Bolt Torque Calculator
Estimate the tightening torque needed to reach a target clamp load on a bolt.
Reviewed by the ToolNestr Editorial Team — July 2026
How the bolt torque calculator works
Turning a bolt with a wrench converts rotational torque into a straight-line clamping force, but most of that torque is lost to friction — under the bolt head, under the nut, and in the threads themselves — rather than stretching the bolt. The widely used short-form torque-tension formula bundles all of that friction into one coefficient K, so that torque T equals K times the nominal diameter D times the target clamp load F.
The K factor, often called the nut factor, depends heavily on surface condition. A plain steel bolt with light lubrication runs around K = 0.2 — this is the number quoted in most general torque charts (SAE J1701 and similar references). Waxed, cadmium-plated, or otherwise low-friction fasteners can drop closer to 0.15, while dry, rusty, or galled threads can push K above 0.3. Because K multiplies directly into the answer, a bolt that looks identical but has different surface friction needs meaningfully different torque for the same clamp load.
Clamp load itself is usually set as a fraction of the bolt's proof load or yield strength — commonly around 75%, per SAE and ASTM guidance — but the right target always depends on the joint design, bolt grade, and application. Because torque is only an indirect proxy for clamp load, critical or safety-related joints often specify torque-plus-angle or direct bolt-stretch methods instead of torque alone. Always follow a manufacturer's or engineer's torque spec when one exists; use this tool only to estimate when no spec is given.
The short-form torque-tension formula and typical K-factor ranges are referenced in SAE J1701 and the Industrial Fastener Institute's torque-tension guidelines.
Enter the bolt size
Nominal diameter in inches.
Set the clamp load
Your target preload, in pounds.
Match the condition
Pick the K factor that fits the fastener finish.
The formula explained
Torque-tension formula
T = K × D × F. A 0.5 in bolt, 8,000 lb clamp load, K = 0.2: 0.2 × 0.5 × 8000 = 800 lb·in.
Convert to foot-pounds
÷ 12 = 800 ÷ 12 = 66.7 lb·ft, the usual torque wrench unit.
Metric conversion
N·m = lb·ft × 1.3558 = 66.7 × 1.3558 ≈ 90.4 N·m.
Worked example
A dry, as-received 0.375 in bolt targeting 5,000 lb clamp load, K = 0.3.
Real-world context
A structural steel connection using a 3/4 in A325 bolt, targeting 75% of its roughly 39,000 lb proof load (about 29,250 lb clamp force) with a plain, lightly lubricated finish (K = 0.2), needs T = 0.2 × 0.75 × 29,250 = 4,387.5 lb·in, or 365.6 lb·ft (about 495.7 N·m). That's the kind of torque figure that shows up on structural steel torque charts for new, clean fasteners of that grade and size.
Reuse that same bolt after it's spent a season exposed to weather, and surface rust can push K up to around 0.3. Applying the identical 365.6 lb·ft torque to the now-rustier threads doesn't deliver the same clamp force — back-solving F = T ÷ (K × D) gives only about 19,500 lb, roughly a third less than the 29,250 lb target. This is exactly why structural codes generally require new, unused fasteners for critical connections rather than reusing bolts with unknown or degraded surface condition.
On the smaller end, a 3/8 in machine bolt with a waxed or cadmium-plated finish (K = 0.15) targeting a 5,000 lb clamp load only needs T = 0.15 × 0.375 × 5,000 = 281.25 lb·in, or about 23.4 lb·ft (roughly 31.8 N·m) — light enough that many mechanics would reach for a smaller torque wrench scale entirely, illustrating how much diameter and finish, not just clamp load, drive the final torque number.
Common misconceptions
"Torque tells you exactly how much a bolt is clamped." Torque is only a friction-dominated proxy. Roughly 90% of applied torque overcomes friction under the head and in the threads — only a small fraction actually stretches the bolt to create clamp load. Two bolts torqued identically but with different surface conditions can end up with very different actual clamp force.
"A torque spec works the same on a reused or corroded bolt." Surface condition changes K, which changes the outcome. Rust, galling, or old thread-locker residue can raise the friction coefficient enough that the same torque produces significantly less clamp force — which is why many specs call for fresh fasteners, new lubrication, or a documented K factor rather than reusing hardware as-is.
Related calculators
Frequently asked questions
How is bolt torque calculated from clamp load?
The standard approximation is T = K · D · F, where T is torque, K is the friction (nut factor) coefficient, D is the nominal bolt diameter, and F is the target clamp (preload) force. A typical K of 0.2 covers plain steel bolts with light lubrication.
What is the K factor (nut factor)?
K bundles together thread friction, bearing-surface friction, and thread geometry into one number. Dry steel-on-steel runs around 0.2, cadmium-plated or waxed bolts can drop to 0.15, and rusty or galled threads can push K above 0.3 — small changes in K swing torque significantly for the same clamp load.
What clamp load should I use?
A common starting point is 75% of a bolt’s proof load or yield strength, per SAE/ASTM guidance, though the exact target depends on the joint, bolt grade, and application. Always use the torque spec from the equipment manual or design drawing when one exists — this calculator is for estimating when no spec is given.
Why does torque accuracy matter?
Under-torquing leaves a joint loose and prone to fatigue or vibration loosening; over-torquing can yield or snap the bolt, or crush a gasket. Because friction dominates the torque-tension relationship, a torque wrench alone is only an approximation — critical joints often use angle-of-turn or bolt-stretch methods instead.
Sources & references
This tool uses standard formulas and reference values from:
- • American Concrete Institute — ACI 318, Building Code Requirements for Structural Concrete. concrete.org
- • ICC — International Residential Code (IRC), span, footing and framing tables. codes.iccsafe.org
- • APA – The Engineered Wood Association, allowable span and load guidance.
Estimates for planning only. Span, load and code values vary by jurisdiction — verify against your local adopted code and a licensed engineer before building.