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TireMath HQ

Overall height

Tire Diameter Calculator

Overall diameter is the number everything else hangs on — speedometer calibration, gearing, clearance and the ±3% substitution rule all come from it. This works it out from any size code, and works backwards too: give it a height and a wheel size and it lists the sizes that land closest.

Enter a size to get its diameter, or use the reverse lookup below.

Common sizes

Overall diameter

32.99″

838.0 mm · 33″ on a tape

Sidewall

6.50″

165.0 mm

Section width

10.83″

275 mm

Revs / mile

611.3

379.8 / km

Also worth knowing

  • Radius, hub center to ground unloaded: 16.50″
  • On a tape measure: 33″
  • Rolling circumference: 103.65″ (2,633 mm)
  • One full turn covers 8.64 ft
Scale drawing of 275/60R20
275/60R20 32.99″

Reverse: find sizes by diameter

Pick the height you want and the wheel you have, and this lists the catalog sizes that land closest.

in
Catalog sizes closest to 33.0 inches
SizeDiameterOff targetWidth
33x12.50R1733.00″0.00%318 mm
295/70R1733.26″+0.79%295 mm
285/70R1732.71″−0.88%285 mm
275/70R1732.16″−2.55%275 mm
315/70R1734.36″+4.13%315 mm
265/70R1731.61″−4.22%265 mm
255/70R1731.06″−5.89%255 mm
35x12.50R1735.00″+6.06%318 mm

See the full size chart sorted by wheel diameter →

Overall diameter is the number every other figure depends on — speedometer, odometer, gearing and the ±3% substitution rule all trace back to it.

Where does the diameter come from?

A tire sits on a wheel with sidewall above and below the hub, so overall diameter is the wheel plus two sidewalls. The wheel is quoted in inches and the sidewall in millimeters, which is the only genuinely awkward part of the calculation: multiply the wheel by 25.4 first, and the rest is addition.

The sidewall itself is not printed on the tire. It is section width × aspect ratio ÷ 100. This is why two tires that share an aspect ratio can have wildly different sidewalls — 70% of 265 mm is 185.5 mm, but 70% of 315 mm is 220.5 mm, a full 35 mm more on each side.

Diameter, radius and circumference

Radius is half the diameter and is what determines how high the axle sits. That is the source of the most common misconception in tire sizing: going up an inch in overall diameter raises the vehicle by half an inch, not a full inch, because half the extra rubber is underneath the axle and half is above it.

Circumference is π × diameter, and revolutions per mile is 63,360 divided by it — 63,360 being the number of inches in a mile. Because revolutions per mile is inversely proportional to diameter, a 3% taller tire turns 3% fewer times, which is precisely the speedometer error.

Common questions about tire diameter

How do you find the diameter of a tire from its size?
Overall diameter equals the wheel diameter plus two sidewalls. Convert the wheel to millimeters by multiplying by 25.4, work out the sidewall as section width times aspect ratio over 100, and add twice that. For 275/60R20: 508 + (2 × 165) = 838 mm, or 33.0 inches.
Is the tire diameter on the sidewall?
Only on flotation sizes. 33x12.50R15 states 33 inches directly. A metric size such as 275/60R20 does not print its diameter anywhere — it has to be derived from the three numbers in the code.
Is calculated diameter the same as measured diameter?
Close, but not identical. The calculated figure is the nominal design diameter unloaded. A mounted, inflated, loaded tire measures a little less at the contact patch, and tread depth varies over the tire's life, so a tape measure on a fitted tire usually reads slightly under.
What is rolling circumference?
The distance covered in one full revolution: π times the overall diameter. It is the figure the odometer is calibrated against, which is why it, and not the diameter, is what actually drives speedometer error.

For a full table of sizes and their diameters sorted by wheel, see the tire size chart.

Other tire maths on this site