Tire Radius Calculator
Calculate the nominal unloaded radius of any tire size in mm, inches and cm, with an estimated loaded radius for clearance and torque math.
Calculate nominal unloaded radius.
Tire Radius Calculator: Tire Size Inputs
Compare two tire sizes here. Use the formulas and reference tables below for this tool's specific calculation.
How Tire Radius Calculator Works?
The Tire Radius Calculator works by halving the overall diameter calculated from the size code. The tool uses 3 steps.
Enter the size
Type a metric size such as 245/40 R18.
Compute the diameter
The calculator returns an overall diameter of 653.2 mm (25.72 in).
Halve the diameter
The calculator returns an unloaded radius of 326.6 mm (12.86 in) and an estimated loaded radius of about 310 mm (12.2 in) at 95% of the unloaded value.
Tire Radius Calculator Formula
The Tire Radius Calculator formula is r = OD ÷ 2 = (W × AR ÷ 100) + (Rim × 12.7).
r = W × AR ÷ 100 + Rim × 12.7r_in = r ÷ 25.4r_loaded ≈ 0.95 × r- r
- Unloaded radius from axle center to tread (mm)
- 12.7
- Half of 25.4, millimeters per half inch
- 0.95
- Typical loaded-to-unloaded ratio for passenger tires
Worked example
A 275/55 R20 tire has a radius of 275 × 0.55 + 20 × 12.7 = 151.25 + 254 = 405.3 mm (15.96 in). The estimated loaded radius is 385 mm (15.2 in).
Tire Radius Calculator Conversion Table
The Tire Radius Calculator conversion table lists the unloaded and estimated loaded radius of 10 common sizes.
| Size | Unloaded radius (mm) | Unloaded radius (in) | Est. loaded radius (mm) | Unloaded radius (cm) |
|---|---|---|---|---|
| 195/65 R15 | 317.2 mm | 12.49 in | 301.4 mm | 31.7 cm |
| 205/55 R16 | 315.9 mm | 12.44 in | 300.2 mm | 31.6 cm |
| 215/60 R16 | 332.2 mm | 13.08 in | 315.6 mm | 33.2 cm |
| 225/45 R17 | 317.1 mm | 12.49 in | 301.3 mm | 31.7 cm |
| 225/65 R17 | 362.1 mm | 14.26 in | 344.0 mm | 36.2 cm |
| 235/45 R18 | 334.4 mm | 13.16 in | 317.6 mm | 33.4 cm |
| 245/40 R18 | 326.6 mm | 12.86 in | 310.3 mm | 32.7 cm |
| 255/35 R19 | 330.5 mm | 13.01 in | 314.0 mm | 33.1 cm |
| 265/70 R17 | 401.4 mm | 15.80 in | 381.3 mm | 40.1 cm |
| 275/55 R20 | 405.2 mm | 15.95 in | 385.0 mm | 40.5 cm |
What is Tire Radius Calculator?
A Tire Radius Calculator is a tool that returns the distance from the center of the wheel to the outer tread surface. Radius equals half of the overall diameter.
Radius is the lever arm between the axle and the road. Wheel torque, tractive force and ground clearance calculations all use the radius, not the diameter.
The unloaded radius is a nominal value. The loaded radius, called static loaded radius (SLR), is shorter and appears in tire maker data books.
What are the uses of Tire Radius Calculator?
There are 4 main uses of the Tire Radius Calculator, including torque math and ground clearance estimates.
Torque and force math
Divide axle torque by tire radius to get force at the road.
Ground clearance estimates
Add the radius change to the stock clearance figure.
Suspension geometry
Locate the wheel center height for alignment and scrub calculations.
Brake and gearing models
Supply the rolling radius to braking distance and gear ratio models.
Related Core Tire Dimensions Tools
View All 134 Subtools →Compare two tire sizes.
Compare several sizes together.
Estimate nominal overall diameter.
Calculate absolute and percentage differences.
Estimate nominal circumference.
Compare estimated rolling distances.