Bike Tyre Size & Rollout Calculator
Calculate wheel diameter, circumference, distance per wheel revolution, and optional gearing speed estimates.
Why bike tyre size matters
Tyre size affects much more than appearance. Even a small change in width can influence comfort, handling, grip, and your effective gearing. When you move from a narrower tyre to a wider one, total wheel circumference usually increases. That means each wheel revolution covers more distance, which can slightly change your real speed for the same cadence.
This bike tyre size calculator helps you estimate those changes quickly using the ETRTO method (width + bead seat diameter). It can also estimate rollout and speed if you enter chainring, cog, and cadence values.
How to read bike tyre sizes
ETRTO format (recommended)
ETRTO gives two numbers: width - BSD, such as 32-622.
- 32 = nominal tyre width in millimeters
- 622 = rim bead seat diameter in millimeters
This is the most reliable way to match tyres and rims.
Inch and French labels
Labels like 700x32c, 29x2.2, or 26x1.95 are common and useful, but they can be less precise than ETRTO sizing. Two tyres with similar marketing labels may still have different measured widths depending on rim internal width and casing construction.
What this calculator computes
- Estimated inflated tyre height (based on width and profile ratio)
- Estimated outer wheel diameter (mm and inches)
- Estimated wheel circumference
- Wheel revolutions per kilometer and mile
- Optional rollout and speed using drivetrain inputs
- Optional comparison between current and new tyre width
Common BSD reference chart
| Common Name | BSD (mm) | Typical Use |
|---|---|---|
| 700C / 29" | 622 | Road, gravel, cyclocross, many MTBs |
| 650B / 27.5" | 584 | Trail bikes, gravel, all-road |
| 650C | 571 | Triathlon and smaller road frames |
| 26" MTB | 559 | Legacy mountain bikes, touring |
| 20" | 406 / 451 | BMX, folding bikes, recumbents |
Fit and safety tips before changing tyre size
1) Check frame and fork clearance
Always confirm clearance at the chainstays, seatstays, fork crown, and under fenders. As a practical guideline, leave extra room for mud, wheel flex, and debris.
2) Verify rim compatibility
Wider tyres generally require an appropriate rim internal width. Review the tyre and rim manufacturer charts to avoid a squirmy or overly stretched profile.
3) Recheck pressure and handling
When changing tyre volume, recommended pressure changes too. Larger tyres are often run at lower pressure, improving comfort and control while reducing vibration losses on rough pavement.
4) Speedometer and distance recording
If your bike computer uses wheel circumference, update it after changing tyres. Otherwise, speed and distance may be slightly over- or under-reported.
Quick example
Suppose your bike uses BSD 622 with a 32 mm tyre, and you switch to a 38 mm tyre. With the same profile ratio:
- Outer diameter increases
- Circumference increases
- Distance per pedal stroke increases slightly (same gear)
- At fixed cadence, actual speed is slightly higher
This can feel like a tiny gearing “hardening,” though comfort and traction usually improve.
FAQ
Is this an exact measurement?
No. It is an engineering estimate. Real inflated size depends on rim width, tyre model, pressure, and manufacturing tolerance.
Why use profile ratio for bicycle tyres?
Bicycle tyres rarely publish an explicit aspect ratio, so using a default 100% gives a practical approximation for most use cases.
Can I use this as a bike wheel circumference calculator?
Yes. Enter BSD and tyre width (plus ratio), then use the circumference output for bike computer setup or gearing calculations.