TireSizeDecoder turns the many ways bike tires are labelled – French (700x25c), inch decimal (29x2.2), inch fractional (26 x 1 3/8) – into the one number that decides whether a tire fits: the ISO 5775 / ETRTO bead seat diameter. Two tires with the same bead seat diameter fit the same rim, whatever the sidewall calls them.
Where the numbers come from
Bead seat diameters follow the ISO 5775 standard and the ETRTO tables used by tire and rim makers. Outer diameters are estimates (bead seat + twice the tire width); real tires vary by a few millimetres between brands and with pressure. Always check the ETRTO number moulded on the sidewall before you buy.
Shopping links
Buttons such as “Find tubes” open an Amazon search for the size. If we earn a commission from a purchase we will say so on the page; it never changes what the decoder tells you.
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