Calculator
Tire Size Calculator: Convert a Tire Size to Inches
A metric tire size is a recipe for its dimensions. Enter it and see the diameter, width, sidewall and circumference in inches and millimeters.
What changed
- : First published.
On this page
How a metric tire size works
Take 225/65R17 as an example:
| Part | Meaning | Example |
|---|---|---|
| 225 | Section width in millimeters, sidewall to sidewall | 225 mm = 8.86 in |
| 65 | Aspect ratio: sidewall height as a percentage of width | 65% of 225 mm = 146 mm |
| R | Radial construction | Almost all modern car tires |
| 17 | Wheel (rim) diameter in inches | 17 in |
The mix of millimeters and inches is historical, and it is why the conversion by 25.4 (millimeters per inch) appears in every formula.
The formulas
- Sidewall height (in) = width (mm) × aspect ratio ÷ 100 ÷ 25.4
- Overall diameter (in) = 2 × sidewall height + wheel diameter
- Circumference (in) = π × diameter
- Revolutions per mile = 63,360 ÷ circumference (there are 63,360 inches in a mile)
For 225/65R17: 225 × 0.65 = 146.25 mm = 5.758 in. Diameter = 2 × 5.758 + 17 = 28.52 in. Circumference = 89.58 in. Revolutions per mile ≈ 707.
Flotation sizes
Off-road and light truck tires are often sold in flotation sizes such as 33x12.50R15. These state the overall diameter (33 in) and section width (12.50 in) directly, then the wheel size (15 in). The sidewall height is (33 − 15) ÷ 2 = 9 in. The calculator accepts both styles.
Calculated vs measured size
The calculator gives the nominal size implied by the code. Real tires differ a little:
- Tread design: aggressive tread blocks can add height; tire makers publish the measured overall diameter for each size of each model.
- Measuring rim width: section width is measured on a specified rim width. On a wider or narrower wheel the tire's width changes slightly.
- Loaded radius: under the car's weight, the distance from axle to ground is less than half the free diameter.
- Wear: as tread wears toward the 2/32 inch replacement point NHTSA gives, the diameter shrinks a little.
For a precise figure, look up the revolutions per mile on the tire maker's spec sheet for the exact model and size.
Where to find your tire size
NHTSA says to check the owner's manual or the Tire and Loading Information label on the driver's side door edge or post for the correct size for your vehicle. The size on your current tires is molded on the sidewall, but it may not be the original size if a previous owner changed it. Our sidewall guide explains every marking.
Comparing sizes
Changing size changes diameter, which affects speedometer readings and clearance. Use the comparison tool to see two sizes side by side, or the speedometer calculator to see what your speedometer reads after a change. Our plus sizing guide covers the load, speed rating and diameter rules from a tire maker's technical bulletin.
Common questions
How tall is a 225/65R17 tire?
About 28.5 inches. The sidewall is 225 × 0.65 = 146 mm, or 5.76 inches; two sidewalls plus the 17 inch wheel make 28.52 inches.
What does the P or LT before the size mean?
P is a passenger car tire and LT is a light truck tire. Sizes with no letter are often called Euro-metric. The dimensions are calculated the same way, but load and inflation tables differ.
What is a 33x12.50R15 in metric terms?
Flotation sizes give the diameter (33 in) and width (12.50 in) directly, on a 15 inch wheel. The nearest metric sizes have a similar diameter; use the comparison tool to check any pair.
Why does my tire measure smaller than the calculator says?
Tire makers' actual dimensions vary by model, and the loaded radius under the vehicle is smaller than the free diameter. Worn tread also reduces diameter. The spec sheet for your tire lists its measured diameter and revolutions per mile.
Sources
- NHTSA, Tires: TireWise (tire size and labeling)
- NHTSA, Tire Safety: Everything Rides On It
- NIST, Unit conversion (1 inch = 25.4 mm exactly)
Checked Nov 11, 2026. Spot an error? Tell us.