Bolt Preload Calculator finds the clamp load a tightening torque puts in a bolt and the share of its proof load used. It also works back from a target preload to the torque needed.
Pick the bolt and enter the torque you tightened it to. The tool gives the clamp load that torque produced.
Custom Bolt
Filled in by the size and grade above. Change any of them for a bolt that is not in the lists.
Stretch, Scatter and Relaxation
Grip length is the clamped thickness between the head and the nut. The method sets how far preload can scatter.
Same Torque, Other Thread Conditions
| Thread Condition | K | Preload (Share of Proof) |
|---|---|---|
| Dry or zinc plated | 0.20 | 9,000 lbf (75%) |
| Anti-seize or threadlocker | 0.17 | 10,588 lbf (88%) |
| Oiled or lubricated | 0.15 | 12,000 lbf (99%) |
| PTFE coated | 0.12 | 15,000 lbf (124%) |
Using the Bolt Preload Calculator Two Ways
Bolt preload is the tension left in a bolt after it is tightened. That tension is what clamps the parts together, so it is also called clamp load. A wrench measures torque, not tension, and the Bolt Preload Calculator converts between the two.
The first mode starts from the torque you applied and returns the preload it produced. The second starts from a target share of the bolt's proof load and returns the torque to apply. Both need the bolt size, the grade and the condition of the threads.
With the default entries, a 1/2-13 Grade 5 bolt is tightened dry to 75 lb-ft. The preload is 9,000 lbf, which is 74.6% of the bolt's 12,061 lbf proof load. About 8,100 lbf is left once the joint settles.
Bolt Preload Formula
The link between torque and preload is the short-form torque equation, and it is the one the Bolt Preload Calculator uses. Torque equals a nut factor times the bolt diameter times the preload. Turned around, it gives the preload from a known torque.
T is the torque, d is the nominal bolt diameter and K is the nut factor, which stands for all the friction in the threads and under the head. For the example, 75 lb-ft is 900 lb-in. Dividing by 0.20 and by 0.5 in gives 9,000 lbf.
The result means little until it is compared with what the bolt can carry. Proof load is the highest tension a bolt holds without taking a permanent stretch. It is the proof strength of the grade times the tensile stress area of the thread.
A 1/2-13 thread has a stress area of 0.1419 in², and Grade 5 has a proof strength of 85,000 psi up to 1 in diameter. Their product is 12,061 lbf. The Bolt Area Calculator gives the stress area for any size and pitch.
Recommended Preload for a Bolt
The usual targets come from Shigley's Mechanical Engineering Design. Preload a bolt to 75% of its proof load when the joint will be taken apart and reused. Preload it to 90% when the joint is permanent.
Proof strength is about 85% of yield strength, so those two targets are roughly 64% and 77% of yield. The preset list offers both, along with 50% and 60% for lighter joints. A preload under 50% of proof draws a notice, because a lightly loaded joint can loosen or fatigue.
The 90% target leaves little room for error. A torque wrench can be off by 25% either way, and 90% plus a quarter is 112% of proof. The Bolt Preload Calculator flags that case and shows the high end of the range. Reserve 90% for methods that control tension more closely than torque does.
Bolt Preload Table
The table gives the preload at 75% of proof and the torque that produces it with a nut factor of 0.20. Inch sizes are SAE Grade 5 coarse thread, and metric sizes are Class 8.8. Set the Bolt Preload Calculator to Target share of proof load to get the same figures for any other bolt.
| Bolt | Proof Load | Preload at 75% | Torque at K 0.20 |
|---|---|---|---|
| 1/4-20 | 2,705 lbf | 2,029 lbf | 8.5 lb-ft |
| 5/16-18 | 4,457 lbf | 3,342 lbf | 17.4 lb-ft |
| 3/8-16 | 6,587 lbf | 4,940 lbf | 30.9 lb-ft |
| 1/2-13 | 12,061 lbf | 9,046 lbf | 75.4 lb-ft |
| 5/8-11 | 19,210 lbf | 14,408 lbf | 150.1 lb-ft |
| 3/4-10 | 28,429 lbf | 21,322 lbf | 266.5 lb-ft |
| 1-8 | 51,488 lbf | 38,616 lbf | 643.6 lb-ft |
| M8 × 1.25 | 21.23 kN | 15.92 kN | 25.5 N·m |
| M10 × 1.5 | 33.63 kN | 25.23 kN | 50.5 N·m |
| M12 × 1.75 | 48.87 kN | 36.66 kN | 88.0 N·m |
| M16 × 2 | 90.87 kN | 68.15 kN | 218.1 N·m |
| M20 × 2.5 | 146.88 kN | 110.16 kN | 440.6 N·m |
| M30 × 3.5 | 336.35 kN | 252.26 kN | 1,513.6 N·m |
So the recommended preload for an M10 Class 8.8 bolt in a reused joint is about 25.2 kN, reached at 50.5 N·m with dry threads. A 1 in Grade 5 bolt takes 643.6 lb-ft, and the same bolt in Grade 8 takes 908.6 lb-ft. For a full torque chart by size and grade, use the Bolt Torque Calculator.
