Free tire size calculator

Tire size calculator and size comparison

Type the size from the sidewall to get overall diameter, sidewall height, section width, circumference and revolutions per mile. Add a second size to compare them and see the speedometer change.

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Overall diameter-
-

Section width-
Sidewall height-
Circumference-
Revs per mile (calc.)-
Size comparison-
Size ASize BDifference

These are nominal sizes from the numbers on the sidewall. A real tire's diameter and width vary by brand, tread depth, rim width and load, so check the maker's spec sheet before you sell a size change.

In short

A metric tire size gives width in millimeters, sidewall height as a percent of that width, and rim size in inches. For 225/45R17: sidewall = 225 × 0.45 = 101.25 mm (3.99 in), and overall diameter = 2 × 3.99 + 17 = 24.97 in. Circumference is π × diameter, and revolutions per mile are 63,360 ÷ circumference, about 808.

How it works

How to use the Tire Size Calculator

1

Type the tire size

Metric (225/45R17, P215/65R15, LT265/70R17) or flotation (33x12.50R15). Load and speed ratings are ignored.

2

Read the dimensions

Overall diameter, sidewall height, section width, circumference and calculated revs per mile.

3

Compare a second size

See the difference in each dimension and what the speedometer shows at 60 mph.

What it does

What the tire size calculator works out

Metric and flotation sizes

P-metric, Euro-metric, LT-metric, ST and flotation sizes such as 33x12.50R15.

Diameter and sidewall

In inches and millimeters, from the size on the sidewall.

Revs per mile

Calculated from the nominal diameter, so you can compare sizes.

Side-by-side comparison

Diameter, width, sidewall, circumference and revs per mile, with the difference in % .

Speedometer effect

What an indicated 60 mph really is on the new size.

Sidewall height
section width (mm) × aspect ratio ÷ 100
Overall diameter
2 × sidewall height + rim diameter (25.4 mm per inch)
Revs per mile
63,360 in ÷ (π × diameter), calculated
Flotation sizes
33x12.50R15 = 33 in diameter, 12.50 in width, 15 in rim
Accuracy
Nominal sizes; makers' published diameters and revs per mile differ
FAQ

Questions about the tire size calculator

How do you read a tire size like 225/45R17?
225 is the section width in millimeters, 45 is the aspect ratio (sidewall height as 45% of the width), R means radial construction, and 17 is the wheel diameter in inches. A P in front marks a passenger tire and LT a light truck tire.
How do you calculate tire diameter?
Sidewall height = width × aspect ratio ÷ 100, in millimeters. Divide by 25.4 for inches, double it (there's sidewall above and below the wheel) and add the rim diameter. 225/45R17 = 2 × 3.99 + 17 = 24.97 in.
Why don't my numbers match the tire maker's spec sheet?
The size is a nominal designation. Real tires vary by brand, tread depth and the rim width they're measured on, and published revs per mile are measured under load, where the tire squats. For example, one maker lists an LT265/70R17 at 31.97 in, against 31.61 in nominal.
How do flotation sizes like 33x12.50R15 work?
They're in inches: 33 in overall diameter, 12.50 in section width, for a 15-inch wheel. LT after the size marks a light truck tire.
Which tire size should a customer's vehicle have?
The size on the vehicle's Tire and Loading Information label (usually on the driver's door edge or post) or in the owner's manual. Changing size can affect the speedometer, ABS and stability systems, and clearance.
Keep going

Guides and features for this

Tire size chart: common factory sizesBayRobinBrake and tire shopsBayRobinBrake job pricing for shopsGuideVehicles, VINs and mileage in BayRobinHelp centerDigital vehicle inspection guideGuideAll guides on running an auto repair shopTopic guides
Sources

Where the formulas and rules come from

Checked October 2026. Results are nominal dimensions from the size designation; a real tire's measurements differ.

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