Tread width is an often misinterpreted measurement. If you look at the sidewall of your tire, you will find a set of large numbers that look like "225/45R16." The first number is your tire width measured in millimetres, and for sizing purposes, that's all you need for purchasing an appropriate size tire. However, people may incorrectly assume this is also the tread width. Tread width can vary widely among manufacturers and models, even when the tire width is the same. Knowing the actual tread width can be forensically useful when matching tracks with tires or when purchasing very specific tire chains.
Look up the tire specifications at the manufacturer's website or in a manual. Most tires already have the tread width specified, so measurement may not be necessary. If you cannot locate the measurement, then you must measure it directly.
Measure the tread width with a tape measure laid across the width of the tire. Measure from the outer edge of the tread on one side to the outer edge on the other side.
Convert the measurement, if necessary, to allow convenient comparisons with prior forensic track measurements or tire chain requirements. Convert inches to millimetres by multiplying the measurement by 25.4. Convert millimetres to inches by dividing by 25.4.