Tire Radius Calculator
Find the tire's radius. That is half its height, with no load on it. Use the calculator below. Results update as you type.
Radius = Overall Diameter / 2How the Tire Radius Calculator Works
This calculator works by finding the overall diameter from the size code and dividing it by 2. The radius is the distance from the hub center to the tread surface of an unloaded tire.
- Enter the size code, for example 235/40R19.
- Read the diameter: 26.40 in (670.6 mm).
- Read the radius: 13.20 in (335.3 mm).
- Compare the radius with a second size to see how far the axle rises or drops.
The tool reports radius in inches, millimeters and centimeters. The axle-to-ground height of a vehicle changes by the radius difference, not the diameter difference.
The Formula
The formula is Radius = Overall Diameter ÷ 2. The overall diameter comes from the standard sidewall equation.
Radius = Overall Diameter ÷ 2 Radius (mm) = (Rim × 25.4 ÷ 2) + (Width × Aspect Ratio ÷ 100)The second line is a shortcut: half the rim plus one sidewall. Worked example for 255/55R18: half the rim is 228.6 mm, the sidewall is 140.25 mm, and the radius is 368.85 mm (14.52 in, 36.89 cm).
The radius from this formula is the unloaded radius. The static loaded radius, measured with the vehicle sitting on the tire, is roughly 5–10% smaller on passenger car tires.
Conversion Table
The conversion table lists the unloaded radius of 12 sizes in inches, millimeters and centimeters. Values run from 11.55 in (293.3 mm) for a 165/70R14 to 16.50 in (419.0 mm) for a 275/60R20.
Subtract one radius from another to get the change in axle height. Moving from 255/55R18 (14.52 in) to 265/60R18 (15.26 in) lifts the axle 0.74 in (18.7 mm).
| Tire Size | Radius (in) | Radius (mm) | Radius (cm) |
|---|---|---|---|
| 165/70R14 | 11.55 | 293.3 | 29.33 |
| 185/60R15 | 11.87 | 301.5 | 30.15 |
| 195/55R16 | 12.22 | 310.4 | 31.04 |
| 205/50R17 | 12.54 | 318.4 | 31.84 |
| 215/45R17 | 12.31 | 312.6 | 31.26 |
| 225/55R18 | 13.87 | 352.4 | 35.23 |
| 235/40R19 | 13.20 | 335.3 | 33.53 |
| 245/50R20 | 14.82 | 376.5 | 37.65 |
| 255/55R18 | 14.52 | 368.9 | 36.89 |
| 265/60R18 | 15.26 | 387.6 | 38.76 |
| 275/60R20 | 16.50 | 419.0 | 41.90 |
| 305/45R22 | 16.40 | 416.6 | 41.66 |
What Is the Tire Radius Calculator?
The Tire Radius Calculator is a tool that returns the distance from wheel center to tread surface for any metric tire size. Engineers use radius more often than diameter in physics and drivetrain math.
Radius sets 3 values directly: axle height, wheel torque converted to pushing force, and the lever arm of braking force. A larger radius reduces pushing force at the same axle torque and raises the vehicle by the same amount the radius grows.
The calculator gives the nominal unloaded radius. Use the loaded-radius tools for figures measured under vehicle weight.
Common Uses
This calculator has 5 uses: axle-height changes, tractive-force math, brake torque checks, suspension geometry and scale drawings.
- Measure axle-height change: Find how much ground clearance a new size adds under the differential.
- Compute tractive force: Divide wheel torque by radius to get pushing force in pounds-force (lbf) or newtons (N).
- Check brake torque: Relate braking force at the contact patch to torque at the rotor.
- Set suspension geometry: Use radius for roll-center and scrub-radius calculations.
- Draw to scale: Plot tire outlines in CAD or on paper at an exact radius.