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
Results
- Sag
- 1.1 in
- Starting pressure
- 77.7 psi
The Result is indicative and educational. It does not constitute a design, a technical opinion or medical advice and does not replace calculations or verification by a person with the appropriate qualifications and authorisations. Before purchase, installation or use in practice – in particular in electrical installations – the Result should be verified by a qualified specialist. Details: Terms of Service, point 4.
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. Shown in bar, or in psi with imperial units. Example: 75 kg (165.3 lb) on a 140 mm fork at 20% → 28 mm and 77.7 psi = 5.36 bar.
The 0.47 psi-per-pound factor is a generic mid-travel guess, not a Fox or RockShox chart. Air-shaft size and volume spacers 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?
Push the O-ring against the dust wiper, then get into your riding position: seated for XC and trail, standing in attack position for enduro and downhill. Step off without bouncing and measure the gap. Compare it with the sag figure here.
XC versus enduro sag?
Fox sets its forks at 15–20%: 15% firm, 20% plush. XC racers sit at the firm end, and some frame makers ask for up to 25% on trail bikes. Figures of 25–33% are for rear shocks, not forks.
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. For comparison, the 2026 Fox 36 chart starts a 160–170 lb rider at 82 psi, about 0.5 psi per pound.
Example products
Product links are Affiliate Links to Amazon websites. Using them does not cost you anything extra, and if you make a purchase we may receive a commission. Details: Terms of Service, point 5.
- Shock pump
A frame pump will not meter a fork air spring in small PSI steps.
$25–$60