Cycling Tire Pressure Calculator

Bike Tire Pressure Calculation

Modern tire pressure science (Silca, ENVE, Frank Berto) recommends much lower pressures than the sidewall maximum. Lower pressure = better grip, comfort, and often lower rolling resistance on real-world rough roads.

The formula

Pressure scales with the load and falls off steeply as the tyre gets wider, because a wider tyre holds up the same rider on a bigger air volume:

Rear PSI = 1.084 × total weight in kg × (25 ÷ tyre width in mm)^1.71

Front PSI = 0.85 × Rear PSI, since the front wheel carries less of you.

The exponent is what people underestimate. Going from 25 mm to 32 mm is only a 28% wider tyre, but it drops the correct pressure by a third.

What that gives for an 83 kg rider and bike (tubed, smooth road). Every row here comes straight out of the calculator, so you can reproduce any of them:

Adjustments:

• Tubeless: Drop 5-10 PSI from tubed values (less pinch flat risk)
• Rough surface / gravel: Drop 10-20% for more grip and less fatigue
• Wet conditions: Drop 5-10% for grip
• Heavy rider (over 200 lb): narrow the front-to-rear gap, front at about 0.9× rear rather than 0.85×, to stop the front deflecting under braking. The calculator does this for you once total weight passes 200 lb
• Bikepacking with full load: Add 10-15 PSI to baseline values

Sweet spots by discipline, quoted the way riders quote them, by rider weight rather than rider plus bike. Add about 8 kg for the bike before comparing them with the table above:

• Road racing 700×25c, 75 kg rider: 75-85 PSI front / 85-95 PSI rear
• Gravel 700×40c, 75 kg rider: 35-45 PSI front / 40-50 PSI rear
• XC MTB 29×2.25, 75 kg rider: 18-22 PSI front / 20-24 PSI rear
• Trail MTB 27.5×2.5, 75 kg rider: 22-26 PSI front / 24-28 PSI rear

Where this formula stops working

The exponent is fitted to road and gravel tyres, and it keeps falling past the point where real riders stop following it. On anything wider than about 50 mm it reads several PSI low: for the 2.5 inch trail tyre above it returns roughly 16 front and 18 rear, while the riders in that discipline run 22 to 26. Casing thickness is the reason. A trail tyre has a stiff sidewall and an insert holding the shape, so it does not need the air pressure a supple road casing does to stay off the rim.

So on wide MTB tyres, take the number here as a floor and use the discipline figures above as the target. The calculator flags this on screen whenever you pick a tyre wide enough for it to matter.

Cold mornings undo all of this

Nothing on this page knows the temperature, and a bike tyre is more sensitive to it than a car tyre because the pressure is higher to begin with. Set 90 PSI in a 68°F garage and roll out into a 35°F morning and the gauge reads about 83. Two PSI per 10°F is the going rate at road pressures.
An MTB tyre loses less in absolute terms, about 0.8 PSI per 10°F at 25 PSI, but that is 10% of the pressure rather than 7%, so it is worth just as much attention. Inflate where you will ride, or at least inflate last thing before you leave.

The tire pressure adjustment calculator does that gas-law arithmetic for any starting pressure. It is written for cars, but the physics does not care what the tyre is bolted to.

The “drop test”: Sit on the bike in the kit you ride in. The tyre should squash about 15% of its inflated height at the contact patch. That is the sweet spot where the tyre rolls easily and still absorbs a hit. Less squash than that means you are too hard, more means too soft. It beats any calculator, this one included, because it accounts for your casing and your rim width.

Dangers of running too low:

• Pinch flats (tubed) or burping (tubeless)
• Rim damage on hard impacts
• Sidewall cuts from cornering forces
• Squirmy handling at speed

Dangers of running too high:

• Hard ride and chatter on anything but glass-smooth tarmac
• Less cornering grip, because the contact patch shrinks
• Slower on rough roads, which surprises people. Every bump the tyre cannot absorb throws the bike upward, and that energy comes out of your legs
• More force into the rim on a square-edged hit, not less. A hard tyre bottoms out against the rim rather than wrapping around the edge