Bolt Torque Calculator
Work out the tightening torque for a bolt from its diameter, grade and friction (nut factor), with the clamp load it gives, in imperial or metric units. SAE Grade 2, 5 and 8 or metric class 8.8, 10.9 and 12.9, with the stress area and proof load shown.
Your numbers
Diameter and threads per inch (UNC or UNF), or metric coarse diameter × pitch.
The head marking: Grade 5 has three radial lines, Grade 8 six; metric heads are stamped 8.8, 10.9 or 12.9.
Fastenal's values. Lubrication lowers K, so the same torque gives more clamp load.
Share of the bolt's proof load. Fastenal's charts use 75%.
Tightening torque
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- Clamp load (preload)
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- Proof load
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- Proof strength
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- Tensile stress area
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- Nut factor used
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Torque is only an indirect measure of bolt tension: friction under the head and in the threads varies, so the clamp load from a given torque scatters.
Fastenal: these values are reachable only if the nut or tapped hole has a proof load at least equal to the bolt's minimum tensile strength.
The calculation runs in your browser and ParrotNotes does not store your numbers. They sit in the page address so you can bookmark or share the result.
How bolt torque is worked out
Tightening a bolt stretches it, and the stretch is what clamps the joint. The calculator starts from that clamp load: a share of the bolt's proof load, 75% by default as in Fastenal's charts. The proof load is the grade's proof strength times the thread's tensile stress area.
The torque that gets there comes from the short-form formula T = K × D × F, with K the nut factor, D the nominal diameter and F the clamp load. In inch units D in inches times F in pounds gives lbf·in, divided by 12 for lbf·ft; in metric, D in millimeters times F in newtons gives N·mm, divided by 1,000 for N·m.
Tensile stress area
A bolt's strength is figured on the tensile stress area of its thread, which is smaller than the area of the plain shank. For inch threads it is 0.7854 × (D − 0.9743 ÷ n)², with n the threads per inch: 0.1419 sq in for 1/2-13 and 0.1600 for 1/2-20. For metric threads it is 0.7854 × (d − 0.9382 × P)² with the pitch P in millimeters: 58.0 mm² for M10 × 1.5.
Pick a size from the list, or choose "Other size" and enter the diameter and thread; SAE J429 grades cover 1/4 to 1 1/2 in and the metric classes here M5 to M39.
Proof strength by grade
Minimum proof strength, from Fastenal's SAE J429 table and its ISO 898-1 torque chart. The proof load is this times the tensile stress area.
| Grade or class | Sizes | Proof strength |
|---|---|---|
| SAE Grade 2 | 1/4 to 3/4 in | 55,000 psi |
| SAE Grade 2 | 7/8 to 1 1/2 in | 33,000 psi |
| SAE Grade 5 | 1/4 to 1 in | 85,000 psi |
| SAE Grade 5 | 1 1/8 to 1 1/2 in | 74,000 psi |
| SAE Grade 8 | 1/4 to 1 1/2 in | 120,000 psi |
| ISO 8.8 | M5 to M16 | 580 MPa |
| ISO 8.8 | M18 to M39 | 600 MPa |
| ISO 10.9 | M5 to M39 | 830 MPa |
| ISO 12.9 | M5 to M39 | 970 MPa |
The nut factor K
K sums up how much of the torque ends up as bolt tension, and it changes with the finish and the lubrication, so it moves the answer more than anything else. Fastenal uses 0.20 for plain and dry, 0.17 for zinc plated and dry and 0.15 for lubricated threads.
Use the same condition the torque was meant for. Lubricating a bolt and then tightening it to a dry torque can push it past its proof load; tightening a dry bolt to a lubricated torque leaves it short of clamp load.
For industrial distributors and manufacturers' reps
A fastener question usually arrives as a call from the line: what torque for this bolt, dry or with anti-seize, will a Grade 8 hold where a Grade 5 stretched. Checking the number while the customer is on the phone, and noting which grade, finish and lubricant they said, saves a second call and a wrong order.
Sources, each read on 2026-10-29. Results are estimates; the fastener maker's or the joint designer's torque spec governs:
- Fastenal, Torque-Tension Relationship for A307A, Grade 5, 8 & 9 Bolts: T = KDF, K values, clamp load at 75% of proof
- Fastenal, Mechanical Properties of Inch Fasteners: SAE J429 proof strengths, tensile stress area formula and table
- Fastenal, Torque-Tension Relationship ISO 898-1 Property Class 4.6, 8.8, 10.9 & 12.9: proof loads, clamp loads, torques
- Engineered Parts, Torque Tensioning chart: metric tensile stress area formula
More for distributors and manufacturers' reps: Industrial distributor sales, Manufacturers' rep, Distributor sales, Sales engineer.
The torque question comes in on a call
Which bolt, which grade, dry or lubricated, and what it is clamping: customers say it on the phone, and the details decide the answer. Record the call in ParrotNotes and get a transcript, a summary and the action items, so the follow-up and the order match what was asked.
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