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Tire Size Calculator

AutomotiveTyres and Wheels

First number on the sidewall, e.g. 225 in 225/45R17.

Sidewall height as a percentage of the width.

Overall diameter (inches) —
Size
—
Sidewall height (mm)
—
Overall diameter (mm)
—
Circumference (mm)
—
Revolutions per mile
—
New tire diameter (in)
—
Diameter difference
—
True speed at an indicated 60 mph
—

A metric tire size like 225/45R17 means a 225 mm section width, a sidewall 45% of that width, and a 17-inch rim. The sidewall is 101.25 mm, so the overall diameter is 17 × 25.4 + 2 × 101.25 = 634.3 mm, or 24.97 inches, and the tire turns about 807 times per mile.

About this tool

Fitting a different wheel, choosing winter tires or checking a used car's fitment all come down to one question: how tall is the tire, and how far does it travel per turn? This calculator reads the three numbers on a metric sidewall and returns sidewall height, overall diameter in inches and millimetres, rolling circumference and revolutions per mile. Enter a second size alongside it and see the diameter difference and the true road speed when the speedometer shows 60 mph. The figures are nominal design dimensions; a real tire grows slightly when inflated, squats under load and loses diameter as tread wears, so measured rolling circumference is usually 2–3% smaller than the geometric value.

How to use it

  1. Read the sidewall

    Find the size code, e.g. 225/45R17: width 225, aspect ratio 45, rim 17.

  2. Enter the three numbers

    Type the section width in millimetres, the aspect ratio in percent and the rim diameter in inches.

  3. Enter the new size

    Fill in the second tire to see the diameter change and speedometer effect; repeat the first size if you only need one.

Examples

225/45R17

Section width
225
Aspect ratio
45
Rim diameter
17
New section width
225
New aspect ratio
45
New rim diameter
17

Result Overall diameter (inches): 24.97
Size: 225/45R17
Sidewall height (mm): 101.3
Overall diameter (mm): 634.3
Circumference (mm): 1,993
Revolutions per mile: 807.6
New tire diameter (in): 24.97
Diameter difference: 0%
True speed at an indicated 60 mph: 60

  1. Sidewall = 225 mm × 45% = 101.3 mm
  2. Diameter = 17 in × 25.4 + 2 × 101.3 = 634.3 mm = 24.97 in
  3. Revolutions per mile = 63,360 ÷ (π × 24.97) = 807.6
  4. New tire 225/45R17 diameter = 24.97 in
  5. Difference = (634.3 − 634.3) ÷ 634.3 = 0%
  6. At an indicated 60 mph the true speed is 60 × 1 = 60 mph

A common sedan size: a 101 mm sidewall on a 17-inch rim gives a tire just under 25 inches tall.

265/70R17 pickup tire

Section width
265
Aspect ratio
70
Rim diameter
17
New section width
265
New aspect ratio
70
New rim diameter
17

Result Overall diameter (inches): 31.61
Size: 265/70R17
Sidewall height (mm): 185.5
Overall diameter (mm): 802.8
Circumference (mm): 2,522
Revolutions per mile: 638.1
New tire diameter (in): 31.61
Diameter difference: 0%
True speed at an indicated 60 mph: 60

  1. Sidewall = 265 mm × 70% = 185.5 mm
  2. Diameter = 17 in × 25.4 + 2 × 185.5 = 802.8 mm = 31.61 in
  3. Revolutions per mile = 63,360 ÷ (π × 31.61) = 638.1
  4. New tire 265/70R17 diameter = 31.61 in
  5. Difference = (802.8 − 802.8) ÷ 802.8 = 0%
  6. At an indicated 60 mph the true speed is 60 × 1 = 60 mph

A tall sidewall on a light-truck tire puts the overall height at about 31.6 inches.

205/55R16 to 225/45R17

Section width
205
Aspect ratio
55
Rim diameter
16
New section width
225
New aspect ratio
45
New rim diameter
17

Result Overall diameter (inches): 24.88
Size: 205/55R16
Sidewall height (mm): 112.8
Overall diameter (mm): 631.9
Circumference (mm): 1,985
Revolutions per mile: 810.7
New tire diameter (in): 24.97
Diameter difference: 0.38%
True speed at an indicated 60 mph: 60.2

  1. Sidewall = 205 mm × 55% = 112.8 mm
  2. Diameter = 16 in × 25.4 + 2 × 112.8 = 631.9 mm = 24.88 in
  3. Revolutions per mile = 63,360 ÷ (π × 24.88) = 810.7
  4. New tire 225/45R17 diameter = 24.97 in
  5. Difference = (634.3 − 631.9) ÷ 631.9 = 0.38%
  6. At an indicated 60 mph the true speed is 60 × 1.0038 = 60.2 mph

A classic plus-one fitment: the diameter grows by under half a percent, so the speedometer barely changes.

How it is calculated

S = W × A ÷ 100; D = R × 25.4 + 2S; revs per mile = 63,360 ÷ (π × D in inches)

W
Section width in mm
A
Aspect ratio in percent
R
Rim diameter in inches
S
Sidewall height in mm
D
Overall diameter in mm

The metric size code fixes the sidewall as a percentage of the width. Adding two sidewalls to the rim diameter, converted at exactly 25.4 mm per inch, gives the unloaded tire height. Circumference is π times the diameter, and dividing the 63,360 inches in a mile by the circumference gives revolutions per mile. The ratio of new to old diameter scales the true speed for any indicated speed.

When not to use it

  • Flotation sizes such as 33×12.50R15 already state the diameter in inches and do not use this formula.
  • It does not check load index, speed rating or wheel offset clearance.

Common mistakes

  • Treating the aspect ratio as millimetres instead of a percentage of the width.
  • Assuming a wider tire is taller; a lower aspect ratio can make it shorter overall.
  • Comparing sizes without keeping the diameter change within about 3%.

Frequently asked questions

What does the R in 225/45R17 mean?

R stands for radial construction, the ply layout used by nearly all modern passenger tires. It does not affect the size calculation; only the width, aspect ratio and rim diameter do.

How much diameter change is acceptable when changing tire size?

A common fitment guideline is to keep the new overall diameter within about 3% of the original. Larger changes shift speedometer and odometer readings, can confuse ABS and stability control, and may cause rubbing on suspension or arches.

Why does my speedometer read differently after fitting bigger tires?

The speedometer counts wheel revolutions. A taller tire covers more ground per turn, so at an indicated 60 mph you are travelling faster. With a 0.38% larger diameter, the true speed is about 60.2 mph.

Is the calculated diameter what I will measure on the car?

Not exactly. Under the car's weight the loaded radius shrinks, and tread wear removes up to around 8 mm of diameter over the tire's life. Manufacturers' published revolutions per mile are measured and usually run a few percent higher than the geometric figure.

Can a wider tire with a lower aspect ratio keep the same height?

Yes, that is how plus-sizing works. Going from 205/55R16 to 225/45R17 adds 20 mm of width and an inch of rim, yet the overall diameter changes by only about 2.4 mm.

What do revolutions per mile tell me?

It is how many times the tire turns over one mile. Gearing, odometer calibration and differential checks on four-wheel-drive vehicles all rely on it, which is why tires on the same axle should have matching values.

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 Size Calculator. Tools Directory. Retrieved October 28, 2026, from https://tools.milunir.com/automotive/tyres-and-wheels/tire-size-calculator/

MLA

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

Chicago

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

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