While there are many ways to measure a compound bow string, there is one tried and true method that just simply works every time.
To begin with you will need to remove the bow string from the bow itself, or if you have purchased a new one then you are already set to go.
This is one of the most accurate ways to measure the string providing that you make sure the string is to the exact mm on the tape measure as even 1mm out can mean you will order the wrong size and it may not fit properly.
Other Steps To Measuring
If you’re wanting to measure the string while attached to the bow all I can say is that its no a good idea. trying to measure it while its still attached to the compound bow is not only incredibly difficult, you will never get the right measurements, so its best to avoid doing it that way at all costs.
If you know what type of string you want, a lot of the time you can simply Google search the string brand and type and a lot of the retailers that sell the string will have the exact length measurements listed on their website.
Just ensure that thy type of string you are going to buy is suitable for your type of compound bow frame, as some string that are thinner may break on a bow that has a higher, more tense bow draw, so you will want to choose the right one that fits your bow.