Bicycle Wheel Size Calculator

Calculator Inputs

Wheel standard

Rim bead seat diameter

Use millimeters.

Tire width

Use measured or labeled width in millimeters.

Sidewall height factor

Use 1.00 when tire height roughly equals tire width.

Tire growth adjustment

Use percent. Positive values increase tire height.

Loaded sag adjustment

Use percent. Sag reduces effective rolling radius.

Manufacturing tolerance

Use millimeters. Negative values are allowed.

Front chainring teeth

Rear cog teeth

Cadence

Use crank revolutions per minute.

Target bicycle speed

Target speed unit

Formula Used

The calculator begins with ISO bead seat diameter. This is the rim diameter where the tire beads sit. Tire height is estimated from width and sidewall factor.

Tire height = tire width × sidewall factor × growth adjustment × sag adjustment
Wheel diameter = bead seat diameter + 2 × tire height + tolerance
Circumference = π × wheel diameter
Gear ratio = chainring teeth ÷ rear cog teeth
Rollout = circumference × gear ratio
Speed = rollout × cadence × 60 ÷ 1,000,000
Angular velocity = wheel rpm × 2π ÷ 60

All wheel measurements are calculated in millimeters first. Then selected results are converted to inches, meters, kilometers per hour, and miles per hour.

How To Use This Calculator

Select a common wheel standard or choose a custom rim size.
Enter the ISO bead seat diameter when using a custom rim.
Enter tire width in millimeters.
Set the sidewall factor. Use 1.00 for a normal round tire.
Add growth or sag adjustments when measured data is available.
Enter chainring, cog, and cadence values for rollout and speed.
Press the calculate button to view the result above the form.
Use the CSV or PDF button to save the calculated data.

Example Data Table

Wheel label	ETRTO example	Bead seat diameter	Tire width	Approx outside diameter	Approx circumference
700 × 25C	25-622	622 mm	25 mm	672 mm	2111 mm
700 × 32C	32-622	622 mm	32 mm	686 mm	2155 mm
29 × 2.25	57-622	622 mm	57 mm	736 mm	2312 mm
27.5 × 2.20	56-584	584 mm	56 mm	696 mm	2187 mm
26 × 2.00	51-559	559 mm	51 mm	661 mm	2077 mm
20 × 1.75	44-406	406 mm	44 mm	494 mm	1552 mm

Bicycle Wheel Size And Riding Physics

A bicycle wheel is more than a labeled size. The real outside diameter changes with rim diameter, tire width, pressure, casing shape, and rider load. A small change can affect speed readings, gear rollout, frame clearance, and handling. This calculator uses the bead seat diameter as the fixed rim base. It then adds two tire sidewalls to estimate the finished wheel.

Why Wheel Size Matters

Circumference is the key value for physics. Each full wheel turn moves the bicycle forward by one circumference, assuming no slip. Larger wheels cover more ground per rotation. Smaller wheels accelerate quickly and can feel more agile. Tire height also changes bottom bracket height, toe overlap, and mudguard clearance. That is why a marked 700x32 tire may not match another 700x32 tire exactly.

Rollout And Speed

Rollout connects the wheel to the drivetrain. It equals wheel circumference multiplied by the chainring to cog ratio. A higher rollout means the bike travels farther for each crank turn. Cadence then converts rollout into speed. Riders can use this value to compare gearing before changing chainrings, cassettes, or tires. It is also useful for indoor trainers and bike computers.

Measurement Tips

Measure inflated tire width with calipers when possible. Use the exact ISO bead seat diameter for the rim. Common road rims use 622 mm. Many modern mountain wheels also use 622 mm, even when called 29 inch. A 27.5 inch wheel usually uses 584 mm. A classic 26 inch mountain wheel often uses 559 mm. These names are helpful, but ISO values are better for calculation.

Practical Use

Use the sidewall factor when the tire height differs from its width. A round tire may use 1.00. A low profile tire may use 0.90. A tall tire may use 1.05. Add growth for high pressure or measured expansion. Add sag when estimating loaded rolling radius. The final result helps you set computer circumference, compare wheel swaps, plan gearing, and check fit before buying parts.

For best accuracy, compare the calculated circumference with a rollout test. Mark the tire, roll one full revolution, and measure the ground distance. Use that measured value when precision matters most for race setup.

FAQs

What is bead seat diameter?

It is the rim diameter where the tire beads sit. It is the best starting value because marketing wheel names can describe different real diameters.

Why is tire width used twice?

The tire adds height above and below the rim. The calculator estimates total diameter by adding two adjusted tire heights to the rim bead seat diameter.

What sidewall factor should I use?

Use 1.00 when tire height is close to tire width. Use a lower value for low profile tires and a higher value for tall round tires.

Can this calculate bicycle computer circumference?

Yes. Use the calculated circumference in millimeters as a starting value. For best accuracy, confirm it with a real rollout measurement.

Why do 700C and 29 inch share a rim value?

Many 700C and 29 inch wheels use a 622 mm bead seat diameter. Tire width makes the finished outside diameter very different.

What does rollout mean?

Rollout is the distance the bicycle travels for one full crank revolution. It depends on wheel circumference and the selected gear ratio.

Does tire pressure affect wheel size?

Yes. Higher pressure can increase measured tire height slightly. Rider load can reduce the effective rolling radius through tire sag.

Can I compare gearing with this calculator?

Yes. Enter chainring teeth, cog teeth, and cadence. The calculator estimates gear ratio, rollout, and speed for that setup.