Bike Tire Pressure Calculator

This bike tire pressure calculator works out a separate pressure for your front and rear tires from three things: how much you, your bike and your luggage weigh, how wide your tires are, and how that weight splits between the wheels. It’s built on one published method, Frank Berto’s 15% tire drop chart, and it shows you the chart line it used for every answer. Nothing here is a hidden formula.

About the bike tire pressure calculator

Most riders pump both tires to the maximum printed on the sidewall. For a light rider on wide tires that’s far too hard. For a heavy rider on narrow tires it can be too soft. The right pressure depends on the load each tire carries and on the tire’s width, and this calculator uses both.

• Front and rear, separately. The rear wheel carries more weight, so it needs more air. You get two pressures, not one.
• psi and bar. Both are shown, whichever your pump reads.
• Every width at once. A table shows what your loads would need on each tire width from 20 to 37 mm, which helps when you’re choosing tires.
• Honest limits. It warns you when a pressure goes past the top of the chart, past the part Berto measured, or past the 72.5 psi limit for hookless rims.

It runs in your browser. Your weight is never sent anywhere or saved.

How to use the bike tire pressure calculator

1. Enter the weight. Weigh yourself dressed to ride, then the bike with bottles and tools. Add any luggage. Kilograms or pounds both work.
2. Enter the tire widths. Type the front and rear widths in millimetres. If you can, measure the inflated tire with calipers rather than trusting the label.
3. Pick the weight split. Choose road racing, touring or gravel, or city. If you’ve weighed each wheel, choose “I measured mine” and type the front percentage.
4. Read and pump. Inflate each tire to its own number, check it with a reliable gauge, and never exceed the limit on the tire or rim.

Why tire pressure matters

Pressure changes four things at once, and Berto’s tests put numbers on some of them.

• Rolling resistance. For a 170 lb rider on a 30 lb bike, going from 75 to 150 psi cut rolling resistance by about 10% on his test drum. That sounds like a lot, but wind resistance dominates, so it’s only about 4% of the total effort to keep the bike moving.
• Cornering. On the same drum, cornering force fell from about 130 lb at 75 psi to 123 lb at 150 psi. On real roads, overinflated tires corner worse still.
• Comfort. A tire at the right drop soaks up small bumps before they reach you. An overinflated one passes them straight through.
• Punctures. An underinflated tire can squash so far on a pothole edge that the tube is pinched against the rim, leaving two small holes side by side, the “snake bite”.

A bike tire pressure calculator can’t make all four perfect at once. The 15% tire drop target is the compromise Berto and the tire makers settled on.

What 15% tire drop means

Tire drop is how far the tire squashes when you sit on the bike: the distance the rim moves toward the ground. Too little drop and the tire is hard and harsh. Too much and it feels vague, wears the sidewalls and risks pinch flats, where the tube is trapped between the rim and a bump.

Frank Berto, a retired engineer and long-time engineering editor at Bicycling magazine, talked to the tire experts at Michelin, National, IRC and Continental. They agreed tire drop is the key measure, and that the best compromise is a drop of about 15% of the tire’s width. Berto then built a simple rig, a hydraulic jack to load the wheel, a bathroom scale to weigh the load and a dial gauge to measure the drop, and tested 50 road tires at 7 pressures and 8 loads.

From those measurements he drew one line per tire width: the pressure that gives 15% drop at each wheel load. Bicycle Quarterly later reprinted the chart in an article that Berto reviewed. Its own tire tests found that the “cut-off” pressure, beyond which a tire rolls no faster, lands close to the 15% line. That chart is what this bike tire pressure calculator reads.

The chart behind the calculator

Here’s the chart as numbers, in psi, for wheel loads from 30 to 70 kg. Remember these are loads on one wheel, not the whole bike.

Two patterns stand out. Pressure rises in a straight line with load, and it falls steeply as tires get wider. At a 50 kg wheel load, a 20 mm tire needs 139 psi while a 37 mm tire needs 49. That’s why heavy riders are better off on wider tires: a narrow tire can’t reach 15% drop at a pressure its rim will hold.

How we read it. We took each line from the vector drawing in Bicycle Quarterly’s PDF and calibrated it against the chart’s own axis marks. The lines reproduce the article’s worked example to within 1 psi, and Berto’s own figures for a 100 lb wheel load (126, 106, 88, 76, 60 and 46 psi for 20 to 37 mm) to within about 2 psi. For widths between two lines, such as 30 mm, the bike tire pressure calculator takes the value in between at your load.

Front and rear pressure

Your weight doesn’t split evenly. Bicycle Quarterly weighed each wheel on a range of bikes:

Because the rear tire carries more, it needs more pressure for the same 15% drop. Berto’s rule of thumb was about 10% less than the average at the front and 10% more at the rear. A softer front also grips better in corners and feels more comfortable through the bars.

To measure your own split, put one wheel on a bathroom scale and the other on a block of the same height. Sit on the bike in your normal riding position while a helper holds it upright, and note the reading. Turn the bike around and weigh the other wheel. Front reading divided by the total gives your front percentage. Loaded touring bikes and bikes with a child seat can be quite different from the presets.

Bike tire pressure calculator example: a 75 kg rider on 28 mm tires

A 75 kg rider on a 9 kg road bike, riding a racing position with a 40/60 split, on 28 mm tires.

