# How to properly calculate wheel diameter (rim+tire)?

Is it true that 26" rim actual diameter is 25.625"? How does tire size affect wheel diameter? Is there any special formula to calculate wheel diameter?

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You could calculate the effective diameter by measuring rollout and dividing that number by pi. To measure the rollout start with your valve stem in the lowest possible position and make a mark on the floor next to the valve stem. Ride the bike one revolution until the valve stem is at the bottom of it's rotation again and make another mark on the floor. Measure the distance between the two marks to find the effective circumference.

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Way to provide the physical/math answer and satisfy my math jones (see bicycles.stackexchange.com/questions/1549/…) – Gary.Ray May 3 '11 at 19:50
This is actually the only accurate way to know. There are too many variables to allow you to read numbers off the tyre and get an accurate answer. @Mike Baranczak's link to Sheldon Brown is a good discussion of the issue. – Мסž May 4 '11 at 2:11
@Gary.Ray, that link goes to a question about calculating power output and doesn't reference tyre diameter at all. – Мסž May 4 '11 at 2:13
@moz - my link is to another "math" problem related to cycling - I upvoted Russ for providing both the precise method, and the proper formumla. – Gary.Ray May 4 '11 at 14:46
I wanted a pure mathematical way to calculate a wheel diameter, but it seems an appropriate way to do this. Thanks. – zetdotpi May 4 '11 at 23:20

Your wheel diameter can vary depending on:

• tire model you are using
• tire wear
• air pressure
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I wouldn't trust any formula, since the actual outside diameter can be affected by so many things, and since the advertised tire width isn't always accurate. If you want the diameter, do like Russ said and measure it yourself. You might also be interested in what Sheldon Brown had to say on this subject.

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Summary: In theory 26" was the diameter of the wheel with tyre installed and correctly inflated. That used to be how the sizing worked. So a 26" x 1" tyre needed a different rim size than a 26" x 1 1/2" tyre. The larger tyre needs a smaller rim to get the same outside diameter. Obviously this makes it hard to find tyres, so now we use ISO "beat seat diameter", so any 559 tyre will fit any 559 rim (that size is commonly called 26" today), even though it may not fit into your frame and the outside diameter will change with tyre size. – Мסž May 4 '11 at 2:18