Tire Diameter Calculator

Overall diameter (inches) —

Overall diameter (mm): —
Sidewall height (mm): —
Sidewall height (in): —
Section width (in): —
Circumference (in): —
Revolutions per mile: —

Overall tire diameter equals the rim diameter plus two sidewalls, where each sidewall is the section width times the aspect ratio. For 205/55R16, the sidewall is 205 × 0.55 = 112.75 mm (4.44 in), so the diameter is 16 + 2 × 4.44 = 24.88 inches, or about 632 mm.

About this tool

Tire sizes are printed as a mix of millimetres, percent and inches, which makes the actual height of the tire hard to read off the sidewall. This calculator turns the three numbers of a metric size code into the unloaded overall diameter in inches and millimetres, along with sidewall height, section width in inches, rolling circumference and revolutions per mile. It is useful when checking whether a replacement or plus-size fitment stays close to the original height, estimating ground-clearance changes, or confirming fit in a wheel well. The figures are nominal design dimensions: a mounted, loaded tire sits a little shorter, and real tires of the same size vary by brand and tread depth.

How to use it

Read the size code
Find the size on the sidewall, for example 225/45R17: width 225, aspect 45, rim 17.
Enter the three numbers
Type the section width in mm, the aspect ratio in percent and the rim diameter in inches.
Read the diameter
The overall diameter appears in inches, with millimetres, sidewall height, circumference and revolutions per mile below it.
Compare sizes
Enter an alternative size and compare its diameter with the original; most fitment guides keep the difference within about 3%.

Examples

205/55R16 compact-car tire

Section width: 205
Aspect ratio: 55
Rim diameter: 16

Result Overall diameter (inches): 24.88
Overall diameter (mm): 631.9
Sidewall height (mm): 112.8
Sidewall height (in): 4.44
Section width (in): 8.07
Circumference (in): 78.16
Revolutions per mile: 811

Sidewall height = 205 mm x 55% = 112.75 mm = 4.439 in
Diameter = 16 in + 2 x 4.439 in = 24.88 in (631.9 mm)
Circumference = pi x 24.88 = 78.16 in
Revolutions per mile = 63,360 / 78.16 = 811

A common size on compact sedans; each sidewall adds about 4.44 in to the 16 in rim.

225/45R17 plus-one size

Section width: 225
Aspect ratio: 45
Rim diameter: 17

Result Overall diameter (inches): 24.97
Overall diameter (mm): 634.3
Sidewall height (mm): 101.3
Sidewall height (in): 3.99
Section width (in): 8.86
Circumference (in): 78.45
Revolutions per mile: 808

Sidewall height = 225 mm x 45% = 101.25 mm = 3.986 in
Diameter = 17 in + 2 x 3.986 in = 24.97 in (634.3 mm)
Circumference = pi x 24.97 = 78.45 in
Revolutions per mile = 63,360 / 78.45 = 808

Going up one rim inch with a lower profile keeps the overall diameter within 0.1 in of the 205/55R16.

265/70R17 light-truck tire

Section width: 265
Aspect ratio: 70
Rim diameter: 17

Result Overall diameter (inches): 31.61
Overall diameter (mm): 802.8
Sidewall height (mm): 185.5
Sidewall height (in): 7.3
Section width (in): 10.43
Circumference (in): 99.29
Revolutions per mile: 638

Sidewall height = 265 mm x 70% = 185.5 mm = 7.303 in
Diameter = 17 in + 2 x 7.303 in = 31.61 in (802.8 mm)
Circumference = pi x 31.61 = 99.29 in
Revolutions per mile = 63,360 / 99.29 = 638

A tall sidewall of 185.5 mm makes this pickup size about 31.6 in tall.

How it is calculated

D = R + 2 × (W × A ÷ 100) ÷ 25.4

D: Overall diameter in inches
R: Rim diameter in inches
W: Section width in millimetres
A: Aspect ratio in percent

The aspect ratio gives sidewall height as a percentage of section width, so W × A ÷ 100 is one sidewall in millimetres. Dividing by 25.4 converts it to inches, and there is one sidewall above the rim and one below, hence the factor of two. Circumference is π × D, and revolutions per mile is 63,360 inches divided by that circumference. These are unloaded nominal values from the size designation.

When not to use it

Flotation sizes such as 33×12.50R15 already state the diameter in inches.
For speedometer calibration use the manufacturer's published revolutions per mile, which accounts for load.
Clearance on a lifted or lowered vehicle needs a physical measurement, not just nominal diameter.

Common mistakes

Treating the aspect ratio as a height in millimetres instead of a percentage of width.
Adding only one sidewall to the rim diameter rather than two.
Mixing the rim size in inches with the width in mm without converting units.
Assuming two brands of the same size have identical measured diameters.

Frequently asked questions

How tall is a 205/55R16 tire?

About 24.88 inches, or roughly 632 mm. Each sidewall is 205 × 0.55 = 112.75 mm, and two of them added to the 16-inch rim give the total.

How much can tire diameter change before it causes problems?

Many fitment guides suggest keeping a replacement within about 3% of the original diameter. Larger changes shift speedometer readings, can confuse ABS and stability systems, and may cause rubbing in the wheel well.

Why does my measured tire come out shorter than the calculation?

The formula gives the unloaded design diameter. On the car, the vehicle's weight flattens the contact patch, so the loaded radius is smaller, and worn tread removes several millimetres more. Manufacturer spec sheets list both figures for that reason.

What does the R in a tire size mean?

R stands for radial construction, the ply layout used on almost all modern passenger tires. It is not the radius; the number after it is the rim diameter in inches.

How does diameter affect the speedometer?

The speedometer counts wheel revolutions. A taller tire covers more ground per turn, so the car is moving faster than the dial shows; a shorter tire makes it read high. The error scales with the diameter ratio.

What is revolutions per mile used for?

It is the number of times the tire turns while travelling one mile, 63,360 inches divided by the circumference. Gearing, odometer and speedometer calibration all rely on it.

Cite this tool

Using this tool in an assignment, article or report? Copy a citation, or a link for your website.

APA

Tools Directory Editorial Team. (2026, October 27). Tire Diameter Calculator. Tools Directory. Retrieved October 28, 2026, from https://tools.milunir.com/automotive/tyres-and-wheels/tire-diameter-calculator/

MLA

Tools Directory Editorial Team. “Tire Diameter Calculator.” Tools Directory, 27 Oct. 2026, https://tools.milunir.com/automotive/tyres-and-wheels/tire-diameter-calculator/. Accessed 28 Oct. 2026.

Chicago

Tools Directory Editorial Team. “Tire Diameter Calculator.” Tools Directory. Last modified October 27, 2026. Accessed October 28, 2026. https://tools.milunir.com/automotive/tyres-and-wheels/tire-diameter-calculator/.

Link

<a href="https://tools.milunir.com/automotive/tyres-and-wheels/tire-diameter-calculator/">Tire Diameter Calculator – Tools Directory</a>