CalculatorsFork Sag and PSI Calculator

Fork Sag and PSI Calculator

Sag is how far the fork sits into its travel with you in riding position. Trail bikes often start near 20%. The PSI figure is a generic air-spring guess for a mid-travel fork, not a Fox or RockShox chart.

Inputs

Rider weight (lb)Riding kit included.Fork travel (in)Target sag (%)

Results

Sag: 1 in
Starting pressure: 77.71 psi

How the number is made

Sag mm = travel × sag% ÷ 100. Starting PSI ≈ rider pounds × 0.47 × (20 ÷ sag%). A higher sag target lowers the suggested pressure. Displayed in bar.

The 0.47 psi-per-pound factor is a generic mid-travel guess, not a Fox or RockShox chart. Tokens, volume spacers and a different air shaft move the real pressure. Use the brand decal once the fork is named.

Related calculators

Rear Shock Spring Rate Calculator
The rear shock is a different spring, often a coil rather than air.
Brake Rotor Size Calculator
More travel and more speed usually want a larger front rotor.

Questions

How do I measure sag?

O-ring against the seal, sit in attack position, step off gently, measure the ring gap. Compare it to the millimetres here.

XC versus enduro sag?

XC often likes 15–20%. Trail 20–25%. Enduro can go toward 30% if the fork has the volume spacers to hold it up.

Is the 0.47 factor a Fox or RockShox chart?

No. 0.47 is a generic mid-travel starting guess so the sag target has a pressure to try. It is not a manufacturer air-spring table.

Related gear

Shock pump
A frame pump will not meter a fork air spring in small PSI steps.
$25–$60

Disclaimer — no professional advice, no liability

These calculators are simplified estimates for education. They are not a bike fit, a structural inspection, or a torque specification that overrides the number printed on a part. Confirm chain length, spoke length, tire fit and bolt torque on the bicycle before you ride.

These calculators, pages and any related content are provided “as is” and “as available”, solely for general educational and informational purposes. They are simplified mathematical estimates, not a professional bike fit, a structural inspection, a manufacturer torque specification or a training, nutrition or medical plan.

Results can be wrong, incomplete or inapplicable to your frame, wheels, tires, drivetrain or suspension. You must independently verify every figure, assume all risk, and confirm chain length before you cut, spoke length before you lace, tire and rim compatibility before you inflate, and the torque printed on the part before you tighten it.

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