Calculate the optimal sag and starting air pressure (PSI) for your mountain bike fork and rear shock. Enter your rider weight, suspension travel, and riding style, this sag calculator MTB riders trust shows your target sag in mm, acceptable range, and recommended PSI starting point. The sag percentages follow Fox, which gives one figure for both ends of the bike, and the pressures are this site's own starting points because no maker publishes a general chart by rider weight. Works as both a fork sag calculator and rear suspension PSI calculator, covering XC, Trail, Enduro, and Downhill setups for any air fork or air shock.
38 mm
Optimal sag
35 to 42 mm
Acceptable range
55 to 75
PSI starting point
Air pressure
Sag zone on 140 mm travel
0 mm27% sag, optimal for Trail140 mm
Target range: 25 to 30% sag (35 to 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.
Fork Air Pressure Chart, Starting PSI by Rider Weight
Starting pressures for an air fork by rider weight and travel, the same figures the calculator above returns. A 75 kg rider on a 140 mm fork starts around 55 to 75 psi, then sets pressure up or down until the sag measures right. The number that matters is the sag, not the pressure that got you there.
Rider Weight (kg / lbs)
120mm (XC)
140mm (Trail)
150mm (Enduro)
170mm (DH)
55 / 121
40 to 60
35 to 55
32 to 52
28 to 48
65 / 143
50 to 70
45 to 65
42 to 62
37 to 57
75 / 165
60 to 80
55 to 75
52 to 72
47 to 67
85 / 187
70 to 90
65 to 85
62 to 82
57 to 77
95 / 209
80 to 100
75 to 95
72 to 92
67 to 87
105 / 231
95 to 115
90 to 110
87 to 107
82 to 102
These are this site's starting points, not a manufacturer chart. More PSI means less sag (firmer), less PSI means more sag (plusher). Err high rather than low: too little pressure bottoms the fork out on a hit, which is the failure that hurts.
MTB Rear Shock Air Pressure Chart
Starting pressures for an air shock by rider weight and stroke, again the same figures the calculator returns. A 75 kg rider on a 55 mm stroke starts around 145 to 165 psi. Rear pressures run roughly three times fork pressures because the shock works through the frame's leverage.
Rider Weight (kg / lbs)
40mm stroke
55mm stroke
60mm stroke
55 / 121
120 to 140
105 to 125
100 to 120
65 / 143
140 to 160
125 to 145
120 to 140
75 / 165
160 to 180
145 to 165
140 to 160
85 / 187
180 to 200
165 to 185
160 to 180
95 / 209
200 to 220
185 to 205
180 to 200
105 / 231
220 to 240
205 to 225
200 to 220
Rear pressures vary more than fork pressures do, because air can size and the frame's leverage ratio change what a given pressure produces. That is exactly why no maker publishes a general chart. Use the sag gradations on the shock stanchion for the final setting.
Where these numbers come from
The pressures are ours. Neither Fox nor RockShox publishes a general air pressure chart by rider weight, and RockShox states the reason: frame design, leverage ratio and how a rider's weight sits on the bike change what a given pressure produces, so no single chart can cover a shock across the bikes it is fitted to. Its own tool asks which fork or shock you own. Fox tells you to change pressure in 5 psi steps until the sag measures right, and gives no weight table at all. So the figures here are this site's starting points, arrived at from common setup practice, and they are not a manufacturer recommendation.
Which way to be wrong. A pressure that is too high costs you grip and a harsh ride, and you feel it immediately. A pressure that is too low lets the fork or shock bottom out on a hit, which is how people get hurt, and you may not find out until the hit arrives. So treat every figure here as a floor to start from rather than a target, and measure the sag before you ride anything serious.
The sag percentages are a convention, and the makers disagree. Fox says 25 percent of travel for a fork or a rear shock, one figure for both ends, and tells you to move a few points either way for your discipline. RockShox says 15 to 20 percent at the front and 30 percent at the rear. This page uses one band per riding style, applied to both ends, which follows Fox rather than RockShox: XC 20 to 25 percent, Trail 25 to 30, Enduro 28 to 32, Downhill 30 to 35. If you run RockShox, its own guidance would put your fork firmer than the figure here. Either way the band is a place to start from, not a specification, and no standard defines it.
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.
5-Step Sag Measurement
Put a zip-tie around the fork stanchion or shock shaft and push it against the wiper seal.
Sit on the bike in your normal riding position, hands on bars, feet on pedals, weight centred.
Get off carefully without compressing the suspension further.
Measure the distance the zip-tie moved from the seal.
Divide by total travel and multiply by 100 to get sag percent.
If sag exceeds the target range, add air pressure 5 PSI at a time. If the ride feels harsh over small bumps, release 5 PSI and measure again. That 5 psi step is what Fox tells riders to do, and it is the method both makers describe instead of publishing a pressure chart.
Neither Brand Publishes a Sag Table by Riding Style
Fox gives one number: “As a starting point, set sag for your fork or rear shock to 25% of active travel”, one figure for both ends, with a few points either way for downhill or cross country, and no table by riding style. RockShox publishes 30 percent for rear shocks and 25 to 30 percent for coil, published for rear shocks, with 5 percent either way for riding style and terrain, and no fork table by riding style.
So the two do not even agree on the shape of the advice, let alone on a table by riding style. The four bands this calculator uses, 20 to 25 for XC through 30 to 35 for downhill, are this site’s own convention, and it aims at the middle of each rounded down to a whole percent.
