How to Find the Diameter of Radial Tires

Measuring the Diameter


Lay the tire down onto a flat surface.


Stretch your measuring tape over the tire, from the left side to the right.


Measure from the outside of the tire (not from the inside). The diameter is the distance across the tire.

Knowing the Circumference


Understand what circumference is. When you purchase your tires, the packaging (or sticker on the tire) will tell you how many inches the circumference is. The circumference is the distance around the tire, normally in inches. Knowing the circumference will allow you to figure out the diameter.


Write down the formula for determining the diameter of a circle when the circumference is given: C = pi x d. Remember that pi = 3.14.


Plug in your numbers. Let's say that the circumference given is 21.31 inches.

C = pi x d

21.31 inches = 3.14 x d


Divide pi by both sides to solve for "d."

21.31 inches / 3.14 = 3.14 x d / 3.14 (Remember that when pi is divided by itself it cancels out, leaving "d" alone.)

21.31 inches / 3.14 = d


Divide 21.31 by 3.14 to find the diameter of the tire.

21.31 inches / 3.14 = d

6.79 inches = d

The diameter of the tire is 6.79 inches.

Knowing the Radius


Understand what the radius is. In some cases, the tire company may give the radius of the tire. The radius is half the distance of the diameter. The formula for finding the diameter, when the radius is given, is: d = 2 x r.


Plug in your numbers. Let's say that the radius is 4.7 inches.

d = 2 x 3.7 inches


Multiply 2 by the radius.

d = 7.4 inches

The diameter is 7.4 inches.

Radial tires are considered to be the standard in almost all vehicle tires. According to the Michelin website, the advantages of radial tires--over bias tires--include better traction, better distribution of pressure, fuel savings and longer tread life. With these advantages, radial tires last longer and are safer than bias tires. Finding the diameter of your tires is easy and may take you back to sophomore year geometry.