Tire Size Calculator

About the Tire Size Calculator

Every tire carries its size on the sidewall in a code such as 225/45R17. The code tells you the tire's width, how tall its sidewall is and the wheel it fits, and from those three numbers you can work out its overall height. That matters whenever you change tires or wheels: a replacement that is noticeably taller or shorter than the original changes the speedometer reading, the gearing and how the tire fits in the wheel arch.

This tire size calculator works out a tire's overall diameter, sidewall height, circumference and revolutions per kilometre and per mile from its size. Choose compare two sizes and it also shows how a new size differs from the original, whether it stays within the usual 3 percent guideline, and what your true speed will be at a given speedometer reading.

How to Use the Tire Size Calculator

Choose One size or Compare two sizes.

Enter the width in millimetres — the first number, 225 in 225/45R17.

Enter the aspect ratio — the second number, 45. It is the sidewall height as a percentage of the width.

Enter the rim diameter in inches — the number after R, 17.

To compare, enter the new width, new aspect ratio and new rim diameter, and optionally a speedometer reading to see the true speed.

How Tire Size Is Worked Out

  sidewall       =  width × aspect ÷ 100
  diameter       =  rim × 25.4 + 2 × sidewall      (mm)
  circumference  =  π × diameter
  revs per km    =  1,000,000 ÷ circumference
  difference     =  new diameter ÷ old diameter − 1
  true speed     =  indicated speed × new diameter ÷ old diameter

Step-by-Step Example

A 225/45R17 tire.

  Sidewall:       225 × 45% = 101.25 mm
  Rim:            17 × 25.4 = 431.8 mm
  Diameter:       431.8 + 2 × 101.25 = 634.3 mm   (24.97 in)
  Circumference:  π × 634.3 = 1,993 mm
  Revolutions:    1,000,000 ÷ 1,993 = 501.8 per km  (807.6 per mile)

Compared with a 235/40R18.

  Sidewall:       235 × 40% = 94 mm
  Diameter:       18 × 25.4 + 2 × 94 = 645.2 mm
  Difference:     645.2 ÷ 634.3 − 1 = +1.72%
  At 60 indicated: 60 × 1.0172 = 61.0 true

The larger wheel with a lower sidewall ends up 1.72 percent taller, within the usual 3 percent guideline. With it fitted, the speedometer would read a little low: at an indicated 60, you would actually be doing about 61.

Reading the Rest of the Sidewall

A full tire marking often looks like 225/45 R17 94W. After the size come the load index and speed rating. The load index, 94 here, is a code for the maximum weight each tire can carry — 670 kg for 94. The speed rating, W, is the highest sustained speed the tire is designed for — 270 km/h for W, 240 km/h for V and 210 km/h for H. Replacement tires should match or exceed the load index and speed rating the vehicle maker specifies.

Other markings include the manufacturing date as a four-digit week and year code, such as 2324 for the 23rd week of 2024, and letters such as XL for extra load or M+S for mud and snow.

Speedometers and Tire Size

A speedometer does not measure speed directly; it counts how fast the wheels, or the gearbox, are turning and assumes a certain tire circumference. Fit a taller tire and each turn covers more ground, so the car travels faster than the speedometer shows. A shorter tire does the opposite. The odometer is affected by exactly the same percentage, so mileage records drift too.

Many countries require speedometers never to read below the true speed, so makers set them to read slightly high. A modest increase in tire diameter can use up that margin, which is one reason to stay within about 3 percent.

Plus Sizing

Plus sizing means fitting a larger-diameter wheel with a lower-profile tire so the overall diameter barely changes. Going from 205/55R16 to 225/45R17 is a typical plus-one change: the diameter moves from about 631.9 mm to 634.3 mm, well under 1 percent. The shorter sidewall flexes less, sharpening steering response, while the wider tread can add grip — at the cost of a firmer ride, more road noise and wheels more exposed to kerb and pothole damage.

Checking Fit and Clearance

Diameter is only part of the fit. A wider tire needs room inside the wheel arch and clearance from the suspension, and the wheel's offset changes where the tire sits. A taller tire can rub the arch on full steering lock or when the suspension compresses. Always check the sizes approved for your vehicle, often listed in the handbook or on a placard inside the driver's door, and ask a tire fitter when moving away from them.

Understanding Your Result

The headline is the tire's overall diameter in millimetres and inches.

The sidewall line gives the sidewall height, and the new tire's when comparing.

The circumference is the distance covered in one turn.

The revolutions line gives turns per kilometre and per mile.

The comparison shows the new size's diameter, the percentage difference, whether it is within 3 percent, and the true speed at your speedometer reading.

When Should You Use This Calculator?

Use it before buying new wheels or tires in a different size.

Use it to check how far off your speedometer will be after a size change.

Use it to compare winter tire sizes, which are often narrower or on smaller wheels.

Use it to understand the numbers on a tire sidewall.

Common Mistakes

Treating the aspect ratio as a height. It is a percentage of the width, so the same ratio gives a taller sidewall on a wider tire.

Mixing units. Width is in millimetres but the rim is in inches; convert the rim by multiplying by 25.4.

Ignoring load and speed ratings. A tire of the right size can still be unsuitable.

Going beyond 3 percent. Large changes affect the speedometer, ABS and gearing.

Checking diameter but not width. A wider tire may rub even when the height matches.