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 based on Fox and RockShox setup guidelines. 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
35–42 mm
Acceptable range
55–75
PSI starting point
Air pressure
Sag zone on 140 mm 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.
Fox Fork Air Pressure Chart — Starting PSI by Rider Weight
Brand-specific starting PSI for Fox forks across common travel ranges. Use this as a fox sag calculator quick lookup.
Rider Weight (kg / lbs)
120mm (XC)
140mm (Trail)
150mm (Enduro)
160mm+ (DH)
55 / 121
45-55
40-50
38-48
35-45
65 / 143
55-65
50-60
48-58
45-55
75 / 165
65-75
60-70
55-65
50-60
85 / 187
75-85
70-80
65-75
60-70
95 / 209
85-95
80-90
75-85
70-80
105 / 231
95-105
90-100
85-95
80-90
Values are starting points from Fox Racing Shox setup guides. Fine-tune based on your riding preference — more PSI = less sag (firmer), less PSI = more sag (plusher).
RockShox Fork Air Pressure Chart — Starting PSI by Rider Weight
Use this rockshox sag calculator chart for Pike, Lyrik, Zeb and similar air forks. Pressures shown are approximate.
Rider Weight (kg / lbs)
120mm (XC)
140mm (Trail)
150mm (Enduro)
160mm+ (DH)
55 / 121
42-52
38-48
36-46
33-43
65 / 143
52-62
48-58
46-56
43-53
75 / 165
62-72
58-68
53-63
48-58
85 / 187
72-82
68-78
63-73
58-68
95 / 209
82-92
78-88
73-83
68-78
105 / 231
92-102
88-98
83-93
78-88
Values based on SRAM/RockShox Suspension Welcome Guide. RockShox forks may require different pressures than Fox for the same rider weight due to air spring design differences.
MTB Rear Shock Air Pressure Chart
MTB shock pressure starting points by rider weight and riding style. Use as a reference alongside the calculator above.
Rider Weight (kg / lbs)
Trail (30% sag)
Enduro (30% sag)
DH (35% sag)
55 / 121
100-120
95-115
90-110
65 / 143
120-140
115-135
110-130
75 / 165
140-160
135-155
125-145
85 / 187
160-180
155-175
145-165
95 / 209
180-200
175-195
165-185
105 / 231
200-220
195-215
185-205
Rear shock pressures vary significantly between shock designs (air can size, leverage ratio). These are approximate starting points. Always use the sag band on your shock stanchion for final setup.
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 %.
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 re-measure.
Both Brands Target the Same Sag Values
Both Fox and RockShox target similar sag percentages for equivalent riding styles. Fox recommends 20–25% for XC, 25–30% for Trail, and 30% for Enduro. RockShox targets are within 1–2% of Fox values. The calculator uses the midpoint of each range as the optimal target.
The difference between brands is in the feel (damping character), not the setup target. A trail bike should run 25–30% sag regardless of whether you have Fox or RockShox suspension.
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: 20–25% sag — firmer platform for climbing efficiency, less travel used on descents. Trail: 25–30% sag — balanced setup for climbing and descending, good small bump sensitivity. Enduro: 28–32% sag — plusher for aggressive descending, still pedals well uphill. Downhill: 30–35% 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 5–10 PSI. If the suspension feels harsh and is not using full travel, remove 5–10 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 fork starting pressure depends on rider weight and travel. For a 75 kg rider with 140mm travel trail fork, start around 60–70 PSI. Use the calculator above or the Fox pressure chart for your exact weight and travel combination. Fox recommends 25–30% sag for trail riding. This page works as a fox sag calculator for any Fox 32, 34, 36, 38 model.
What PSI should I run in my RockShox fork?
RockShox starting pressure varies by model and rider weight. For a 75 kg rider with a RockShox Pike (140mm), start around 55–65 PSI. RockShox air springs may require slightly different pressures than Fox due to design differences. Use the calculator above as a rockshox sag calculator for a precise recommendation.
How much sag should a mountain bike fork have?
Most MTB forks should have 25–30% sag for trail riding. This means if your fork has 140mm of travel, you want 35–42mm of sag. XC riders go firmer (20–25%), enduro/DH riders go plusher (28–35%).
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 5–10 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 25–30% 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. For overall suspension knowledge, visit our suspension setup guide.
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.