Enter a tire size like 205/55R16 to get its diameter, circumference and sidewall height — and compare two sizes.
What Is a Tire Size?
A tire size calculator decodes markings like 205/55R16: 205 mm wide, sidewall 55% of width (112.8 mm), 16-inch rim. Total diameter = 2 × 112.8 + 406.4 = 631.9 mm (24.88 in); circumference 78.16 in. When upsizing, keep the diameter within ±3% so the speedometer and gearing stay accurate — that is what the comparison use is for.
How to Use
- Enter the tire width in mm.
- Enter the aspect ratio %.
- Enter the rim diameter in inches.
- Click “Calculate Tire Size”.
Formula
Sidewall = width × aspect% ÷ 100. Diameter = 2 × sidewall + rim × 25.4 (mm).
Worked Example
205/55R16: sidewall 112.8 mm; diameter 631.9 mm = 24.88 in; circumference 78.16 in.
How It Works, In Plain Words
The marking on a tire’s sidewall is a recipe in three numbers, and the tool cooks it in order. The width comes first: 205 mm across the tread. The aspect ratio turns that width into the sidewall height — it is a percentage, not a measurement, so the sidewall is 205 × 55 ÷ 100 = 112.8 mm. The last number is the rim the tire fits, given in inches, so the tool converts it at 25.4 mm to the inch: the 16-inch rim contributes 406.4 mm. Stack the parts — sidewall, rim, sidewall again — and the diameter formula gives 2 × 112.8 + 406.4 = 631.9 mm, or 24.88 in.
From the diameter comes the circumference, 78.16 in for the example: the distance the tire rolls in one full turn. Diameter is the figure that matters when you compare two sizes, because the car’s speedometer and gearing were set up for the original one. That is why this page repeats its ±3% guidance — a replacement size whose diameter stays within ±3% of the original keeps those instruments honest, and the tool lets you check any candidate size against that yardstick before money changes hands.
Real-World Uses
A driver whose tires are worn out reads the sidewall code, decodes it here, and orders the same size with confidence instead of squinting at the tire in a car park. Someone fitting larger wheels for looks runs both the old and new sizes and checks the diameter shift stays inside the ±3% band this page recommends, so the speedometer keeps telling the truth. And a buyer offered a second-hand set of wheels checks the decoded diameter and sidewall against their car’s size first — the few minutes that separate a bargain from a set that was never going to suit the car.
Benefits
- Decodes the sidewall marking plainly
- Diameter in mm and inches
- The ±3% upsizing rule explained
- Useful before buying new wheels
Features of Our Tool
- Instant results in your browser
- No sign-up, no download, 100% free
- Works on mobile, tablet and desktop
- Clear step-by-step explanation below the tool
- Private — your entries never leave your device
Common Mistakes
- Changing diameter more than ±3% — speedometer and gearing drift
- Forgetting the aspect ratio is a % of width, not mm
- Mixing load/speed ratings (separate from size)
Common Mistakes to Avoid
The costliest error is chasing a look and letting the overall diameter drift beyond ±3%: the speedometer and gearing were calibrated for the original diameter, and they drift with it. Just as common is reading the aspect ratio as millimetres — it is a percentage of the width, which is why 55 on a 205 mm tire means a 112.8 mm sidewall. Do not let a matching size distract you from the load and speed ratings either; they are separate from size and this tool does not check them. Finally, compare diameters, not widths, when judging whether a different size will behave like the original.
FAQ
What does 205/55R16 mean?
205 mm wide, sidewall 55% of width, radial, 16-inch rim.
What is the diameter of 205/55R16?
631.9 mm (24.88 in).
Can I fit bigger tires?
Yes, within ±3% of the original diameter — check clearance and speedometer error.
Does tire size affect the speedometer?
Yes — a larger diameter reads slow, a smaller one reads fast.
What is aspect ratio?
Sidewall height as a percentage of tire width — 55 means the sidewall is 55% of 205 mm.
Why does the tool show circumference as well?
Because circumference is the distance the tire covers in one full revolution — 78.16 in for the example — and it is the link between tire size and the speedometer: the car counts revolutions, so a different circumference means the dial no longer matches the road, which is why a larger diameter reads slow and a smaller one reads fast.
What does the R in 205/55R16 stand for?
Radial construction — as the first answer on this page puts it, the marking decodes as 205 mm wide, sidewall 55% of width, radial, 16-inch rim. The tool’s three inputs are the numbers around that letter: width, aspect ratio and rim diameter.