One: the ETRTO numbers on your tire
Two numbers on the sidewall. The second — the bead seat diameter — is what a tire or a tube must match. 622 is what most people call 700c; 584 is 27.5 inch.
Tubes cover a range of widths for one bead seat diameter.
Two: the speed count on your drivetrain
Count the sprockets on the rear wheel. That number decides the chain and the cassette, and it is the only figure that has to match on a chain.
Three: the freehub standard on your rear hub
Shimano HG, SRAM XD or XDR, Campagnolo, or a microspline variant. Different shapes, and no adapter bridges them.
Published by your wheel manufacturer, by model. Establish it before buying a cassette.
Four: the seatpost diameter
Read it off the post already in your frame rather than measuring — the steps between 27.2, 30.9 and 31.6 mm (1.24 in) are under a millimeter.
Five: the caliper model, for brake pads
Printed on the caliper body. Pad compatibility is model by model, and a good pad listing names the calipers it fits — Alphatrail names eleven codes, which is a materially stronger statement than “fits most”.
The bar diameters, which are two numbers not one
Many flat bars are 31.8 mm (1.25 in) at the stem clamp and 22.2 mm (0.87 in) at the grips. Measure where the accessory will actually sit.
Write them down
A note on your phone with those figures turns most purchases into a two-minute decision, permanently.
The questions riders ask
Where is the ETRTO size?
On the tire sidewall, as two numbers such as 32-622. The first is the width in millimeters, the second is the bead seat diameter, and it is the second that must match your rim.
How do I find my freehub standard?
From your wheel manufacturer, by model. It is not written on the hub in a way most people can read, and it decides whether a cassette fits at all.
Last reviewed 30 August 2026