A 305 tire is 305 millimetres wide, which is 12.01 inches. The 305 only tells you the width. How tall the tire is depends on the other two numbers in the size, the aspect ratio and the rim diameter. Use the calculator below to get all of it.
Tire size calculator
Enter the three numbers from the tire sidewall, for example 305/70R16 is width 305, aspect ratio 70, rim 16.
What does a 305 tire mean in inches?
The first number in a metric tire size is the width in millimetres. To change millimetres to inches, divide by 25.4. So 305 ÷ 25.4 = 12.01 inches. That is the width of the tire from sidewall to sidewall, measured on a specific rim width, so the real tire can differ a little between brands.
The height is a separate question. The same 305 mm width can be a short tire or a tall one:
| Size | Sidewall (in) | Rim (in) | Overall height (in) |
|---|---|---|---|
| LT305/70R16 | 8.41 | 16 | 32.81 |
| LT305/65R17 | 7.81 | 17 | 32.61 |
| LT305/55R20 | 6.60 | 20 | 33.21 |
These three are all close to 33 inches tall, but they get there in different ways: a tall sidewall on a small rim, or a short sidewall on a big rim.
How to read a tire size
Here is a size as it appears on the sidewall, with each part labelled:
You may also see LT (light truck) or ST (special trailer) at the start instead of P. Some light truck sizes end with two load numbers, such as 118/115. The first is the load index for a single tire and the second is for tires used in pairs.
How to work out the size by hand
Here is LT305/70R16 step by step:
- Width in inches: 305 ÷ 25.4 = 12.01 in.
- Sidewall height: 305 × 70% = 213.5 mm, and 213.5 ÷ 25.4 = 8.4055 in.
- Overall height: the rim plus two sidewalls, 16 + (2 × 8.4055) = 32.81 in.
- Circumference: 32.81 × 3.1416 = 103.08 in.
- Revolutions per mile: a mile is 63,360 inches, so 63,360 ÷ 103.08 = about 615.
Compare two tire sizes
If you change tire size, the real height changes, and your speedometer and odometer are no longer exact. Enter your current size and the new one to see the difference. The defaults compare 285/75R16 with 305/70R16, a pair that is nearly identical in height.
The new tire is within 3% of the current height, a common rule of thumb for swapping sizes.
Height is only one check. A wider or taller tire also needs room in the wheel well, a rim of the right width, and a load rating at least as high as the original. Use the size on your door-jamb placard or in the owner’s manual as the starting point, and ask a tire shop about anything different.
Tire width in millimetres and inches
| Width (mm) | Width (in) |
|---|---|
| 155 | 6.10 |
| 165 | 6.50 |
| 175 | 6.89 |
| 185 | 7.28 |
| 195 | 7.68 |
| 205 | 8.07 |
| 215 | 8.46 |
| 225 | 8.86 |
| 235 | 9.25 |
| 245 | 9.65 |
| 255 | 10.04 |
| 265 | 10.43 |
| 275 | 10.83 |
| 285 | 11.22 |
| 295 | 11.61 |
| 305 | 12.01 |
| 315 | 12.40 |
| 325 | 12.80 |
| 335 | 13.19 |
Common tire sizes in inches
Heights are calculated from the size with the formulas above. Real tires vary a little by brand, so check the maker’s spec sheet when the exact figure matters.
| Size | Width (in) | Sidewall (in) | Overall height (in) | Revs per mile |
|---|---|---|---|---|
| 205/55R16 | 8.07 | 4.44 | 24.88 | 811 |
| 225/65R17 | 8.86 | 5.76 | 28.52 | 707 |
| 235/65R17 | 9.25 | 6.01 | 29.03 | 695 |
| 265/70R17 | 10.43 | 7.30 | 31.61 | 638 |
| 275/65R18 | 10.83 | 7.04 | 32.07 | 629 |
| 285/75R16 | 11.22 | 8.42 | 32.83 | 614 |
| 285/70R17 | 11.22 | 7.85 | 32.71 | 617 |
| 305/70R16 | 12.01 | 8.41 | 32.81 | 615 |
| 315/70R17 | 12.40 | 8.68 | 34.36 | 587 |
| 305/55R20 | 12.01 | 6.60 | 33.21 | 607 |
Sizes written in inches, like 35×12.50R17
Some truck tires use a flotation size that is already in inches. In 35×12.50R17, the 35 is the overall height, 12.50 is the width and 17 is the rim, all in inches. The sidewall is the overall height minus the rim, divided by two: (35 − 17) ÷ 2 = 9 inches.
| Size | Overall height (in) | Width (in) | Width (mm) | Sidewall (in) |
|---|---|---|---|---|
| 31×10.50R15 | 31 | 10.5 | 266.7 | 8 |
| 33×12.50R15 | 33 | 12.5 | 317.5 | 9 |
| 35×12.50R17 | 35 | 12.5 | 317.5 | 9 |
| 37×12.50R17 | 37 | 12.5 | 317.5 | 10 |
Load index and speed rating
The last two parts of a size, such as 102H, show how much weight the tire can carry and how fast it is rated for. Never go below the load index or speed rating that your vehicle maker specifies.
| Load index | Max load (kg) | Max load (lb) |
|---|---|---|
| 85 | 515 | 1,135 |
| 91 | 615 | 1,356 |
| 95 | 690 | 1,521 |
| 100 | 800 | 1,764 |
| 102 | 850 | 1,874 |
| 105 | 925 | 2,039 |
| 110 | 1,060 | 2,337 |
| 115 | 1,215 | 2,679 |
| 120 | 1,400 | 3,086 |
| Speed rating | km/h | mph |
|---|---|---|
| Q | 160 | 99 |
| R | 170 | 106 |
| S | 180 | 112 |
| T | 190 | 118 |
| H | 210 | 130 |
| V | 240 | 149 |
| W | 270 | 168 |
| Y | 300 | 186 |
Questions people ask
What does 305 mean on a tire?
It is the tire width in millimetres. 305 mm is 12.01 inches.
How tall is a 305/70R16 tire?
About 32.81 inches, using the formula on this page. A 305/65R17 comes out near 32.6 inches and a 305/55R20 near 33.2 inches.
Is a 305 tire the same as a 33 inch tire?
No. 305 is the width, and 33 inches would be the height. Several 305 sizes land close to 33 inches tall, but the height always depends on the aspect ratio and rim size.
How do I convert tire size from mm to inches?
Divide the width by 25.4 for the width in inches. For the overall height, multiply the width by the aspect ratio to get the sidewall, divide by 25.4, double it and add the rim size. The calculator above does all of this for you.
What does the R in a tire size mean?
R means radial construction, which is how almost all modern car and truck tires are built.
Can I fit a different tire size than the one on my vehicle?
Often yes, within limits. Keep the overall height close to the original (within about 3% is a common rule of thumb), keep the load rating the same or higher, and make sure the tire clears the suspension and body. A tire shop can confirm what fits.
