How to Determine Tire/Wheel Diameter

Wheel Size

If you want to know if a particular spare tire cover will fit, or are trying to find out how much clearance you would have when switching to another tire/wheel combo, there is an easy way to find out. Simply read the numbers off the sidewall of your current vehicle or trailer tire.

Common Tire Size Parts

The most common tire sizes will have three parts, just like the 205/75-R15 tire pictured above. This number will tell you three things about the tire and wheel combination that you are trying to fit.

Section Width

The First Number

The number 205 represents the section width of the tire, in millimeters. This is the measurement of the tire at its thickest point. If you want to convert centimeters to inches, divide the number of centimeters by 2.54. 1 Inch is about equal to 2.54 centimeters.

Example: 20.5 cm / 2.54 = 8.070 in

A 205/75-R15 tire would have a section width of 8.070 inches.

Aspect Ratio

The Second Number

The number 75 refers to the sidewall aspect ratio. This will help you find out the height of the sidewall, from rim to tread. This is also called section height. The second number in a tire size indicates the ratio between tire width and height.

Example: 8.070 in x 0.75 = 6.053 in

A 205/75-R15 tire would have a section height that is 75 percent of 8.070 inches.

Wheel Diameter

The Third Number

The last number, 15 in our example, represents the diameter, in inches, of the wheel on which the tire is mounted.

Determining Total Diameter

Total Diameter

Now that you know the section width, section height and rim size of the 205/75-R15 tire and wheel, you are able to determine the total diameter. The section height, or sidewall on the tire, is 6.053 inches. Multiply this by 2 to determine the amount of tire extending out on both sides of the wheel. Imagine drawing a straight line across the tire and wheel, directly across the center.

Example: 6.053 in x 2 = 12.106 in

Add the combined section heights of the tire (12.106 in) to the wheel size to get the combined diameter of the tire and wheel.

15 in + 12.106 in = 27.106 in

Section Height

Section Height = Section Width x Aspect Ratio

Tire and Wheel Diameters

Total Diameter = (Section Height x 2) + Wheel Diameter


Wheel Exception

Not every trailer tire will have a size like the example above. We also offer sizes such as 4.80-12 that do not automatically include all the information you need to determine the total tire and wheel diameter.

In the case of a size like 4.80-12, 4.80 is the section width of the tire in inches. The final number indicates the wheel diameter, similar to the more common tire size notation. If the first number of the size is 3 or 4 digits separated by a decimal point, you need to look at the last digit to determine the aspect ratio.

For numbers that end in 0, the aspect ratio is 92 percent. Numbers that do not end in 0 have an aspect ratio of 82 percent.

Example: 4.80 in x 0.92 = 4.416 in

A 4.80-12 tire has a section width of 4.8 inches, a section height of 4.416 inches and a wheel diameter of 12 inches. The combined diameter of the tire and wheel assembly is 20.832 inches.

Updated by: Raymond P.

Last updated: 7/3/18

Questions and Comments about this Article

John N.

In the beginning you spoke of millimeters and then referred to it as centimeters. Is that a typo? It seems easier to keep everything in millimeters for our conversion.

Etrailer Expert
Reply from Jon G.

It wasn't a typo. Since it's easier to convert from centimeters to inches we jump over to centimeters. The initial 205 number you see on your tire sidewall is for millimeters which is why we mention that.

Reply from John N.

@JonG OK thank you very much for your reply.

Jerson A.

How to get the diameter of 235\65R17 104H in Hyundai santafi

Etrailer Expert
Reply from Jon G.

Is that tire size designed to fit a Hyundai Santa Fe? Passenger vehicles are quite a bit more specific on what can fit and work due to clearances behind tires. Trailers are a bit more generic though so they can be more flexible on what they fit. I would check with your local Hyundai dealer or tire shop (or even your owner's manual) to see if it would be safe to fit.

Khurram K.

For the tire size of 235/55r18, Do I need to see only the diameter (18") and the bolt pattern while buying the wheels (rims) or need to check something else too? Will this tire fit all 18-inch rims? Thanks

Etrailer Expert
Reply from Jon G.

Generally speaking, all you have to do is look at those last digits (the 18 like you noted) to know that it will fit on 18" wheels. In this case you'd just need to get the bolt pattern correct for the wheels. There are some tires out there though that will only fit with specific wheel widths. I doubt this is a factor in your setup but it never hurts to double check what wheel widths your tire fits (with the manufacturer) just to be on the safe side of things.

West T.

What size Chevy Rally Wheel is correct for P235 60R15 tire.

Etrailer Expert
Reply from Jon G.

The last digit in that tire size is the wheel diameter that you need (15"). With that being said, we only deal with trailer tires so you will need to make sure that you do some research on the passenger car tires that you decide to use.


This equation is concrete and proven. With cars you would usually alternate 2.54, to 25.4...dividing it by the first number on the tire. There is also a rule of the for rim width. For every .2 in. In width of the rim changes the rim width by .5 in wider or narrower.

Etrailer Expert
Reply from Jon G.

I'm a little confused by that last sentence. Would you be able to maybe rephrase it a little? It sounds like you said for every 0.2" of rim width, the rim width changes by 0.5".

Abdel M.

Please I use 255/50r19 instead of 235/65r19

Etrailer Expert
Reply from Jon G.

I'm pretty sure you're referring to passenger vehicle tires as those sizes are pretty large for a trailer. When it comes to vehicles you need to look in your owner's manual and/or speak with your local dealer because there's a lot more that goes into using a different tire size on a vehicle as opposed to a trailer. This is due to there being more components directly behind the tires so you run into clearance issues where the tire can potentially cause damage.

John M.

The formula for circumference of a circle is diameter times pi (3.14). Therefore a 21 inch diameter tire would travel a little more than 63 inches. This formula is sometimes expressed 2*r*pi. I think the author multiplied the radius times pi, rather than the diameter times pi.

Etrailer Expert
Reply from Jon G.

You are correct (although I think you meant to post this in the thread between @RavenF and @ChrisR). If using the radius the formula is C = 2 x pi x radius If using circumference the formula is C = pi x diameter

Cat C.

Will a 2017 Nissan Altima 5x114.3 16" wheel fit a 2001 Ford Mustang 5x114 16"

Etrailer Expert
Reply from Jon G.

That's something you're going to have to speak with an auto shop or Ford dealer about. From the numbers side of things I would imagine that it would fit but vehicle tires/wheels fit a lot more snug than trailer tires/wheels so you're going to want the opinion of someone who works with the vehicle side of things for this.

Raven F.

Your article was very helpful but i just have a question for one of my assignments. So the combined diameter of the tire and wheel is 20.832in then how far does that tire travel in 1 rotation?

Etrailer Expert
Reply from Chris R.

If a tire/wheel has an overall diameter of 20.832", then it's circumference would be 32.70624". This is the distance that it would travel in one rotation.

Reply from Raven

Thank you so much and thank you for responding. I have tried things like this before with other articles that I commented on and had a question about and no one got back to me so thank you.

Etrailer Expert
Reply from Chris R.

No problem!

Etrailer Expert
Reply from Jon G.

If using the radius the formula is C = 2 x pi x radius If using circumference the formula is C = pi x diameter. This means that for your 20.832 diameter tire the circumference is actually 65.41248".



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