What does a tire size like 225/45R17 mean?

225 is the section width in millimetres, 45 is the aspect ratio (sidewall height is 45% of 225 mm = 101.25 mm), R means radial construction and 17 is the rim diameter in inches. A P or LT prefix marks a passenger or light-truck tire and does not change the dimensions.

How do I convert a tire size to inches?

Diameter in inches = (2 × width × aspect ÷ 100) ÷ 25.4 + rim. For 265/70R17: 2 × 265 × 0.70 = 371 mm = 14.61 in, plus 17 = 31.61 in. Width in inches = 265 ÷ 25.4 = 10.43 in. So 265/70R17 is roughly a 31.6x10.4R17.

How do flotation sizes like 33x12.50R15 work?

Flotation sizes give the overall diameter (33 in), the section width (12.50 in) and the rim diameter (15 in) directly. The sidewall is (33 − 15) ÷ 2 = 9 in, which is an aspect ratio of 72. A 285/75R16 is about the same height at 32.8 in.

What is revolutions per mile and why does it matter?

Revolutions per mile = 63,360 ÷ tire circumference in inches. A bigger tire turns fewer times per mile, so the speedometer and odometer read low. At an indicated 60 mph a tire 3% taller is really doing about 61.8 mph.

How much bigger can I go without problems?

A common guideline is to stay within about 3% of the original overall diameter to limit speedometer error and ABS or stability-control issues, but it is not a manufacturer rule. Use the size on the door placard or one the vehicle maker approves, and check load index, speed rating, rim width and clearance.

What do 81H at the end of 175/60R15 81H mean?

81 is the load index, a maximum of 462 kg per tire, and H is the speed rating, 210 km/h (130 mph). Neither changes the dimensions; the calculator ignores them when you paste the full size.

Are these calculated dimensions exact?

No. They are nominal geometric estimates. Actual mounted dimensions vary with tire model, approved rim width, inflation, load, tread depth and wear.

Tire Size Calculator

Type a size like 225/45R17, LT275/65R18 or 33x12.50R15 to get diameter, width and sidewall in inches and mm, circumference and revs per mile. Compare up to three sizes side by side with ride-height and speedometer changes, and see alternative sizes within 3% of your original diameter.

Overall tire diameter = 2 × sidewall height + rim diameter, where sidewall height = section width × aspect ratio ÷ 100 and the rim size in inches × 25.4 gives millimeters.

Tire Size Input

Tire size (225/45R17, LT275/65R18 or 33x12.50R15)

Type or paste the size from the sidewall; the fields below fill in and results update as you type.

Width (mm)

Aspect %

Rim (in)

Enter a tire size and click Calculate to see dimensions.

Tire A Selected size

Difference (B vs A)

Speedometer estimate:

Diameter change: — beyond the ±3% comparison threshold; verify vehicle requirements

Indicated	Estimated actual	Difference

Alternative sizes within 3% of Tire A

Same and larger wheels, closest overall diameter first. Select a size to compare it as Tire B.

Size	Diameter	Difference	At 60 mph indicated

Cross-section Preview

Fitment note: These are nominal geometry estimates, not clearance or safety approval. NHTSA advises using the original tire size or another size recommended by the vehicle manufacturer. Check the driver-door Tire and Loading Information label or owner’s manual, and verify load index, speed rating, rim-width approval, inflation pressure, and clearance with a qualified tire professional.

Quick answer

Overall tire diameter = 2 × sidewall height + rim diameter, where sidewall height = section width × aspect ratio ÷ 100 and the rim size in inches × 25.4 gives millimeters. Divide by 25.4 for inches: 265/70R17 is 31.6 in tall and 10.4 in wide, or about a 31.6x10.4R17. Revolutions per mile = 63,360 ÷ (diameter × π in inches).

Examples

Tire size	Diameter	Width	Revs per mile
205/55R16	24.88 in (631.9 mm)	8.07 in	811
225/45R17	24.97 in (634.3 mm)	8.86 in	808
265/70R17	31.61 in (802.8 mm)	10.43 in	638
285/75R16	32.83 in (833.9 mm)	11.22 in	614
33x12.50R15	33.00 in (838.2 mm)	12.50 in	611

Enter your size, add up to two more to compare, or pick an alternative size within 3% of the original.

Summary

A metric tire code such as 225/45R17 gives the section width in millimetres (225), the sidewall height as a percentage of that width (45) and the rim diameter in inches (17). From these, overall diameter = 2 × 225 × 0.45 + 17 × 25.4 = 634.3 mm (24.97 in). Flotation sizes such as 33x12.50R15 state the diameter and width in inches directly. This calculator accepts both, shows every dimension in inches and millimetres, and compares up to three sizes for diameter, ride height, circumference, revolutions per mile and speedometer error. It also lists alternative sizes within 3% of the original diameter at the same and larger rim sizes. Results are nominal geometry; actual mounted size varies by tire model, rim width, inflation and wear, and fitment must follow the vehicle placard or manufacturer approval.

How it works

Type or paste a tire size such as 225/45R17, P205/55R16, LT275/65R18 or 33x12.50R15; the width, aspect and rim fields fill automatically and results update as you type.
Read diameter, section width, sidewall, circumference, revolutions per mile and the inch size; switch between millimetres and inches.
Open the compare panel and add a second and third size to see them side by side with the difference from the first tire.
Check the speedometer table: with a larger tire the speedometer reads low, so the actual speed is higher than indicated.
Use the alternative sizes list to find sizes within 3% of the original diameter on the same or a larger wheel, and click one to compare it.

Use cases

Convert a metric tire size to inches, for example 265/70R17 to about 31.6 inches tall.
Compare OEM and upgraded tire sizes before buying.
Find plus-size options for larger wheels that keep overall diameter within 3%.
Check how much a bigger tire will lift the vehicle and change speedometer readings.
Compare a metric truck tire with a flotation size such as 33x12.50R15.
Calculate revolutions per mile for gearing and fuel economy calculations.

Frequently Asked Questions

Last updated: 2026-10-10 · Reviewed by Nham Vu