To measure men's chest size accurately, follow these steps:

1. Use a flexible, soft measuring tape for accuracy.
2. Stand up straight with arms relaxed at the sides.
3. Wrap the tape measure around the fullest part of the chest, usually at nipple level, across the shoulder blades and under the armpits.
4. Ensure the tape is horizontal, parallel to the ground, snug but not too tight—comfortable enough to breathe naturally and allow a finger between the tape and skin.
5. Record the measurement where the tape meets its end.
6. Take a couple of measurements and average them for best accuracy.

This method gives the chest circumference, commonly used for sizing shirts, jackets, and suits.