A ruler
i would use inches
To measure crank arm length accurately, use a ruler or measuring tape to measure the distance from the center of the bottom bracket to the center of the pedal spindle. This measurement will give you the crank arm length.
I would use feet
You would use meters.
You would answer it with cubic feet
To measure your arm length, stand up straight with your arms relaxed at your sides. Use a measuring tape and extend it from the top of your shoulder (where it meets your neck) down to your wrist bone. Make sure your arm is straight when measuring for accuracy. Alternatively, you can measure from your shoulder to your fingertips for a longer measurement.
meters
You would use either inches or centimeters.
I would use centimetres and millimetres
centimeter
The best method to accurately measure crank arm length is to use a ruler or measuring tape to measure the distance from the center of the bottom bracket to the center of the pedal spindle. This measurement should be taken in millimeters for precision.
The metric unit of measurement that would best be used to measure the length of your arm is centimeters. This unit provides a more precise measurement compared to using meters or kilometers for such a small length.