How Are Tires Measured? | Size Codes Explained

Tire sizing combines width, sidewall height, construction type, wheel diameter, load index, and speed rating into one sidewall code.

Most tire measurements are stamped right on the sidewall. Once you can read that line of letters and numbers, buying replacement tires gets a lot easier. You can tell whether a tire is narrow or wide, tall or short, built for a passenger car or a light truck, and matched to the right wheel.

A common size code looks like P225/65R17 102H. That one line tells you the tire’s width, aspect ratio, internal build, rim diameter, load index, and speed symbol. Around it, you’ll see more markings that cover max load, inflation data, and the DOT date code.

How Are Tires Measured? Sidewall Numbers Decoded

Start with the size code on the sidewall. Each part has a job.

• P = passenger tire. You may also see LT for light truck, ST for trailer, or no letter at all on some metric sizes.
• 225 = section width in millimeters.
• 65 = aspect ratio, which is the sidewall height as a percent of the width.
• R = radial construction.
• 17 = wheel diameter in inches.
• 102 = load index.
• H = speed rating.

So if your tire reads P225/65R17 102H, it is a passenger tire that is 225 mm wide, has a sidewall height equal to 65% of that width, fits a 17-inch wheel, carries load index 102, and has an H speed symbol.

The Measurements Most Drivers Mean

When people ask how tires are measured, they usually mean three things: width, sidewall height, and wheel diameter. Width is measured in millimeters on most passenger tires. Wheel diameter is almost always shown in inches. Sidewall height is not listed as a stand-alone number. It is tied to the aspect ratio.

That mix of metric and inch units is why tire sizing can feel odd at first. A tire can be 225 millimeters wide and still fit a 17-inch wheel. That is normal.

Metric Sizes, LT Sizes, And Flotation Sizes

Not every tire uses the same style of code. Passenger-car tires usually use the metric format. Light-truck tires may use metric or flotation sizing, and flotation sizes read very differently.

A metric size looks like 225/65R17 or LT265/70R18. A flotation size looks like 33×12.50R17LT. In that second style, the first number is the tire’s overall diameter in inches, the second is width in inches, and the last is wheel diameter in inches.

That means a 33×12.50R17LT tire is about 33 inches tall, about 12.5 inches wide, built with radial construction, and made for a 17-inch wheel. Off-road and truck shoppers run into this format a lot.

What Changes Between These Systems

The biggest shift is the first number. In metric sizing, the first number is width. In flotation sizing, the first number is overall diameter. If you mix those up, it is easy to buy the wrong tire.

Load handling can shift too. Passenger tires, LT tires, and trailer tires are built for different jobs, so the letters before or after the size matter just as much as the numbers.

How Tire Measurements Work In The Real World

The sidewall code gives you the raw data, but it helps to know what each measurement looks like on the vehicle.

Code Part	What It Means	How It Is Measured
P / LT / ST	Tire service type	Shows the class of tire, such as passenger, light truck, or trailer
225	Section width	Measured sidewall to sidewall at the widest point, in millimeters
65	Aspect ratio	Sidewall height shown as a percent of the section width
R	Construction type	Radial construction on nearly all modern road tires
17	Wheel diameter	Measured where the tire bead seats on the rim, in inches
102	Load index	A coded number tied to the tire’s carrying capacity
H	Speed rating	A symbol tied to the tire’s top speed capability
DOT Code	Tire identification and date	The last four digits show the week and year of manufacture

If you want the same type of breakdown from a manufacturer and a federal safety source, Goodyear’s tire size breakdown and NHTSA’s tire safety page line up on the basics: width, aspect ratio, rim diameter, and sidewall identification all matter when you replace a tire.

Section Width Is Not Tread Width

This trips up a lot of people. The width in the size code is section width, not tread width. Section width measures the tire at its widest sidewall point when mounted and inflated on a specified rim width. The tread that actually touches the road is often narrower.

So a 225 tire is not a promise that the tread is 225 millimeters wide. It is the overall width of the tire body at the sidewalls.

Aspect Ratio Controls Sidewall Height

The aspect ratio tells you how tall the sidewall is compared with the width. On a 225/65R17 tire, the sidewall height is 65% of 225 mm. That works out to 146.25 mm, or about 5.76 inches.

Lower aspect ratios create shorter sidewalls. A 225/45R17 tire will be much shorter than a 225/65R17 even though both are 225 mm wide and fit a 17-inch wheel.

How To Figure Out Overall Tire Diameter

You can estimate a tire’s overall height with a simple formula:

Overall diameter = wheel diameter + 2 × sidewall height

Using 225/65R17 as the sample:

1. Take the width: 225 mm
2. Multiply by the aspect ratio: 225 × 0.65 = 146.25 mm sidewall height
3. Convert sidewall height to inches: 146.25 ÷ 25.4 = about 5.76 inches
4. Double it for the top and bottom sidewall: 5.76 × 2 = about 11.52 inches
5. Add the wheel diameter: 11.52 + 17 = about 28.52 inches

That gives you a tire that stands about 28.5 inches tall. This number matters when you compare replacement sizes, check wheel-well clearance, or want a rough feel for how a plus-size or minus-size change will alter ride height.

Still, treat it as a close estimate. Actual mounted size can shift a bit by brand, tread design, load range, and wheel width.

What Load Index And Speed Rating Tell You

The numbers and letters after the main size code matter just as much as the width and diameter. Load index tells you how much weight the tire can carry when properly inflated. Speed rating tells you the tire’s tested speed class.

If you replace a tire, the new one should match or exceed the vehicle maker’s required load capacity and speed category. Dropping below that can create fitment and safety problems even when the raw size looks right.

Tire Size	Plain-English Reading	What Stands Out
225/65R17	225 mm wide, 65-series sidewall, radial, 17-inch wheel	Taller sidewall for a smoother ride
245/45R18	245 mm wide, 45-series sidewall, radial, 18-inch wheel	Wider and shorter sidewall
LT265/70R17	Light-truck tire, 265 mm wide, 70-series sidewall, 17-inch wheel	Built for truck-duty use
33×12.50R17LT	About 33 inches tall, 12.5 inches wide, radial, 17-inch wheel	Flotation-style sizing in inches

What The Sidewall Does Not Tell You By Itself

The sidewall gives you a lot, but not the full story. It does not tell you the exact tread width, how a tire will ride, how noisy it will be, or how it will grip in rain or snow. It also does not tell you the correct pressure for your vehicle. That comes from the door-jamb placard or owner’s manual, not the max-pressure stamp on the tire.

It also will not tell you whether a different size will clear your suspension, fenders, or steering lock. Two tires with similar published sizes can still fit a little differently once mounted.

A Simple Check Before You Buy

Before you order replacement tires, compare these items with your current setup or the vehicle placard:

• Service type: P, LT, or ST
• Width and aspect ratio
• Wheel diameter
• Load index
• Speed rating
• Overall diameter if you are changing sizes
• Manufacture date if you are buying older stock

Once you know how tire measurements work, the sidewall stops looking like random code. It becomes a clean label that tells you the tire’s size, shape, wheel fit, carrying ability, and age. Read it line by line, and you can shop with a lot more confidence and a lot fewer mistakes.