Tire Size Calculator
Calculate tire dimensions, plus sizing, and tire comparisons
Tire Size
Load & Speed Ratings
Overall Diameter
676 mm
Sidewall Height
135 mm
Circumference
212.5 cm
Revolutions per Mile
1219 rev/km
Load Capacity
615 kg
Speed Rating
240 km/h
How it works
A tire size calculator decodes a tire's sidewall code and computes its overall diameter, useful when comparing or changing sizes. The code gives width, aspect ratio (sidewall height as a percent of width), and wheel diameter, which combine into the full tire diameter.
Tire diameter
Diameter = wheel diameter + 2 × (width × aspect ratio ÷ 100)
- width
- tread width in mm (e.g. 225)
- aspect ratio
- sidewall height as % of width (e.g. 45)
- wheel diameter
- rim size in inches
Worked example
- Tire 225/45R17
- Sidewall = 225 × 0.45 = 101.25 mm ≈ 3.99 in
- Diameter = 17 + 2 × 3.99
≈ 24.98 inches overall diameter.
Good to know
- Changing tire size alters your speedometer reading unless the overall diameter stays close (within ~3%).
- A lower aspect ratio means a shorter, stiffer sidewall — sportier feel, harsher ride.
- Match the load and speed ratings when changing sizes.
Related Calculators
Frequently Asked Questions
What do tire size numbers like 225/45R17 mean?
225 is the tread width in millimeters, 45 is the aspect ratio (sidewall height as a percent of width), R means radial construction, and 17 is the wheel diameter in inches.
How do I calculate a tire's overall diameter?
Sidewall height = width x aspect ratio; overall diameter = rim diameter + 2 sidewalls. A 225/45R17 has 101 mm sidewalls (about 4 inches), giving roughly a 25-inch overall diameter.
How different can a replacement tire size safely be?
Keep the new overall diameter within about ±3% of the original. Larger deviations skew the speedometer and odometer, and can upset ABS, traction control, and gearing.
What is plus sizing?
Fitting a larger wheel with a lower-profile tire while keeping the overall diameter nearly constant — for example moving from 205/65R15 to 225/45R17. Handling sharpens, but ride comfort and pothole resilience drop.
How does tire size affect my speedometer?
A larger-diameter tire covers more ground per revolution, so the speedometer reads slower than your true speed; smaller tires make it read fast. The error percentage equals the diameter change percentage.