Tire Size Calculator
Enter any tire code — 205/55R16, 275/35R20 — and instantly get width, sidewall height, overall diameter, and circumference.
Tyre Size Calculator
Enter code → dimensions in mm and inches
How It Works
A tire code encodes width, aspect ratio, and rim diameter. Sidewall height = width × (aspect ratio ÷ 100). Overall diameter = rim diameter × 25.4 + 2 × sidewall height.
How to Use This Calculator
- Read the tire code from the sidewall (e.g., 205/55R16).
- Enter the full code in the format WIDTH/ASPECTRRIM — e.g. 205/55R16.
- Click Calculate — all dimensions appear instantly.
- Use the diameter to compare against another tire size.
- Use the circumference in the RPM or speedometer calculators.
Worked Example
Reference Table
| Tire Code | Width | Sidewall | Diameter | Circumference |
|---|---|---|---|---|
| 195/65R15 | 195 mm | 126.75 mm | 635.9 mm | 1997 mm |
| 205/55R16 | 205 mm | 112.75 mm | 631.9 mm | 1985 mm |
| 225/45R17 | 225 mm | 101.25 mm | 634.9 mm | 1994 mm |
| 235/35R19 | 235 mm | 82.25 mm | 647.3 mm | 2034 mm |
| 265/70R17 | 265 mm | 185.5 mm | 803.4 mm | 2524 mm |
| 275/35R20 | 275 mm | 96.25 mm | 700.9 mm | 2202 mm |
Frequently Asked Questions
What does the ‘R’ in a tire code mean?
R stands for Radial construction — the internal cords run radially from bead to bead. Almost all modern passenger car tyres are radial. Older bias-ply tyres use a ‘B’ or ‘-‘ designation.
How much does tire diameter affect speedometer accuracy?
A 3% increase in diameter makes your speedometer read 3% low — so if the speedo shows 100 km/h you are actually doing 103 km/h. Use the Speedometer Error Calculator for precise correction.
What is the ‘+1 fitment’ rule for upgrading wheel size?
When increasing rim diameter by 1 inch, reduce tyre aspect ratio to keep overall diameter within 3% of stock. For example, moving from 205/55R16 to 225/45R17 keeps diameter nearly the same (631 mm vs 635 mm).
Why do low-profile tires have a harsher ride?
A lower aspect ratio means a shorter, stiffer sidewall with less flex to absorb road impacts. A 35-series tyre has far less cushioning than a 65-series, directly transmitting more road texture and pothole impacts into the cabin.
