MTB Suspension Sag Calculator

Find your optimal sag in mm and starting air pressure. Based on Fox and RockShox setup guidelines.

38 mm
Optimal sag
35–42 mm
Acceptable range
55–75
PSI starting point
Air pressure
Sag zone on 140mm travel
0 mm27% sag — optimal for Trail140 mm
Target range: 25–30% sag (35–42 mm)
⚠ For reference only. These are starting points based on Fox Racing Shox and RockShox setup manuals. Correct sag requires measuring with a zip-tie on the stanchion while sitting in your normal riding position. Fine-tune by feel — add air if sag exceeds target, release if too firm.

Sources: Fox Owner's Manuals and RockShox Suspension Setup Guide

How it works

Sag — the Foundation of Suspension Setup

Sag is how much your suspension compresses under your weight in a neutral riding position. It determines how much travel is available for both bump absorption (extension) and compression. Too little sag makes the bike harsh and loses traction. Too much sag uses up travel before you hit a bump.

Why sag matters

With correct sag, your suspension sits in the middle of its travel when you're sitting on the bike in a normal riding position. This gives equal travel for absorbing bumps below you (compression) and for extending over dips and drops (extension). Without correct sag, the suspension is biased in one direction and cannot do both jobs effectively.

Frequently asked questions

What sag should I run for trail riding?

Trail riding works best with 25–30% sag. For a 150mm fork this is 37–45mm. Start at 27% (40mm) and adjust based on feel — if you bottom out frequently, add 5 PSI. If the ride feels harsh over small bumps, release 5 PSI.

Why does rider weight affect sag pressure?

Heavier riders compress the air spring more for the same amount of travel. A 90kg rider needs more air pressure than a 60kg rider to achieve the same sag percentage. The relationship is roughly linear — 10kg more weight requires approximately 8–12 PSI more air.

What is the difference between fork sag and shock sag?

Fork sag is measured on the front stanchion and targets the same percentage as rear shock sag for most riding styles. Some riders prefer the rear shock 2–3% lower (more sag) than the fork to keep the bike level and improve traction on climbs.

How often should I check my sag?

Check sag at the start of each riding season and whenever air pressure feels noticeably different. Air temperature affects pressure — cold weather reduces pressure by approximately 1 PSI per 5°C drop. Fox and RockShox recommend checking before every ride in competition.

Can I use sag with coil suspension?

Coil springs use different setup procedures — sag is controlled by spring rate (kg/mm) not air pressure. This calculator is for air suspension only. For coil setup, consult your spring manufacturer's sag chart.

What does it mean if I can't reach the recommended sag?

If you reach maximum pressure (200+ PSI fork, 300+ PSI shock) before achieving target sag, your rider weight may exceed the suspension's design range. Consider a higher-volume air spring, a negative spring volume reducer, or a heavier-duty suspension model.

Is Fox sag the same as RockShox sag?

Yes — both Fox and RockShox target the same sag percentages for equivalent riding styles. The difference is in the feel (damping character) not the setup target. A trail bike should run 25–30% sag regardless of brand.

How do I measure sag without a zip-tie?

Mark the stanchion with a dry-erase marker at the wiper seal, sit on the bike, get off, and measure the distance from the mark to the seal. A dedicated sag scale tool (available from Fox, RockShox, and third-party brands) clips onto the stanchion and reads directly in mm.