Tire Size Calculator · Practical guide
What Do the Numbers on a Tire Mean?
Every size like 205/55R16 encodes width, sidewall height, construction, and wheel diameter — here's how to read it.
At a glance
Decoding a Size Like 205/55R16
The three key numbers describe the tire's dimensions: 205 is the width in millimeters, 55 is the aspect ratio — the sidewall height as a percentage of the width, so 55% of 205 = 112.75 mm — R means radial construction, and 16 is the wheel diameter in inches.
| Marking | Meaning | Example Value |
|---|---|---|
| 205 | Section width | 205 mm (8.07 in) |
| 55 | Aspect ratio | Sidewall = 112.75 mm |
| R | Construction | Radial |
| 16 | Rim diameter | 16 inches |
Interactive tool
Tire Size Calculator
Enter your own values, then compare the result with the examples below.
Results
From Numbers to Overall Diameter
Overall diameter combines both sidewalls plus the rim: (width × aspect ÷ 100 × 2 ÷ 25.4) + rim. For 205/55R16 that's (205 × 0.55 × 2 ÷ 25.4) + 16 = 8.88 + 16 = 24.88 inches tall. Enter the three numbers in the calculator above to get the diameter of any size — and to compare it against another.
Where to Find Your Size
The size is molded into the tire sidewall, printed on the sticker inside the driver's door jamb, and listed in the owner's manual. When replacing tires, match the door-jamb sticker rather than whatever is currently mounted — a previous owner may have changed sizes.
Frequently Asked Questions
What does the R mean in a tire size?
R stands for radial construction, where the internal cords run perpendicular to the direction of travel. Nearly all modern passenger tires are radials; older bias-ply tires are marked B or D.
What do the load index and speed rating after the size mean?
A marking like 91V pairs a load index (91 = 1,356 lbs per tire) with a speed rating (V = up to 149 mph). Match or exceed the vehicle's original ratings when replacing tires.
Why are width and rim diameter in different units?
It's a historical hybrid: tire width is metric (millimeters) while wheel diameter stayed in inches. The conversion factor 25.4 mm per inch bridges the two in diameter calculations.
Can I change the aspect ratio when buying tires?
Yes, as long as the overall diameter stays within about 3% of stock — a lower aspect ratio on a wider tire can keep the same height, which is the idea behind plus sizing.