Suspension Sag Calculator (Mountain Bike and Motorcycle Fork and Shock Setup)
How much a fork or shock compresses under your weight, as millimetres and as a share of its travel, compared with the sag you are aiming for.
Results
The dashed line marks the value you entered.
What this tool does
Sag is how far the suspension sinks under the rider's weight alone, and setting it is the first step of any suspension setup: too little and the wheel does not follow the ground, too much and it bottoms out and wallows. Measure the fork or shock fully extended, then again with you on the bike in riding gear and riding position, and subtract. A common starting point is roughly 25–35 % of travel, with the right figure depending on discipline and the maker's guidance. If the sag is too big, add spring preload or air pressure; too small, reduce it.
Formula
sag = extended − loaded · sag % = sag ÷ travel × 100 · excess = sag − target × travel
Variables
| Symbol | Meaning | Unit |
|---|---|---|
tv | Total suspension travel | mm |
l0 | Measurement with the suspension fully extended (no rider) | mm |
l1 | Measurement with you on the bike in riding position | mm |
tg | Sag you are aiming for | % |
SP | Sag as a share of the travel | % |
SG | Sag | mm |
TM | Target sag | mm |
AD | Excess of sag over the target (positive = too soft) | mm |
RM | Travel left to absorb bumps | mm |
Worked example
- Total suspension travel150 mm
- Measurement with the suspension fully extended (no rider)500 mm
- Measurement with you on the bike in riding position455 mm
- Sag you are aiming for30 %
- Sag as a share of the travel30.0 %
- Sag45.0 mm
- Target sag45.0 mm
- Excess of sag over the target (positive = too soft)0.0 mm
- Travel left to absorb bumps105.0 mm
Limitations
- The default values are typical reference figures, not measurements of your situation. Replace them with your own data whenever you have it.
- Mixing units is the most common source of error. Convert every input to the units shown next to each field before calculating.