Explainer · updated 2026-08-22
How to Read E-Bike Tire Sizes (20x4, 26x2.35, 700x45c and More)
Tire sizing is the one bike topic that makes even experienced mechanics reach for a chart. The same tire can be described three ways on its sidewall, two 26-inch tires can be incompatible with each other, and a tube that says it fits your size may be wrong for the width. E-bikes add their own wrinkles with 4-inch fat tires, small 20-inch wheels on folders, and weight ratings that regular bike tires do not carry.
This guide teaches you to read a sidewall and order the right replacement the first time. The short version is: trust the ISO number in millimeters over the inch size, match the width to your rim's range, and buy tubes by the width range printed on the box. The long version explains why, which is useful when the number on your tire does not look like any of the options on a listing.
The three ways a size is written
Look at a tire sidewall and you will usually find two or three numbers. The inch size, like 26 x 2.35 or 20 x 4.0, is what most e-bikes and Amazon listings use; the first number is the nominal wheel diameter and the second is the tire width. The French size, like 700 x 45c, is used for road and many commuter wheels; 700c is a nominal outer diameter and 45 is the width in millimeters. The ISO or ETRTO size, like 60-559 or 45-622, is the precise one: width in millimeters, then the rim bead seat diameter in millimeters.
ISO is the number that never lies. A tire marked 559 fits a 559 rim regardless of what the inch label says. Most 26-inch mountain and fat bike rims are 559; most 27.5 or 650b rims are 584; most 29er and 700c rims are 622; most 20-inch e-bike and folding rims are 406, and some BMX-style 20-inch wheels are 451 and will not interchange. When in doubt, find the ISO number on your existing tire and match it.
Why 26 inch is not always 26 inch
The inch system evolved from outer diameter, which changes with tire width, so several bead seat diameters ended up being called 26 inch. The common one on modern e-bikes and fat bikes is 559, but there are older 26 x 1 3/8 tires at 590 and others at 597 that look similar and will not mount. The same happened with 20 inch (406 vs 451) and 24 inch (507 vs 540).
Fractions versus decimals also matter: 26 x 1 3/8 and 26 x 1.375 are different tires on different rims, despite being the same arithmetic. On e-bikes you will mostly see decimal sizes, but if you ever see a fraction, stop and check the ISO number. This is also why we recommend ordering by ISO when a listing offers it.
Fat tires and plus sizes
Fat-tire e-bikes use 20 x 4.0 (about 100-406), 26 x 4.0 (about 100-559) and occasionally 24 x 4.0 or 26 x 4.8. The 4.0 width is nominal; actual measured width varies by casing and rim, and some 4.0 tires run closer to 3.8 inches. Plus-size tires between 2.8 and 3.0 inches sit in between and usually need a rim of at least 35 to 40 mm internal width.
Rim width sets the tire widths you can safely run. A rough rule: the tire should be about 1.8 to 2.5 times the internal rim width. A 65 mm fat rim is happy with 3.5 to 4.5 inch tires; a 25 mm commuter rim suits 1.75 to 2.4 inch tires. Going narrower than the rim's range makes the tire squared-off and prone to burping; going wider makes it lightbulb-shaped and vulnerable to sidewall cuts. Your bike's manual or the rim's sidewall sometimes lists the range.
Check frame and fork clearance too. Many fat-tire e-bikes have fenders and chainstays that barely clear a 4.0 tire; a 4.5 may rub, and mud or a slightly out-of-true wheel will make it worse. Going a half size narrower on a fat bike is usually fine and can improve efficiency noticeably on pavement.
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Matching tubes, valves and rim tape
Tubes are sold by wheel diameter and a width range, like 20 x 3.5–4.0 or 26 x 1.9–2.35. Buy the range that brackets your tire width; a too-narrow tube stretches thin and pinch flats, and a too-wide tube folds and chafes. Fat-tire tubes are heavy and cost more, and many owners carry a spare because fat tubes are not stocked at every shop or gas station.
Valves come in Schrader (the car-style valve common on fat and budget e-bikes) and Presta (the thin valve common on commuter and premium wheels). Your rim is drilled for one; Presta tubes can use a Schrader-drilled rim with a small adapter or grommet, but a Schrader tube will not fit a Presta-drilled rim without drilling. Valve length matters on deep rims; 48 mm is the safe default for most e-bike rims.
When you replace a tire after repeated flats, look at the rim tape. A strip that has shifted or cracked exposes spoke holes and causes mystery punctures on the inside of the tube. Replacing it is cheap and quick, and our tire keeps going flat page covers the other usual suspects.
E-bike ratings, pressure and tread
Some tires carry an e-bike rating, such as ECE-R75 or a brand's e-bike label, which indicates extra casing strength and speed rating. It is not legally required in the US, but on a 70 lb bike with a 250 lb rider it is reassuring and worth seeking out. Puncture protection layers add weight but are the best upgrade for commuters in an area with goathead thorns and glass.
Pressure range is printed on the sidewall as a minimum and maximum. Heavier riders and loaded cargo bikes run toward the upper end; fat tires on pavement run toward their maximum for efficiency and lower on gravel or sand for grip. Check pressure weekly with a gauge. Tread choice is simple: smooth or lightly textured for pavement, small knobs for mixed paths, aggressive knobs only for real dirt, since they hum and slow you on concrete.
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Frequently asked questions
Will a 26 x 4.0 tire fit my 26 x 4.0 rim from a different brand?
Almost always, as long as the ISO bead diameter is 559 on both. Double-check the rim's internal width and your frame clearance, since actual tire width varies between brands.
Can I put a 2.5 inch tire on my 2.0 inch commuter rim?
It depends on the rim's internal width and your frame clearance. Most 20–25 mm commuter rims handle up to about 2.4 inches. Check clearance at the chainstays, fork and fenders before ordering.
What does the c in 700x45c mean?
It is a holdover from an old French system that used letters for tire width; today it just marks the size as part of the 700c family, which uses a 622 mm bead seat. The 45 is the width in millimeters.
Is 27.5 the same as 650b?
Yes. Both refer to a 584 mm bead seat diameter. The 27.5 label is used for mountain and many e-bike tires; 650b is the older road and touring term.
Do I need a special tube for a fat tire?
You need a tube sized for the width, for example 26 x 3.5–4.5, and the correct valve type. Standard-width tubes will stretch too thin and flat easily.