Nut Factor and Thread Friction
Every figure above depends on K, and K is the least certain input. The NASA Fastener Design Manual says 0.2 is the commonly assumed value but should not be used blindly. It gives 0.15 as a more realistic value for steel on steel. Its table ties K to the friction coefficient of the surfaces.
| Friction Coefficient, Threads and Head | Nut Factor K |
|---|---|
| 0.05 | 0.074 |
| 0.10 | 0.133 |
| 0.15 | 0.189 |
| 0.20 | 0.250 |
Shigley lists 0.30 for plain black bolts, 0.20 for zinc plated, 0.18 for lubricated and 0.12 with anti-seize. Anti-seize products differ. One maker's data sheet, quoted in a Physics Forums thread on this question, gives 0.172 for its copper-graphite compound. Use the figure from your lubricant's data sheet when it has one.
The reason this matters is that lower friction turns the same torque into more preload. Open Same Torque, Other Thread Conditions under the results to see it. For the 1/2-13 Grade 5 bolt at 75 lb-ft, the numbers are these.
| Thread Condition | K | Preload at 75 lb-ft | Share of Proof |
|---|---|---|---|
| Dry or zinc plated | 0.20 | 9,000 lbf | 75% |
| Anti-seize or threadlocker | 0.17 | 10,588 lbf | 88% |
| Oiled or lubricated | 0.15 | 12,000 lbf | 99% |
| PTFE coated | 0.12 | 15,000 lbf | 124% |
A torque value written for dry threads will overload a lubricated bolt. To keep the same preload, scale the torque by the ratio of the two nut factors. Going from 0.20 to 0.15 cuts 75 lb-ft to 56.3 lb-ft, and going to 0.12 cuts it to 45 lb-ft. If you have a measured torque and tension pair, the Bolt Friction Coefficient Calculator works K out from them.
How Accurate Is Preload From Torque?
Torque only estimates preload, which is why the Bolt Preload Calculator reports a range and not just one number. The NASA manual reproduces the Industrial Fasteners Institute table of how close each tightening method gets, along with what it costs relative to tightening by feel.
| Tightening Method | Preload Accuracy | Relative Cost |
|---|---|---|
| Feel | ±35% | 1 |
| Torque wrench | ±25% | 1.5 |
| Turn of the nut | ±15% | 3 |
| Load-indicating washers | ±10% | 7 |
| Bolt elongation | ±3 to 5% | 15 |
| Strain gages | ±1% | 20 |
Pick the method under Stretch, Scatter and Relaxation and the last card shows the range. With a torque wrench, the 9,000 lbf example could be anywhere from 6,750 to 11,250 lbf. The high end is 93.3% of proof, which is still safe. The low end is what the joint must survive on.
Preload also drops after tightening as surfaces bed in and the twist in the bolt relaxes. A 10% allowance is the general rule and is the default. Applied to the low end, it leaves 6,075 lbf. Design the joint around that figure, not the 9,000 lbf headline.
How to Tell If a Bolt Is Tight Enough
The most direct check is to measure how much the bolt has stretched. A bolt under load lengthens like a stiff spring, and the stretch is the preload times the grip length, divided by the stress area and the elastic modulus.
At 9,000 lbf over a 2 in grip, the 1/2-13 steel bolt stretches 0.0042 in, or about 0.0021 in for each inch of grip. Measure the bolt length with a micrometer before and after tightening and compare. This is the bolt elongation method, good to within 3 to 5%.
The Bolt Preload Calculator bases stretch on the threaded section for the whole grip. A bolt with a plain shank in the grip is stiffer there and stretches less. For the example, a full shank would stretch 0.0031 in, so treat the figure as the upper bound for a partly threaded bolt.
Bolt Preload Calculator in Metric
Switch Units to metric for kN, N·m and MPa. The size list changes to ISO coarse threads from M5 to M39, and the grades change to property classes. Proof strength is 225 MPa for Class 4.6, 830 MPa for Class 10.9 and 970 MPa for Class 12.9. Class 8.8 is 580 MPa up to M16 and 600 MPa above it.
An M12 Class 8.8 bolt has a proof load of 48.87 kN. Tightened dry to 88 N·m it carries 36.66 kN, which is 75% of proof. The same bolt in Class 10.9 reaches 75% at 125.9 N·m. A size or strength that is not listed can be typed under Custom Bolt.