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Cars & mobility

Tyre size calculator

Tyre size calculator: enter your values and get an instant free result.

Your result

Complete the fields and calculate.

Calculation method and formula

25.4 × rim + 2 × width × ratio ÷ 100

Understand the inputs

Old width
Unit: mm. Use a measured value or your own assumption.
Old aspect ratio
Unit: %. Use a measured value or your own assumption.
Old rim
Unit: in. Use a measured value or your own assumption.
New width
Unit: mm. Use a measured value or your own assumption.
New aspect ratio
Unit: %. Use a measured value or your own assumption.
New rim
Unit: in. Use a measured value or your own assumption.

Assumptions and limitations

The unloaded geometric outer diameter is calculated. Real rolling circumference and approved fitments differ. This does not replace approval or speedometer testing.

The formula uses the specified units. Displayed numbers are rounded; whole packs, item counts and full time periods may also be rounded within the calculation.

Worked example

Illustrative values. Replace them with your own inputs for your calculation.

Old width
205 mm
Old aspect ratio
55 %
Old rim
16 in
New width
225 mm
New aspect ratio
45 %
New rim
17 in
Example result0.38 %

Old diameter: 631.9 mm · new: 634.3 mm.

Model revision: October 17, 2026 · How CleverCalc works

FAQ

Frequently asked questions

How does Tyre size calculator work?+

25.4 × rim + 2 × width × ratio ÷ 100. The unloaded geometric outer diameter is calculated. Real rolling circumference and approved fitments differ. This does not replace approval or speedometer testing.

What assumptions should I consider for this result?+

The unloaded geometric outer diameter is calculated. Real rolling circumference and approved fitments differ. This does not replace approval or speedometer testing. The formula uses the specified units. Displayed numbers are rounded; whole packs, item counts and full time periods may also be rounded within the calculation.

How accurate is the result?+

The formula uses the specified units. Displayed numbers are rounded; whole packs, item counts and full time periods may also be rounded within the calculation. Prices, measurements and model assumptions determine how well the result represents your situation.

Is this calculator free?+

Yes. Calculations and PDF export work without an account or registration.

What happens to my inputs?+

Calculations run in your browser. Results are saved locally only when you add them to My results; favourites and history are also local. Shared links contain your values. The currency tool requests the selected currencies from its rate provider without sending your amount.

Toolbox

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