Tyre size designations can be found on the tyre's sidewall and usually look similar to these:

European Metric Tyre Size Designation

(Valid in Europe, standard ETRTO – European Tyre and Rim Technical Organisation)

European Metric Tyre Size Designation

Let's see what each symbol means.

195 mm
Section Width. Nominal section width of the tyre in millimeters.
65
Aspect Ratio. The sidewall height as a percentage of the section width. For example, sidewall height of this tyre will be 195 x 0.65 = 126.75 mm
R
Radial construction. Denotes radial construction which is dominant for passanger tyres.
15
Rim diameter in inches to which the tyre fits.
91
Load Index. Load capacity of the tyre which is the maximum weight the tyre can carry. You may lookup the value in the load index table.
T
Speed Index or Speed Rating. Maximum speed at full load. You may lookup the value in the tire speed rating table.

Tyre dimension may be prefixed by a letter which denotes tyre's intended usage. Here is what each means:

P
Passenger tyre.
LT
Light truck tyre.
T
Temporary (spare) tyre

There are more markings on each tyre, but these are the most important.

Calculate the tyre dimensions (diameter, circumference, revolutuons per mile) from the tyre size designation on a tyre, for any tyre size (in mm and inches).


P-Metric Tire Size Designation

(Valid in North America, standard TRA – Tire and Rim Association)

P-Metric Tire Size Designation

Let's see what each symbol means.

P
P-metric (passenger car tire).
195 mm
Section width in mm.
65
Aspect ratio in percents.
R
Radial construction.
15
Rim diameter in inches.
89
Load index.
H
Speed rating.

High Flotation Tire Size Designation

(Valid in North America, standard TRA – Tire and Rim Association)

High Flotation Tire Size Designation

If a tire size ends with LT, it signifies that a tire is Light Truck and that is designed to be used on the light truck vehicles – vans, SUV's and light duty pickup trucks (LT-High Flotation, Wide Base or LT-Numeric). The example tire size designation refers to the LT-High Flotation tire.

Let's see what each symbol means.

31
Overall tire diameter in inches.
10.50
Section width in inches.
R
Radial construction.
15
Rim diameter in inches.
LT
Light truck tire design.
D
Load range.

Learn more about the tire designations:

Light truck tire size designation and Service description.

Light Truck Tire Designations - Examples.

Motorcycle Tyre Size Designations.

Truck and Bus Tyre Size Designations.