Pressure

How hard should those tires be, really?

Rider & Bike

Rider weight 75 kg
Bike + gear weight 9 kg
System weight: 84.0 kg

Tires

Front width 28 mm
Rear width
Setup

Surface

Weight distribution

Bike type
Front / Rear 45% / 55%
Advanced: casing stiffness
Casing

Recommended pressure

Front
—
PSI · — bar
±— PSI
Rear
—
PSI · — bar
±— PSI
Why this number?

Tire pressure is a 100-year-old debate. This tool fits a curve against published recommendations from Frank Berto's tire-pressure algebra, Bicycle Rolling Resistance's empirical Crr data, and the SRAM / Silca / Specialized public pressure guides — then shows you the math so you can argue with it.

The formula is PSI = (axle_load_kg0.95 × 305) / width_mm1.5 multiplied by modifiers for surface, setup, and casing. No proprietary formula is reused. The constant is fitted independently against published reference ranges and is released as CC0 / public domain. Sources & methodology →

Sources & methodology

Tire pressure depends on system weight, tire width, surface, casing, and setup. No single formula captures every nuance, but the literature converges on a well-defined target: the pressure that produces about 15% tire compression at the contact patch under static load (Berto's "drop test"). This is the methodology behind every modern pressure calculator.

Formula

Per-axle PSI = (axle_load_kg0.95 × 305) ÷ tire_width_mm1.5, then multiplied by surface, setup, and casing modifiers; clamped to [15, 130] PSI and capped at 73 PSI for hookless rims per ETRTO.

The constant 305 is fitted against published pressure recommendations from SRAM AXS, Silca, and Specialized. A 75 kg rider + 9 kg bike on 28 mm tubeless tires (smooth tarmac) yields ~65 PSI front / ~75 PSI rear, within ±5 PSI of the three reference calculators.

Calibrated range

The fit is densest for road and gravel widths (23–50 mm) — that's where the reference calculators overlap most. Output is reliable to ±5 PSI in this range.

For MTB widths (50–90 mm), the formula is conservative-low because MTB pressure is constrained by impact protection and bead retention, not the rolling-efficiency model the formula uses. Published MTB guides typically recommend 4–8 PSI more than this tool's output for trail riding. Treat the result as a starting point; add air based on the surface and how the bike feels.

For fat bikes (>90 mm), the formula extrapolates but isn't calibrated. Published recs vary by 100%+ depending on terrain (snow vs dirt vs sand) — more variance than width alone explains. Not modeled in this version.

Load capacity warning

Separately from the pressure formula, this tool shows a warning when your axle load approaches the tire's typical max-load rating. The estimate is max_load_kg ≈ 3.2 × width_mm − 5, a linear fit through ETRTO and manufacturer load tables across road, gravel, and MTB widths. The warning fires at 90% of capacity and escalates at 100%. Real tire ratings vary ±15% by casing construction — check your specific tire's sidewall for its actual max.

Heavier riders on narrow tires get this warning fast. A 95 kg rider on 21 mm tubulars is right at the limit (which is why pro riders weighing 60-70 kg got away with that setup in the 80s but most modern amateurs shouldn't try it).

Sources

Uncertainty

The ±5% band on each output is a reminder that pressure is preference-sensitive. Skilled riders adjust by feel after the first few miles. Treat the number as a starting point, not a target.

License

Formula and modifier tables are released CC0 / public domain. Copy, fork, embed, fight about it. Source on GitHub.