Bike tyre pressure calculator

How the calculator works

When you sit on your bike, your tyres squash a little. This is called tyre drop. Frank Berto, a bike writer, measured how much dozens of tyres squash at different pressures. His chart shows the pressure that makes a tyre drop by 15% of its height.

Bicycle Quarterly tested how fast tyres roll. It found that pressure is close to the best mix of speed and comfort. With less air you roll slower. With more air you go no faster, but the ride gets much harsher.

The calculator adds your weight to your bike's weight. It splits the total between the two wheels, with more on the back. Then it works out the pressure for each tyre. Wider tyres need less air for the same weight.

Front and back tyres

Most of your weight sits over the back wheel. Bicycle Quarterly weighed a range of bikes. A road bike carried about 40% at the front and 60% at the back. A bike with a bag on the front was closer to 45% and 55%. A town bike with a load on the back rack was about 35% and 65%.

To check your own bike, put one wheel on bathroom scales and the other on a block of the same height. Sit on the bike as you would to ride, and ask a friend to read the scales. Then turn the bike round and do the same for the other wheel.

Rough roads and gravel

The numbers suit average road surfaces. Bicycle Quarterly says a little less air can help on very rough or unmade roads. On very smooth roads, a little more may help.

Safe limits

Your tyre and your rim each have a lowest and a highest pressure printed on them. Always stay inside both. Pump to the answer here, ride, and change it a little at a time until it feels right.

Where the numbers come from

• The 15% tyre drop chart is Frank Berto's. It is printed in the Bicycle Quarterly article on tyre pressure for your weight, which also gives the weight splits.
• Berto's chart is a set of lines, not a formula. Dave Adams fitted a formula to Berto's measured points, shared on the BikeTinker blog. We checked it against the worked example in Bicycle Quarterly. It lands within about 3 psi.
• Berto measured 700c tyres from 20 to 37 mm wide. For wider tyres the answer is a rougher guess. Rene Herse Cycles also points out that the chart uses average tyres from the 1990s, and soft, supple tyres can differ.
• The change from psi to bar uses the NIST conversion table.