m) |
|---|---|---|---|
| #10-24 | 1,116 | 3.5 | 4.8 |
| 1/4″-20 | 2,027 | 8.4 | 11.5 |
| 3/8″-16 | 4,941 | 30.9 | 41.9 |
| 1/2″-13 | 9,046 | 75.4 | 102.2 |
| 5/8″-11 | 14,408 | 150.1 | 203.5 |
| 3/4″-10 | 21,293 | 266.2 | 360.9 |
| 1″-8 | 38,633 | 643.9 | 873.0 |
A lubricated bolt (lower K) needs less torque for the same clamp load. Enter your own size, grade and condition above.
Everything the calculator gives you
One bolt and condition produce a full torque profile, not just a single number.
Bolt torque reference numbers
Built for anyone tightening fasteners
Mechanics, engineers, and assemblers all need the right torque for a bolted joint.
Torquing fasteners on a car, machine or build and needing a sensible figure when there is no spec on hand, for the size, grade and condition in front of them.
- Get a torque in ft·lb, in·lb or N·m
- Adjust for dry vs lubricated threads
- Match the figure to the bolt grade
Designing a bolted joint and setting the preload — the clamp load target, the proof load, and the torque that reaches it across sizes and grades.
- Work out the clamp load from proof and target
- See the proof load and stress area per size
- Hand a reviewer a clean PDF of the figures
Setting torque values for production or maintenance work, where repeatable, documented figures keep joints consistent from one assembly to the next.
- Pick a repeatable torque for a given condition
- Compare torque across the fastener range
- Keep a record with the PDF report
Get a reliable tightening torque
Small habits make a bolt torque figure far more trustworthy.
Friction dominates
The nut factor K is the biggest unknown. Pick the condition that matches your actual threads.
Lubed needs less torque
A waxed bolt can need half the torque of a dry one for the same clamp load. Don’t mix them up.
Use a calibrated wrench
A torque figure is only as good as the tool. Check and calibrate your torque wrench.
Pick the right preload
75% of proof for joints you’ll take apart; 90% for permanent joints where some yield is fine.
Follow the maker’s spec
A published torque and tightening sequence always beats a generic figure for a specific joint.
Clean, undamaged threads
Dirt, rust or damaged threads throw the friction — and the clamp load — well off.
Torque isn’t exact preload
Clamp load can scatter ±25%. For critical joints, use turn-of-nut or bolt-stretch methods.
Frequently asked questions
Common questions about bolt torque, clamp load, nut factor, and preload.
Bolt torque is the twisting effort that stretches the bolt to a target tension, called the clamp load. The common short form is torque equals the nut factor K, times the bolt diameter D, times the clamp load F: T = K x D x F.
The clamp load itself is the target preload — a percentage of the bolt’s proof load, which is the proof strength times the tensile stress area of the thread. So you set the tension you want, then find the torque that produces it.
For example, a 1/2-inch Grade 5 bolt at 75 percent of proof, dry (K about 0.20), needs about 75 ft-lb. This calculator works out the clamp load and the torque in ft-lb, in-lb and N-m for any size, grade and condition.
The nut factor K bundles all the friction in the joint — between the threads and under the bolt head or nut — into a single number used in T = K x D x F. It is the biggest variable in bolt torque.
Typical values are about 0.20 for dry plain steel, 0.22 for zinc-plated, 0.15 for lightly oiled, and 0.10 for waxed or moly-coated threads. A lower K means less torque is needed to reach the same clamp load, because less of the effort is lost to friction.
Because friction varies so much, K is only approximate, and it is the main reason the same torque can give different clamp loads. This calculator lets you pick the condition that matches your fasteners.
The clamp load, or preload, is the tension the tightened bolt holds, squeezing the joint together. It is what actually keeps a joint from loosening or separating — the torque is just the means to get there.
It is usually set as a percentage of the bolt’s proof load. A common target is 75 percent for joints that may be taken apart again, 90 percent for permanent joints where a little yielding is acceptable, and 60 percent as a conservative figure.
For example, a 1/2-inch Grade 5 bolt has a proof load around 12,000 pounds, so 75 percent is about 9,000 pounds of clamp. This calculator works out the clamp load from the grade, size and target you choose.
Proof strength is the maximum stress a bolt can take without permanent stretch, in psi, and it is set by the bolt’s grade. Proof load is that strength times the tensile stress area of the thread — the actual force, in pounds, the bolt can hold without yielding.
Typical proof strengths are about 55,000 psi for SAE Grade 2, 85,000 for Grade 5, 120,000 for Grade 8, and 140,000 for alloy socket-head cap screws. The stress area grows with bolt size, so a bigger bolt of the same grade has a higher proof load.
The clamp load target is a fraction of this proof load. This calculator shows the proof load and the tensile stress area for the size and grade you pick.
Both raise the torque, but through different parts of the formula. A higher grade has a higher proof strength, so the same size bolt can be preloaded to a larger clamp load, which raises the torque in proportion.
A larger size increases the torque two ways: it has a bigger tensile stress area, so a higher clamp load, and a larger diameter D, which is the lever arm in T = K x D x F. That is why torque climbs steeply with bolt size.
For example, a 1-inch Grade 5 bolt needs far more torque than a 3/8-inch one. This calculator covers UNC sizes from #10 to 1 inch and four grades, with a by-size table to compare.
No — torque is an indirect way to reach a clamp load, and it is not precise. Because friction varies, the same torque can produce clamp loads that differ by 25 percent or more, even with identical bolts.
Most of the applied torque is actually spent overcoming friction in the threads and under the head; only a small share goes into stretching the bolt. That is why the nut factor K matters so much and why it is only approximate.
For critical joints, methods that measure the stretch directly — turn-of-nut, bolt-stretch gauges, or load-indicating washers — are more accurate than torque alone. This calculator gives a torque-based estimate; follow the manufacturer’s method for anything safety-critical.
It depends on what the spec calls for — but you must match the nut factor to the actual condition, because lubrication changes the torque dramatically for the same clamp load. A lubed bolt needs much less torque than a dry one to reach the same tension.
Using a dry torque value on a lubricated bolt will over-tighten it, possibly past yield, because the lower friction lets more of the torque turn into tension. Using a lubed value on a dry bolt will leave it loose.
So always tighten to the condition the torque figure assumes. This calculator lets you choose dry, zinc-plated, lightly oiled or waxed, and adjusts the torque to match.
Torque converts simply between the three units. One pound-foot is 12 pound-inches, and one pound-inch is about 0.113 newton-metres, so one pound-foot is about 1.356 newton-metres.
To go from ft-lb to in-lb, multiply by 12; to go from in-lb to N-m, multiply by about 0.113; and to go from ft-lb to N-m, multiply by about 1.356.
For example, 75 ft-lb is 900 in-lb, or about 102 N-m. This calculator reports the tightening torque in all three units at once, so you can match whatever your torque wrench is set in.
Disclaimer: US units (inch bolts, pounds, ft-lb). Torque = K × D × clamp load, with clamp load = target % × proof strength × tensile stress area (UNC coarse thread). The nut factor K is approximate and friction is the biggest source of scatter — real clamp load can vary ±25% or more for the same torque. Values assume clean, undamaged threads and do not cover gasketed, soft, tapped-hole, structural (turn-of-nut / DTI) or safety-critical joints. A planning aid only — always follow the joint or component manufacturer’s published torque spec and sequence.