How to use
- 1Enter your rider weight and the weight of bike and gear (kg or lb).
- 2Type the tire width and the rim’s internal width in millimetres.
- 3Pick the setup (inner tube, hooked tubeless, hookless), the surface and, if needed, wet conditions.
- 4Adjust the front/rear weight split; front and rear pressure appear in bar and psi.
How the calculation works
The tool uses an approximation derived from Frank Berto’s “15% tire drop” approach published in Bicycle Quarterly: the load on each wheel is divided by the tire’s actual width to the power of 1.5. Because wider rims spread the tire, the actual width is corrected for rim internal width. It then suggests about 6% less for tubeless, 5% less on rough tarmac or gravel and 5% less in the wet. Example: a 75 kg (165 lb) rider on a 9 kg bike, 28 mm tires on a 21 mm rim, inner tubes and a 40/60 split gives roughly 50 psi (3.4 bar) front and 75 psi (5.1 bar) rear.
The 72.5 psi limit for hookless rims
Under ETRTO and ISO 4210, hookless road rims must not exceed 5 bar (72.5 psi), and tires narrower than 25 mm are not allowed on them. If the result is above that, the tool caps it at 72.5 psi and shows a warning; a wider tire is then the safer and more comfortable fix. If the limit printed on your tire or rim is lower, always go by the lower figure.
Why front and rear differ
On a road bike roughly 40% of the weight sits on the front wheel and 60% on the rear, so the rear tire needs more pressure. In an aero position or when descending on an MTB the front carries more, so you can move the split up to 45%. Treat the result as a starting point: if the bike chatters and bounces, drop 2–3 psi; if the tire squirms in corners or you hit the rim, add some.
Frequently asked questions
What PSI should 28 mm road tires be?
For a total weight of 75–85 kg (165–190 lb) with tubes on tarmac, about 48–54 psi front and 72–80 psi rear is a good start. Tubeless can go 3–5 psi lower.
How much lower should tubeless pressure be?
Without the pinch-flat risk of a tube, riders usually run 5–10% less. The calculator applies about 6%.
What is the max pressure for hookless rims?
72.5 psi (5 bar) per ETRTO/ISO. If the rim maker states a lower limit, that one applies.
Should I lower tire pressure in the rain?
Yes, about 3–4 psi (roughly 5%) lower increases the contact patch and grip.
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