There is a thing called a tape measure, it is a handy tool where a thin strip of metal is rolled inside a metal canister, there are numbers printed on the strip that correspond with inches and feet. Some even have metric measurement. It should have a small metal tab to hold it against the starting side of what you need to measure, then you just pull the canister and the tape unrolls. Start at one outer side of your tire and pull it to the other side then read the measurement this is done by looking at the number on the strip. Hopefully this will help you. I thought about doing a write-up but this response should be good enough for most.
Hopefully you know this is a J/k I:I