Skip to content
Allin
Sign up · Sign in

Tire Size Calculator

Decode a tire code like 205/55 R16 into overall diameter, circumference and revolutions per kilometre.

Related tools

All Maintenance & tyres tools →
Tire Pressure Temperature GuideAdjust cold tire pressure for a change in ambient temperature using the ideal-gas law.Tire Tread Wear CalculatorEstimate your tyre wear rate from two tread measurements or a known rate, then the distance and months left before the 2/32 in (1.6 mm) legal limit.Wheel circumference calculatorCompute a wheel's circumference from its diameter, for bikes and cycle computers.Luggage Allowance & Volumetric Weight GuideCompute linear dimensions, volumetric weight and chargeable weight for a bag.Gear ratio calculatorCompute the gear ratio from the number of teeth on the driving and driven gears.Car affordability calculatorFind a sensible monthly and yearly car budget from your income.Car depreciation calculatorEstimate a car's residual value after several years.Car lease calculatorEstimate a monthly car lease payment from price, down payment and residual value.

The Tire Size Calculator turns Section width (mm), Aspect ratio (%), Rim diameter (in) into Overall diameter, Circumference, Revolutions per km, instantly and for free. For instance, with Section width (mm) = 205, Aspect ratio (%) = 55 and Rim diameter (in) = 16 it returns Overall diameter = 631.9 mm, Circumference = 1,985 mm and Revolutions per km = 504.

How to use it

  1. Enter your values: Section width (mm), Aspect ratio (%), Rim diameter (in).
  2. Read the result instantly: Overall diameter, Circumference, Revolutions per km.

Frequently asked questions

What does the Tire Size Calculator actually compute?

It takes Section width (mm), Aspect ratio (%) and Rim diameter (in) and derives Overall diameter, Circumference and Revolutions per km from them. The calculation is live as you type, so the result updates on every change.

What information do I need to provide?

3 values: Section width (mm), Aspect ratio (%) and Rim diameter (in). Nothing else is required — no account, no file upload.

Can you show a worked example?

With Section width (mm) = 205, Aspect ratio (%) = 55 and Rim diameter (in) = 16, the calculator returns Overall diameter = 631.9 mm, Circumference = 1,985 mm and Revolutions per km = 504. Those figures come from running this exact tool, so you can reproduce them by entering the same values.

What happens if I enter larger values?

It moves a lot. Using Section width (mm) = 410, Aspect ratio (%) = 110 and Rim diameter (in) = 32 instead, Overall diameter goes from 631.9 mm to 1,715 mm — which is why it is worth testing a few scenarios rather than trusting a single figure.

What does it give for smaller values?

Scaled down to Section width (mm) = 105, Aspect ratio (%) = 30 and Rim diameter (in) = 8, Overall diameter comes out at 266.2 mm. The relationship is worth checking at both ends before you rely on a single result.

When would I actually use this?

Between garage visits: checking a tyre size is an allowed substitute, working out when the next service is due, and knowing what a wear threshold means.

What is the most common mistake?

Changing tyre size without keeping the rolling circumference. More than a couple of percent off and the speedometer, the odometer and the driver aids all read wrong.

What is the difference between the Tire Size Calculator and the Tire Pressure Temperature Guide?

This one returns Overall diameter and Circumference; the Tire Pressure Temperature Guide returns Adjusted pressure. That is the whole difference — open the one whose figure you need.

Is there a tool for the next step?

Tire Tread Wear Calculator is the closest one after this: Estimate your tyre wear rate from two tread measurements or a known rate, then the distance and months left before the 2/32 in (1.6 mm) legal limit.

Where do the figures come from, and how current are they?

Tyre dimensions follow the ISO marking, so the geometry is exact. Service intervals and legal wear limits are set by the manufacturer and by national law and differ between them.

Further reading

All guides →
ExplainerHow to Read Tire Size: What 205/55 R16 Actually MeansThe numbers on your tire sidewall decode into width, aspect ratio and rim diameter. Learn to read 205/55 R16 and work out the real overall diameter.ExplainerTyre Tread Depth: the Legal Limit Is Not the Safe LimitThe legal minimum exists to define an offence, not to describe a safe tyre. Tread does one job — getting water out of the contact patch — so its effect is near zero in the dry and decisive in the wet. Here is what the numbers say, and how to measure your own.GuideGear Ratios, and Why Your Speedometer Lies After New TyresEngine rpm reaches the road through one unbroken multiplication chain, and the last link is the tyre. Change the tyre's rolling circumference and the speedometer goes wrong by exactly that percentage — in a direction the law cares about.ExplainerRolling Circumference Decides More Than You ThinkThe speedometer is the famous consequence, and the least expensive one. The same number sets your odometer and therefore your lease bill, your effective final drive and cruising rpm, and what the ABS believes about wheel slip.ExplainerWhat Tire Pressure Should I Use? Placard vs Max PressureUse the vehicle placard pressure, not the number on the tire. Learn why the sidewall figure is a maximum, why pressure is set cold, and how temperature moves it.ExplainerStopping Distance Is Mostly Not BrakingTotal stopping distance is two distances added together, and they follow different laws — one grows with speed, the other with speed squared. That single fact explains why doubling your speed roughly triples the distance.