You do not tighten a bolt to a torque because the torque matters — you tighten it to create preload, the clamping tension that actually holds a joint together and keeps it from loosening or fatiguing. Torque is just the indirect, and frankly imprecise, way we chase that preload. This guide covers the torque-preload relationship, how to pick a target preload, and a worked example.
The torque-preload equation
The workhorse relationship every fastener engineer uses is:
where T is tightening torque, D is the nominal bolt diameter, F is the target preload (bolt tension), and K is the dimensionless nut factor that lumps together thread friction, under-head friction, and geometry.
| Condition | Typical K | Note |
|---|---|---|
| Plain / dry, as-received | 0.20 | Common default |
| Lightly oiled | 0.15–0.17 | Reduces scatter |
| Molybdenum disulfide / wax | 0.10–0.12 | Low friction, tight control |
| Zinc plated | 0.20–0.25 | Can be erratic |
Choosing the target preload
Preload is set as a fraction of the bolt’s proof load — the maximum tension it can carry with no permanent set:
where Sp is the proof strength of the grade and At is the tensile stress area. Common targets:
| Application | Target preload |
|---|---|
| Reused fasteners | 75% of proof load |
| Permanent connections | 90% of proof load |
| Joints subject to fatigue | High preload is protective — keep it up near 75–90% |
Worked example: 1/2-13 Grade 5 bolt
Find the tightening torque for a 1/2-13 SAE J429 Grade 5 bolt, reused, plain/dry.
Step 1 — proof load. A 1/2-13 has tensile stress area At = 0.1419 in2. Grade 5 proof strength Sp = 85 ksi.
Step 2 — target preload at 75% of proof (reused):
Step 3 — torque with K = 0.20 and D = 0.5 in:
Lubricate the same bolt (K = 0.15) and the required torque drops to about 56 lb·ft for the same preload. Same fastener, same clamp force, 25% less torque — which is exactly why you never apply a dry torque spec to a lubricated bolt. You will massively over-tension it.
Calculate it directly
Skip the arithmetic: our free Bolt Torque Calculator takes diameter, grade, K-factor, and target preload and returns the torque, using the T = KDF model with lubricated and dry presets. For thread engagement and drill sizing, pair it with the Tap Drill Size Lookup, and check joint friction pairs in the Friction Coefficient Lookup — all under Fasteners & Joints.
Beyond torque: better ways to hit preload
Because friction eats most of the input torque, torque control typically delivers preload with ±25–35% scatter. When a joint matters, engineers move up the accuracy ladder:
- Turn-of-nut (angle) control — snug, then rotate a specified angle. Bypasses friction scatter by measuring bolt stretch through rotation.
- Load-indicating washers — crush to a known gap at target tension; a visual, inspectable check.
- Bolt stretch / ultrasonic — measure elongation directly. The most accurate, used on critical flanges and structural connections.
Three mistakes to avoid
- Applying a dry torque to a lubricated bolt. As the worked example shows, the same torque over-tensions a lubed bolt by ~25%. Spec torque and lube condition together.
- Reusing yield-tightened fasteners. Bolts torqued past yield (turn-of-nut on high-strength assemblies) have used up their stretch — replace, do not re-torque.
- Galling stainless. Stainless-on-stainless threads seize under load; use anti-seize and re-rate K, or the bolt shears before it clamps.
Frequently asked questions
What is the nut factor K?
K is a dimensionless coefficient that lumps thread friction, under-head friction, and geometry into the torque-preload equation T = K·D·F. Typical values run 0.20 dry and 0.15–0.17 lightly oiled.
How much should I preload a bolt?
Set preload as a fraction of proof load (Sp × At): about 75% for reused fasteners and up to 90% for permanent connections. High preload also improves fatigue performance.
Can I use the same torque for a lubricated bolt?
No. Lubrication lowers K, so the same torque produces much higher preload — often 25% more — and can over-tension or yield the bolt. Always match the torque spec to the lubrication condition.
Specifying a torque value? Get the right number for your bolt grade, size, and lubrication in one click.
Stop looking it up twice — run the numbers and keep the record.
This reference is free forever. When you need to actually calculate, document, and defend the result, do it in the app — then export a clean report you can hand to a reviewer.
- ✓46 engineering calculators, unlimited
- ✓Save & reload every calculation
- ✓Organize work into project folders
- ✓Branded, PE-stamped PDF reports
Free plan, no credit card. Upgrade only when it's paying for itself.