How to Determine Tire Size

Co-authored by Howard Fleischmann and Sophia Latorre

Last Updated: June 17, 2021

You may need to know the size of your tires for various reasons, such as for a spare tire or when purchasing wheels, rims, or snow tires. However, measuring the tire size manually with a tape measure will not give you accurate results. Luckily, your tire size is easy to figure out. Simply read the series of letters and numbers printed on the tire sidewall—all the information you need is right there!

Part 1

Part 1 of 2:

Understanding the First Half of the Series

1
Find the series of numbers on the sidewall. Tires are manufactured with the tire size printed on the sidewall. The sidewall is the outer wall of the tire, rather than the tread that contacts the road. The size should be printed underneath the tire manufacturer name, just above the rim of the tire.^{[1]
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Research source}^{[2]
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Expert Source

Howard Fleischmann
Automotive Tire & Repair Specialist
Expert Interview. 4 June 2021.}
- For instance, the series may look like this: P 225 / 50 R 17 98 H
2
Recognize that the first letter denotes the service type. On some tires, the series of numbers begins with a letter. “P” stands for “P-metric” and denotes a passenger vehicle.^{[3]
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Expert Source

Howard Fleischmann
Automotive Tire & Repair Specialist
Expert Interview. 4 June 2021.} “LT” stands for light truck, “T” means temporary spare, and “C” stands for commercial. If you're purchasing a spare or new tire, you'll need to choose the same service type as the rest of the tires.^{[4]
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3
Note that the first 3-digit number identifies the tire width. The 3-digit number before the slash denotes the tire width, which is measured from sidewall to sidewall and corresponds to the tread that contacts the road. The measurement is given in millimeters, and all 4 tires must have the same width.^{[5]
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- For instance, if the first number is 225, the tire tread is 225 mm wide.
4
Understand that the following 2-digit number is the aspect ratio. The numbers after the slash indicate the aspect ratio, which compares the tire's section height with the tire's section width.^{[6]
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Expert Source

Howard Fleischmann
Automotive Tire & Repair Specialist
Expert Interview. 4 June 2021.} If you're replacing only 1 tire, make sure it has the same aspect ratio as the others.^{[7]
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Research source}
- For example, if there is a “50” after the slash it means the tire's section height is 50% of the tire's section width.

Part 2

Part 2 of 2:

Figuring out the Second Half of the Series

1
Understand that the next letter corresponds to the tire casing. After the aspect ratio, a single letter will be listed that denotes how the tire was constructed. “R” means radial construction, “B” stands for belted bias, and “D” means diagonal bias construction. Choose a spare or new tire with the same casing and construction as the rest.^{[8]
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2
Recognize that the next 2-digit number tells the rim diameter. After the letter, there will be a 2-digit number. This number gives the measurement, in inches, of the rim diameter, which is essential to know if you're getting new rims or wheels.^{[9]
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- For instance, if the number is 17, your rims are 17 inches (43.2 cm) in diameter.
3
Note that the last combination is the load index and speed rating. The last combination of letters and numbers is called the service description. The load index refers to the load-carrying capabilities of a properly-inflated tire, while the speed rating tells you the maximum speed the tire can handle.^{[10]
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- The load index number corresponds with a load index chart, and does not tell you the weight in pounds. For instance, a load index of 98 can carry 1,653 pounds.^{[11]
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- Speed ratings are lettered A–Z and correspond to a speed rating chart. For example, the letter H indicates a maximum speed of 130 mph.

Community Q&A

Question

What is the difference between 185/14 and 195/14?

Community Answer

The larger number just represents the distance from bead to bead, so 195/14 would be slightly larger.
Question

What's the tire size for 1998 Dodge Neon?

Community Answer

Check the driver's side door. There should be a label there indicating the correct tire pressure and tire sizes. You can also check on the sidewall of the actual tires you have on the car now, it should be clearly printed on the side.
Question

If my vehicle was factory-equipped with 255/55 R18 tires, can I change to 265/55 R18 if I change all four tires?

Logic Johnson Lafontaine

Top Answerer

Can you just swap them? No. You'll need new wheels to put them on because the new tires will have slightly bigger sidewalls. You also need to be sure the factory tires aren't max size, because the new ones are 10 mm wider.