The difference between brands is in the feel, meaning damping character, rather than in a setup target you could look up. A trail bike runs the same sag whether the fork says Fox or RockShox on it.
How to Measure Sag on Your MTB
Sag is how much your suspension compresses under your body weight while sitting on the bike in riding position. To measure: (1) Set your fork and shock to fully open (no lockout). (2) Sit on the bike in your normal riding position, feet on pedals, hands on bars, wearing your riding gear and pack. (3) Have a friend push the rubber o-ring on the stanchion up against the seal. (4) Carefully dismount without bouncing. (5) Measure the distance from the o-ring to the seal, this is your sag in mm. (6) Calculate sag percentage: (sag_mm / total_travel_mm) × 100. The calculator above gives you the target sag and PSI to achieve it. Once your suspension is set, check your tyre pressure, it works together with suspension for optimal grip and comfort.
What Sag Percentage Should I Run?
Optimal sag depends on riding style and terrain. XC racing: 20 to 25% sag, firmer platform for climbing efficiency, less travel used on descents. Trail: 25 to 30% sag, balanced setup for climbing and descending, good small bump sensitivity. Enduro: 28 to 32% sag, plusher for aggressive descending, still pedals well uphill. Downhill: 30 to 35% sag, maximum traction and bump absorption for high-speed descending. These are starting points. Fine-tune based on feel: if you are bottoming out frequently, add 5 to 10 PSI. If the suspension feels harsh and is not using full travel, remove 5 to 10 PSI. Riding e-MTB? Your heavier system weight affects setup, see our eMTB tyre pressure calculator for weight-adjusted recommendations.
Fork Sag vs Rear Shock Sag
Fork and rear shock sag are set independently because they serve different purposes. Fork sag affects steering feel and front-end grip. Too little fork sag makes the bike feel twitchy and loses front traction in corners. Too much makes the bike feel sluggish and dives under braking. Rear shock sag affects pedalling efficiency and rear traction. Most riders benefit from matching front and rear sag percentages, but some prefer slightly more rear sag for better traction or slightly less for better climbing. Pair your sag setup with proper rebound damping for a balanced ride, and learn more about compression damping to fine-tune how your suspension handles impacts. For overall suspension knowledge, visit our suspension setup guide.
Frequently asked questions
What PSI should I run in my Fox fork?
Fox does not publish a starting pressure by rider weight. Its own instruction is to pump the fork, measure the sag and change the pressure in 5 psi steps until the sag is right, with 25 percent of travel as the starting target for a fork or a shock. The calculator above gives a starting figure of our own so that you have somewhere to begin: for a 75 kg rider on a 140 mm fork that is 55 to 75 psi. Treat it as a first guess and let the sag decide, on a Fox 32, 34, 36 or 38 alike.
What PSI should I run in my RockShox fork?
RockShox does not publish a general pressure chart by rider weight, and says why: leverage ratio and weight distribution differ from frame to frame, so the same pressure produces different sag on different bikes. Its own tool asks for your exact fork model. The figure this calculator gives is a starting point of our own, and the pressure is only ever a means to an end. Pump to the starting figure, measure the sag, and adjust until the sag is right. If in doubt go higher rather than lower: too little pressure bottoms the fork out on a hit.
How much sag should a mountain bike fork have?
Most MTB forks should have 25 to 30% sag for trail riding. This means if your fork has 140mm of travel, you want 35 to 42mm of sag. XC riders go firmer, {SAG_TARGETS.xc.min} to {SAG_TARGETS.xc.max}%, enduro {SAG_TARGETS.enduro.min} to {SAG_TARGETS.enduro.max}% and downhill {SAG_TARGETS.downhill.min} to {SAG_TARGETS.downhill.max}%. Those are two separate bands, not one.
What is the difference between sag and air pressure?
Sag is the result, how many mm your suspension compresses under your weight. Air pressure (PSI) is the input, how much air you pump in. Higher PSI = less sag (firmer). The relationship is not linear because air springs are progressive. This bike sag calculator gives you both the target sag and the PSI needed to achieve it.
Does rider weight include gear?
Yes. Enter your total riding weight including clothing, helmet, backpack, hydration pack, and any tools you carry. This "all-up" weight can be 5 to 10 kg more than your body weight alone. Using just body weight will result in too little sag.
What sag should I run for trail riding?
Trail riding works best with 25 to 30% sag. For a 150mm fork this is 37 to 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 close to linear in this calculator: from 55 to 95 kg it adds 10 PSI per 10 kg on every fork travel except the longest, where the first step is 9, and the bracket widens above 95 kg.
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 to 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, and how much depends on the pressure you run, because gas pressure falls in proportion to ABSOLUTE pressure. A 5 degree drop costs roughly two percent of gauge pressure: about 1.0 PSI on a 45 PSI fork and about 3.7 PSI on a 200 PSI shock. A single figure of 1 PSI per 5 degrees only holds near the bottom of that range. 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. For overall suspension knowledge, visit our suspension setup guide.
Is Fox sag the same as RockShox sag?
No, the two makers publish different targets. Fox gives one figure for both ends, 25 percent of travel, and tells you to move a few points either way for your discipline. RockShox gives 15 to 20 percent at the fork and 30 percent at the rear shock. This page uses one band per riding style at both ends, which follows Fox, so a RockShox owner reading their own guidance would run the fork firmer than the figure here. Neither is a standard, and sag is a starting point either way: ride it and adjust.
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.