MTB tire pressure guide

Mountain bike tires are sold in inches, but pressure follows air volume and volume follows the real, mounted width. Multiply the inch figure by 25.4 to get millimetres: 2.4" is about 61 mm, 2.6" about 66 mm.

Then check it. A wide rim can push a 2.4" tire out to 63–64 mm and a narrow one can pull it in to 58 mm. Inflate the tire, leave it overnight, measure the widest point of the casing with a caliper and use that number in the calculator.

Four inputs do almost all the work; everything else is fine-tuning:

• Loaded weight — rider in full kit, bike, water, pack and tools. Body weight alone underestimates by 10–15 kg.
• Measured tire width and wheel size — 29" and 27.5" wheels of the same width hold different volumes, so the same pressure behaves differently.
• Terrain and impact severity — smooth hardpack tolerates more pressure; square-edged rock and repeated hard landings need a casing that can deflect instead of bouncing.
• Pace and riding style — a rider hitting the same rock garden 8 km/h faster loads the casing far harder and needs a few more PSI or a tougher tire.

Lower pressure lets the knobs wrap around rock and root instead of skating off it, and it lets the casing absorb chatter that would otherwise be lost as vibration and arm fatigue. That is why almost every rider is faster on trail at a pressure that feels soft on the driveway.

The floor is set by two failures: the tire folding sideways in hard corners, and the rim hitting through the casing on square edges. Find the pressure where neither happens on your regular trail, then add 1–2 psi as your margin and stop there.

The rear wheel carries roughly 60 % of your loaded weight, takes almost every square-edge hit under power and is the wheel you land on. Run it 2–4 psi (0.15–0.3 bar) above the front.

The front wheel is where cornering grip and steering feel come from, so it is the one that benefits most from the extra millimetre of casing deflection. The calculator splits front and rear for you, and handles a wider rear or front tire if you run mixed sizes.

Tubeless is the baseline for low pressure: with no inner tube there is nothing to pinch, so you can typically run 3–6 psi below the same tire with a tube. If you still ride tubes, keep a real safety margin — pinch flats arrive without warning.

Casing thickness matters as much as pressure. A light XC casing needs more air to stay stable; an enduro or downhill casing supports itself and can be run several PSI lower for the same rider. Foam inserts add rim protection and sidewall support at the bottom of the range, which is why bike park and enduro riders can drop into the mid teens.

If you are burping air in corners or hitting the rim repeatedly, the answer is a tougher casing, an insert or a wider tire — not simply more pressure, which trades away the grip you came for.

Start from the calculator's number, then shift it for the day:

• Wet roots, mud and loose-over-hard: drop 1–2 psi (0.1 bar) for grip.
• Dry, fast, rocky and square-edged: add 1–2 psi to protect rim and casing.
• Bike park or shuttle laps with big landings: add 2–3 psi over your trail setting, or fit an insert.
• Riding with a loaded pack: recalculate with the total weight rather than guessing.
• Cold start: pressure rises as the tire warms up, so setting slightly low in the car park is safe.
• Altitude change on a long descent: check pressure at the top of the trail, not at the bottom.

Use a digital gauge, not the pump dial — at 20 psi a 2 psi error is 10 % of your pressure. Change one wheel at a time, ride the same section twice and notice which failure appears first: vague, folding cornering means too low, skipping and deflection off rocks means too high.

Write the winning numbers down. In Pressure Lab Cycling you can save the setup in the garage, mark it as your current setup and have the tire, width and pressure appended automatically to your Strava rides, so you always know what you actually rode.