Wheel Size Converter
Enter your tire width to compare the real diameter and rollout of 26″, 27.5″, and 29″ wheels, and see how a wheel-size change moves your gearing.
The numbers behind the wheel-size debate
Bigger wheels roll over obstacles more smoothly and hold speed; smaller wheels accelerate and change direction faster. Underneath the feel are two concrete numbers — outer diameter and rollout — that also drive your gearing and your bike computer's calibration. Seeing them side by side at your actual tire width turns a debate into a decision.
Once you know your rollout, the Gear Ratio Calculator shows the gear inches and speed that come with it, and the MTB Tire Pressure Calculator helps you set up the new tire.
Frequently Asked Questions
What is rollout, and why compare it?
Rollout is the distance a wheel covers in one complete turn — pi times its outer diameter. It is the honest way to compare wheel sizes, because the marketing numbers (26, 27.5, 29) refer to rough nominal diameters, not the real size once a tire is fitted. Comparing rollout at the same tire width shows exactly how much further a 29er rolls per turn than a 27.5 or a 26, which is what actually changes how the bike feels and gears.
How is outer diameter calculated?
Each wheel size has a fixed bead-seat diameter — the rim measurement the tire seats onto: 559 mm for 26", 584 mm for 27.5", and 622 mm for 29". The fitted tire adds its width above and below the rim, so the outer diameter is the bead-seat diameter plus twice the tire width. That is why a wide 2.6" tire noticeably enlarges a wheel compared with a narrow one on the same rim.
How does changing wheel size affect my gearing?
Because a bigger wheel rolls further per turn, the same chainring and cog push a taller effective gear on it. Going up a wheel size raises your gear inches by roughly ten percent, so your easiest climbing gear gets harder. Many riders compensate with a larger cassette cog or a smaller chainring. The gearing factor this tool reports is the multiplier that keeps your gear inches the same across wheel sizes.
Is the rollout figure exact?
It is a close geometric estimate, not a rolling-tape measurement. Real rollout is a little shorter than pi times the outer diameter because the tire squashes under load, and it changes with pressure, tread, and rider weight. For comparing wheel sizes and setting up a bike computer's wheel size it is more than close enough; for a precise cyclo-computer calibration, roll the wheel out on the ground with your weight on it.
A geometric estimate for comparing wheel sizes. Real rollout is slightly shorter under load; roll the wheel out with your weight on it for a precise computer calibration.