Tire Size Calculator
Calculate tire diameter, sidewall height, circumference and revolutions per km from size markings.
Tire diameter
24.88 in
632 mm
Circumference
78.2 in
1,985.2 mm
Sidewall height
4.4 in
Revolutions per km
504
Example: a 205/55R16 tire has a 205 mm wide tread, a sidewall that is 55% of the width, and fits a 16-inch rim. Change tire size and the rolling diameter changes too — keep it within 3% of stock.
Tire Size Calculator on True Calculator gives you an instant, accurate answer with no sign-up and no app install. Calculate tire diameter, sidewall height, circumference and revolutions per km from size markings. Every result shows the formula and a worked example so you can verify the calculation yourself, and all values are computed in your own browser — your numbers never leave your device.
Popular uses: tire size calculator · tyre size calculator · tire diameter
How We Calculate
This calculator uses standard physics and chemistry formulas verified by our team. All calculations are performed instantly in your browser using JavaScript — no data is sent to any server.
Formulas follow standard scientific definitions and physical constants, consistent with what is taught in school and university courses.
When to Use This Calculator
Use this calculator when changing tire size for better grip, a smoother ride or a sportier look on your car in India. The diameter and circumference numbers matter for speedometer checks after an upgrade, and revolutions per km is the figure used when calibrating tripmeters or setting up cycle computers. Fleet owners comparing tire sizes across models use the diameter to predict ground clearance changes and fuel economy shifts. The same math applies to motorcycles, scooters and even bicycles with labeled tire sizes. Before you commit to new alloys and tires, run the comparison — a 3% diameter change is fine, but beyond that the odometer, ABS and even the gear ratios all start telling a different story.
How to Use This Calculator
- Step 1: Read the size markings on the tire sidewall, for example 195/60 R15.
- Step 2: Enter the width in mm (195), the aspect ratio in % (60) and the rim diameter in inches (15).
- Step 3: Read the sidewall height, total diameter, circumference and revolutions per km.
- Step 4: Compare with your current size to check the change in rolling diameter.
Worked Example
A 195/60 R15 tire has a sidewall of 195 × 60% = 117 mm. Its total diameter is 15 × 25.4 + 2 × 117 = 615 mm, about 24.21 inches. Each revolution covers π × 615 = 1,932.08 mm, so the tire rotates about 518 times per kilometre. Upgrading to 205/55 R16 changes the diameter to 631.9 mm, a 2.7% increase — still within the 3% rule of thumb for speedometer accuracy.
Tips and Common Mistakes
- •Keep the rolling diameter within 3% of the stock size when upgrading, or recalibrate the speedometer.
- •Check rim width compatibility — a very wide tire on a narrow rim wears unevenly and bulges.
- •Match the new size to the load index and speed rating of the original equipment tire.
- ✗Do not compare only the width — the aspect ratio and rim size together set the diameter.
- ✗Do not fit a tire that rubs the wheel well at full steering lock or when the suspension compresses.
Frequently Asked Questions
What does 205/55 R16 mean?
205 is the tread width in mm, 55 is the aspect ratio (sidewall is 55% of the width = 112.75 mm), and R16 is the rim diameter in inches. Total diameter: 16×25.4 + 2×112.75 = 631.9 mm.
Why does tire size affect my speedometer?
A bigger tire covers more distance per revolution, so the speedometer reads low. When changing sizes, keep the rolling diameter within about 3% of stock to avoid speedometer and ABS errors.
How do I calculate tire diameter?
Diameter = rim × 25.4 + 2 × (width × aspect ÷ 100), in mm. Divide by 25.4 for inches. A 225/45 R17 tire, for example, is 17×25.4 + 2×101.25 = 634.3 mm.
What is the circumference used for?
Circumference (diameter × π) gives revolutions per km — 1,000,000 mm ÷ circumference. A 205/55 R16 tire rotates about 504 times per km. This matters for odometer calibration and gearing feel.
Can I fit a wider tire on the same rim?
Only within limits — each rim width accepts a range of tire widths, and clearance to the body and suspension must be checked. Always verify with a tyre dealer and keep the rolling diameter close to stock.
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