How to read the chart
Each row is a section width in millimetres, the first number of a metric size. Each column is the aspect ratio, the sidewall height as a percentage of the width. The cell where they meet is the overall diameter of that tire on the rim you picked, in inches (or millimetres if you switch to metric). A 285/70R17 sits in row 285, column 70 of the 17 in chart.
The target diameter box highlights every size within 0.5 in of the height you want. That is the quickest way to answer questions like "which sizes are 33 inches on a 17 inch rim". On phones, the list view shows the same sizes sorted by how close they come.
The formula behind every cell
The chart is maths, not a catalogue. Every cell uses the same formula:
diameter (in) = rim (in) + 2 × width (mm) × aspect ÷ 100 ÷ 25.4Worked example for 285/70R17: the sidewall is 285 × 0.70 = 199.5 mm, which is 199.5 ÷ 25.4 = 7.85 in. There are two sidewalls, one above and one below the rim, so the diameter is 17 + 2 × 7.85 = 32.71 in, or 830.8 mm.
Why a size in the chart may not exist
The chart fills in every width from 125 to 405 mm and every aspect ratio from 25 to 85, so you will see combinations no one makes, like a 405/85 on a 14 in rim. Use it to find the height you want, then check that the size is sold in the tire type you need (passenger, LT, all-terrain, mud-terrain).
Nominal size vs measured size
The diameter in the chart is the nominal size from the code. The tire you buy can measure a little different: tread depth, construction and the width of the rim it is mounted on all change the real figure. Tire makers publish a measured overall diameter for each size and model, and that is the number to use for tight clearances.
Before you change size
A different diameter changes your speedometer reading, your gearing and your ground clearance. Whether a size fits your vehicle depends on your wheels, suspension and body: check clearance at full steering lock and full suspension compression. The recommended size and load rating for your vehicle are on the door jamb placard and in the owner's manual.
Questions
What size tire is 33 inches on a 17 inch rim?
By the formula, 255/80R17 (33.06 in), 315/65R17 (33.12 in), 285/70R17 (32.71 in) and 295/70R17 (33.26 in) are all close to 33 inches. Set the rim to 17 and the target to 33 to see every match, then check which of them are actually made.
Is 275/70R18 a 33 inch tire?
Nearly. The sidewall is 275 × 0.70 = 192.5 mm (7.58 in), so the diameter is 18 + 2 × 7.58 = 33.16 in.
Why does the chart differ from the tire maker's spec sheet?
The chart uses the nominal size code. Makers publish a measured diameter for each model, which can differ by a few tenths of an inch because of tread depth, construction and rim width.
How do I convert the chart to millimetres?
Switch the units to metric at the top of the page. Every cell is then shown in millimetres: 285/70R17 becomes 831 mm.