Tire Diameter Calculator

This calculator works in 4 steps: it reads the size code, finds the sidewall height, converts the rim to millimeters, and adds the parts together. Every result comes from the three numbers molded on the sidewall, such as 225/45R17.

1. Enter the section width in millimeters (mm), for example 225.
2. Enter the aspect ratio as a percentage, for example 45.
3. Enter the rim diameter in inches (in), for example 17.
4. Read the overall diameter in inches, millimeters and centimeters (cm) in the results panel.

The tool multiplies width by aspect ratio to get one sidewall, doubles it to cover the top and bottom of the tire, and adds the rim diameter. A 225/45R17 returns 24.97 in (634.3 mm). Results update on every keystroke, so you can test 5 or 6 sizes in under a minute.

The formula is Overall Diameter = (Rim Diameter × 25.4) + 2 × (Width × Aspect Ratio ÷ 100). The result is in millimeters; divide by 25.4 for inches.

The formula has 3 inputs:

• Width is the section width in mm, measured sidewall to sidewall on the measuring rim.
• Aspect Ratio is sidewall height as a percentage of width.
• Rim Diameter is the wheel bead seat diameter in inches; 25.4 converts it to mm.

Worked example for 265/70R17: the sidewall is 265 × 0.70 = 185.5 mm (7.30 in). Two sidewalls give 371 mm. The rim is 17 × 25.4 = 431.8 mm. The overall diameter is 371 + 431.8 = 802.8 mm, or 31.61 in.

The conversion table lists the overall diameter of 12 common sizes in inches, millimeters and centimeters. Sizes run from compact cars (185/65R15, 24.47 in) to full-size trucks (285/75R16, 32.8 in).

Read a row from left to right: find your size, then pick the unit you need. Values are nominal, meaning they come from the size code and not from a tape measure. A mounted, inflated tire measures within 1–3% of these figures.

The Tire Diameter Calculator is a free online tool that returns the nominal overall height of a tire from its size code. Overall diameter is the distance from the ground to the top of the tread on an unloaded tire, passing through the hub center.

Tire diameter controls 4 vehicle measurements: speedometer reading, ground clearance, effective gear ratio and fender clearance. A diameter change of 1 in (25.4 mm) on a 30 in tire changes each of these by about 3.3%.

The calculator follows the European Tyre and Rim Technical Organisation (ETRTO) and Tire and Rim Association (TRA) method for nominal dimensions. Manufacturers publish measured diameters that differ slightly by tread pattern and construction, so treat the result as the reference value for comparisons.

This calculator has 6 main uses: size comparison, speedometer checks, lift planning, plus-sizing, AWD matching and spare-tire checks.

• Compare sizes: Check whether a new size stays within ±3% of the original diameter.
• Predict speedometer error: Use the diameter ratio to find true road speed after a size change.
• Plan lift kits: Match the tire height to the clearance a 2 in, 4 in or 6 in (51, 102 or 152 mm) lift creates.
• Choose plus sizes: Hold diameter constant when moving from a 16 in wheel to a 17 in or 18 in wheel.
• Match AWD axles: Keep front and rear diameters equal on all-wheel-drive (AWD) vehicles.
• Check spares: Confirm a spare's height before mixing it with road tires.