Bicycle wheel size calculator
Estimate wheel geometry from ETRTO, or calculate it from a measured outside diameter.
Fill in the fields and the result will appear here automatically.
ETRTO numbers describe nominal tyre width and rim bead-seat diameter. This tool estimates outside diameter by assuming tyre height equals width; that is an approximation, not the ETRTO definition. Inch mode uses the entered outside diameter as a geometric measurement; an historical label such as “26 inch” is not necessarily that measurement.
How it works
Formula and logic
ETRTO estimate: D≈BSD+2W, where nominal width W substitutes for radial tyre height. Inches: D=d×25.4 mm. Then circumference C=πD, radius=D/2 and revolutions per km=1,000,000/C. Zero width describes a rim circle only, not a usable tyre assembly.
Example
25-622: D≈622+2×25=672 mm, C≈2111.15 mm=2.11115 m and about 473.68 revolutions per km. This estimates geometry rather than measuring a particular tyre’s rollout.
Fields and units
- How the size is given — list option
- Rim bead seat diameter — mm
- Tyre width — mm
- Wheel diameter — in
How to use
- — For marking 25-622, enter width 25 and bead-seat diameter 622 mm.
- — Treat the ETRTO result as an initial geometric estimate.
- — In inch mode enter measured outside diameter, not only a nominal size name.
- — For a bike computer, measure one loaded wheel revolution at working pressure.
Method and limitations
- Calculation method
- Formula and logic
- Data or methodology source
- Schwalbe: ETRTO dimensions and ambiguous inch labels Schwalbe: construction, rim, pressure and load affect rollout
- Limitation
- Preliminary geometry assuming height≈width. It does not establish exact rollout, tyre fit, clearance or safe installation.
FAQ
How do I read an ETRTO tyre size?
25-622 means nominal width 25 mm and bead-seat diameter 622 mm. It helps match the bead size, but does not check all rim, tyre and frame compatibility.
Why is tyre width doubled in this estimate?
Radial height contributes above and below the rim. This model approximates height with width; actual shape depends on tyre and rim.
Why do nominal inches differ from ETRTO?
Historical inch labels are ambiguous. 28 and 29 can both use a 622 mm bead seat; “26” alone does not identify one bead-seat diameter.
Is estimated circumference suitable for a bike computer?
Rollout varies with construction, rim, pressure and load. Measure the distance of one loaded revolution for calibration.
How do I pass circumference to the gear calculator?
Divide millimetres by 1000:2111.15 mm → 2.11115 m. Prefer measured rollout when available.