How do I convert a metric tire size to inches?

Divide the first number by 25.4 for the width, multiply the width in millimetres by the aspect ratio and divide by 25.4 for the sidewall, then add two sidewalls to the rim for the overall diameter. For 285/70R17: 285 mm is 11.22 inches wide, the sidewall is 199.5 mm or 7.85 inches, and 17 plus twice 7.85 is 32.71 inches overall, so in inch terms it is 32.7x11.2R17.

How do I convert an inch size such as 33x12.50R15 to metric?

The width is 12.50 times 25.4, which is 317.5 mm. The sidewall is half of the overall diameter less the rim, 33 minus 15 over 2, which is 9 inches or 228.6 mm, and 228.6 over 317.5 is an aspect ratio of 72 percent. Metric sizes are made in fixed steps of width and aspect ratio, so an exact metric twin rarely exists; compare a candidate against the inch size to see how close it is.

Why is a 285/70R17 called a 33 inch tire?

Because inch sizes are rounded and named for the class they compete in. Worked from its code, a 285/70R17 is 32.71 inches tall, which is close enough to 33 that it is sold and talked about as one. The page gives the exact figure so you can see how far a given metric size is from the name it goes by.

Do ATV, golf cart and mower sizes convert too?

Yes, when they are written as inch codes. 25x8-12 is a 25 inch tire, 8 inches wide, on a 12 inch rim; 18x8.50-8 and 15x6.00-6 read the same way. The dash in those codes is kept as written. Tractor sizes such as 18.4-38 and bicycle sizes use other systems and are not read here.

Is the rim size the same in both systems?

Yes. The rim is in inches on a metric car code and on an inch code alike, so 265/70R17 and 33x12.50R17 both mount on a 17 inch wheel. Only the width and the height are written differently.

Tire Size to Inches Calculator

Type a metric sidewall code and read it as an inch size: overall diameter, section width and rim, each to a tenth of an inch, with the exact diameter and sidewall height under it. Type an inch size such as 33x12.50R15 and the page goes the other way, giving the width in millimetres and the sidewall as a percentage of it. LT before or after the code is read and dropped, and the working is written out beside the result.

Size in inches225/45R17

In inch terms: 25.0x8.9R17
Overall diameter: 24.97 in (634.3 mm)
Section width: 225 mm (8.86 in)
Sidewall (45%): 3.99 in (101.3 mm)
Rim diameter: 17 in (431.8 mm)
Circumference: 78.45 in (1992.7 mm)
Revolutions per mile: 808

How it is worked out225/45R17

sidewall = 225 mm x 45% = 101.25 mm = 101.25 / 25.4 = 3.99 in diameter = 17 + 2 x 3.99 = 24.97 in circumf. = pi x 24.97 = 78.45 in revs/mile = 63360 / 78.45 = 808

Tire size

Units

Reading

One code read out in full: overall diameter, sidewall height, section width, circumference.

225/45R17 in full

In inch termsdiameter x width R rim, each to a tenth of an inch
25.0x8.9R17
Overall diameterrim plus two sidewalls
24.97 in (634.3 mm)
Section widththe first number of the code, across the widest part
225 mm (8.86 in)
Sidewall (45%)45% of 225 mm
3.99 in (101.3 mm)
Rim diameterthe wheel the tire mounts on, in inches even on a metric code
17 in (431.8 mm)
Circumferencepi times the overall diameter
78.45 in (1992.7 mm)
Revolutions per mile63,360 inches in a mile, over the circumference
808

Geometry, not fitment. This tells you what a size measures, not whether it fits your car: it gives no clearance check, no load or speed rating advice and no tire pressure, and it has no vehicle size list. Exact arithmetic on the sidewall code, which is a definition rather than an estimate. It cannot know your wheel well's clearance, whether a load or speed rating suits your vehicle, or the true mounted diameter, which changes with pressure, load and wear.

Common questions

How do I convert a metric tire size to inches?: Divide the first number by 25.4 for the width, multiply the width in millimetres by the aspect ratio and divide by 25.4 for the sidewall, then add two sidewalls to the rim for the overall diameter. For 285/70R17: 285 mm is 11.22 inches wide, the sidewall is 199.5 mm or 7.85 inches, and 17 plus twice 7.85 is 32.71 inches overall, so in inch terms it is 32.7x11.2R17.
How do I convert an inch size such as 33x12.50R15 to metric?: The width is 12.50 times 25.4, which is 317.5 mm. The sidewall is half of the overall diameter less the rim, 33 minus 15 over 2, which is 9 inches or 228.6 mm, and 228.6 over 317.5 is an aspect ratio of 72 percent. Metric sizes are made in fixed steps of width and aspect ratio, so an exact metric twin rarely exists; compare a candidate against the inch size to see how close it is.
Why is a 285/70R17 called a 33 inch tire?: Because inch sizes are rounded and named for the class they compete in. Worked from its code, a 285/70R17 is 32.71 inches tall, which is close enough to 33 that it is sold and talked about as one. The page gives the exact figure so you can see how far a given metric size is from the name it goes by.
Do ATV, golf cart and mower sizes convert too?: Yes, when they are written as inch codes. 25x8-12 is a 25 inch tire, 8 inches wide, on a 12 inch rim; 18x8.50-8 and 15x6.00-6 read the same way. The dash in those codes is kept as written. Tractor sizes such as 18.4-38 and bicycle sizes use other systems and are not read here.
Is the rim size the same in both systems?: Yes. The rim is in inches on a metric car code and on an inch code alike, so 265/70R17 and 33x12.50R17 both mount on a 17 inch wheel. Only the width and the height are written differently.

Exact arithmetic on the sidewall code, which is a definition rather than an estimate. It cannot know your wheel well's clearance, whether a load or speed rating suits your vehicle, or the true mounted diameter, which changes with pressure, load and wear.