Measuring for Pump Tube

Does anyone know why when measuring for a pump tube you can’t use a tailors measuring tape and instead need to use a string?

Here is text from one manufacture:

NOTE: Be sure to use string to measure circumference. A tailors tape or flexible ruler will not provide accurate measurements. Standard cylinder length is 9 inches. Please measure accurately as cylinders cannot be returned or exchanged for health reasons.

Thanks

Just use your tailors tape and let me know your erect base girth and length and I will help you out on the correct cylinder. The manufacturers tend to recommend too close to your actual size.

I am 5” at the base and 6” in length. Thanks for the help!

You should order a 9” X 1.75”.