Suspension Sag Calculator
Enter your fork travel, shock stroke, and target percentages to get the exact sag in millimetres to measure, plus the kitted-up weight to look up on your fork maker’s pressure chart.
Set sag first, then everything else
Riders spend hours chasing rebound clicks and volume spacers while running the wrong sag — which quietly undoes all of it. Setting sag correctly balances the bike front to rear, gives the wheels the travel they need to stay on the ground, and makes every later adjustment behave predictably. It takes five minutes and a shock pump.
Sag is one half of your grip; tire pressure is the other. Once your suspension is set, use the MTB Tire Pressure Calculator to match your tires to the same trails.
Frequently Asked Questions
What is sag, and why does it matter so much?
Sag is how far your fork and shock compress under your static weight, just from sitting on the bike. It sets the suspension's ride height and how much travel is available to extend into holes and soak up hits. Get it right and the wheels track the ground; too little sag and the bike skips and feels harsh, too much and it wallows, bottoms out, and steers slowly. It is the foundation every other suspension setting is built on, so it comes first.
How much sag should I run?
As a starting point, most trail forks are set around 15–20% of travel and most rear shocks around 25–30%. Cross-country riders tend toward the firmer end for efficient pedalling; enduro and downhill riders toward the softer end for grip and bottom-out control. The calculator defaults to 20% front and 30% rear, but you can enter any target and it returns the millimetres to measure.
How do I actually measure sag?
Push the rubber o-ring on the stanchion (or shock shaft) down to the seal, then get on the bike in your riding gear, feet on the pedals, in a neutral standing position — ideally leaning against a wall rather than dabbing a foot. Settle gently, step off without bouncing, and measure how far the o-ring moved. Compare that to the target millimetres this tool gives you, then add or release air to match.
Why doesn't the calculator give me an air pressure?
Because no single rule fits every fork. The old shortcut of 'psi equal to your body weight in pounds' would put a 165 lb rider at 165 psi, while FOX's 36 FLOAT chart starts that rider (the 160-170 lb row) at 82 psi and lists 120 psi as the fork's maximum. Air springs differ that much between models. Start from the pressure chart printed on your fork leg or in the maker's app for your kitted-up weight, never exceed the maximum printed there, then measure sag and fine-tune the pressure until the o-ring shows your target. Volume spacers and rebound are separate settings to dial afterward.
A setup starting point, not a manufacturer specification. Always follow your fork and shock maker's pressure and service guidance, and confirm sag by measurement.