1. Total load: 75 + 9 = 84 kg.
2. Front wheel: 84 x 40% = 33.6 kg. Rear wheel: 50.4 kg.
3. Front, on the 28 mm line at 33.6 kg: 54 psi (3.7 bar).
4. Rear, on the 28 mm line at 50.4 kg: 83 psi (5.7 bar).

The width table in the bike tire pressure calculator then shows what the same loads need on other tires: 62 and 99 psi on 25 mm tires, or 42 and 65 psi on 32 mm tires. The rear figure on 28 mm is above 72.5 psi, so this rider shouldn’t run 28 mm tires at that pressure on hookless rims. A 32 mm tire solves it.

Example: a heavier rider

A 100 kg rider on a 10 kg bike with the same 40/60 split puts 44 kg on the front and 66 kg on the rear.

On 25 mm tires the rear needs 132 psi. Check that against the maximum printed on your tire before you try it. On 37 mm tires it’s 64 psi. Berto’s own advice was blunt: heavier riders should use wider tires. The table makes that easy to see.

Example: loaded touring

An 80 kg rider on a 14 kg touring bike with 12 kg of luggage, on 35 mm tires with a 45/55 split. The total is 106 kg: 47.7 kg on the front and 58.3 kg on the rear. The calculator gives 53 psi front and 64 psi rear. Take the luggage off for a day ride and both numbers drop, so it’s worth running the calculation for each setup.

Reading the results

• Front and Rear are the two pressures to pump to, in psi, with bar and the load each tire carries beneath.
• Warnings appear only when they apply: above the top of Berto’s chart (155 psi), outside the part of a line he drew, or above 72.5 psi for hookless rims.
• The same loads on other tire widths shows every chart width for your weight and split.
• Show the working lists the loads and names the chart line or lines used for each tire.

Adjusting for your roads

Berto’s chart aims at average paved roads. Bicycle Quarterly suggests going a little lower on rough roads or unpaved surfaces, and a little higher on very smooth ones. Berto put numbers on the trade-off: 13% drop means higher pressure, less rolling resistance and a harder ride; 17% means lower pressure, a little more resistance and more comfort. Treat the bike tire pressure calculator result as the middle of that range.

Then test it. Ride a familiar loop at the calculated pressure, then a few psi either side. The right pressure feels supple without the tire squirming in corners or bottoming out on bumps.

Pressure limits you must respect

Never exceed the maximum on the tire or the rim, whichever is lower. If the calculator asks for more than your tire allows, fit a wider tire rather than overinflating.

Hookless rims have no hook to hold the tire bead. The ISO and ETRTO standards cap them at 5 bar, or 72.5 psi, and some wheel makers set lower limits for wider tires. Check your wheel maker’s chart.

Check your gauge. Berto tested floor-pump gauges and found they tend to read high as they wear. A pump showing 100 psi may really be giving 80 or 90. Check yours against a new hand gauge now and then.

Tire width: measured, not labeled

The chart uses the actual width of the tire, and the number on the sidewall isn’t always accurate. Rim width matters too: the same tire measures wider on a wider rim. If you can, inflate the tire to riding pressure and measure across the widest point with calipers. Enter that figure, decimals included.

Tubeless, gravel and mountain bike tires

Berto measured tires with inner tubes. The chart doesn’t model tubeless, but tubeless tires can’t get pinch flats, which is one reason riders run them softer on rough ground. Use the result as a starting point and go lower from there if the surface calls for it.

The calculator stops at 37 mm because that’s the widest line Berto drew. We don’t stretch his lines to wider tires, because that would be a guess. For mountain bike tires, Berto himself said the 15% rule doesn’t apply: off-road you want grip and shock absorption, and pressures are set lower on purpose.

Tips for using a bike tire pressure calculator

1. Weigh yourself in your riding kit. Shoes, helmet, water and a full saddle bag can add a few kilograms.
2. Recalculate when the load changes. Panniers, a child seat or a winter bike all shift the numbers.
3. Pump each tire to its own figure. The rear number is higher for a reason.
4. Check pressure before every ride. Tires lose air slowly even without a puncture, and latex tubes lose it faster.
5. Write your numbers down. Keep a note of the pressures that feel right for each bike, and start from there next time.

How this compares with other calculators

Several tire and wheel makers publish their own calculators, including SILCA, Zipp, ENVE, Rene Herse and Schwalbe. They’re built on each company’s own testing, and they don’t publish the formulas. SILCA, for example, calculates a “breakpoint” pressure for a chosen road surface, the point past which a harder tire gets slower on that surface.

This bike tire pressure calculator does something different. It uses a method you can check yourself: a published chart, read transparently, with the working shown. If you want a second opinion, compare its result with one of those tools and settle somewhere between them after a test ride.

Planning your riding

Counting down to an event? The date duration calculator tells you how many days and weeks you have left to train. For a structured fitness test, our PFT calculator scores the Marine Corps physical fitness test. Road surface matters to tire pressure too, and our bitumen calculator works out the asphalt and binder for a road surface, if you’re curious what you’re riding on.

Sources

• Frank Berto, “All About Tire Inflation” (2006): the 15% tire drop criterion, the tire drop tests, front and rear advice, and pump gauge accuracy.
• Jan Heine, “Optimizing Your Tire Pressure for Your Weight”, Bicycle Quarterly, reviewed by Frank Berto: the 15% tire drop chart and the measured weight distributions.
• ISO and ETRTO standards for hookless rims: 5 bar (72.5 psi) maximum.