Go like this and choose the number of fingers that fit between your muscle and elbow
With your arm bent, put your fingers on the inner part of your elbow and feel for that cable-like tendon. It’s what attaches your upper arm muscle to the bone of your forearm. This tendon is what we’re trying to measure.
Now flex your arm like a body builder, squeezing your muscle hard. Keep your fingers on that cable-like tendon and start moving your fingers just a little bit up the arm. Feel for where that tendon stops and the bicep muscle starts. It’s usually just a few centimeters away from the elbow’s bend.
So making sure that you’re flexing hard while your arm is up at a 90 degree angle (like the corner of a box), see how many fingers (if any at all) can fit easily in that space along the tendon between the elbow’s bend and the beginning of your bicep muscle.
If you have an extra layer of fat in the arm, feel through the soft tissue to where the hard muscle is underneath (Yes! We all have bicep muscles!)
Don’t squish as many fingers in as possible but measure what fits easily.
Your left and right arm should be similar, so if one side is hard to figure out, try the other.
This doesn’t measure how strong you are or how much you work out. It just measures the length of that cable-like tendon.