To measure chest size accurately, use a flexible tape measure and follow these steps:

1. Stand up straight with arms relaxed at your sides, wearing a light or no shirt.
2. Wrap the tape measure horizontally around the fullest part of your chest, usually at nipple height, making sure the tape is parallel to the ground.
3. The tape should be snug but not tight; there should be enough room to slide a finger between the tape and your body.
4. Record the measurement where the tape meets.
5. For accuracy, take the measurement a couple of times and average the results.

This method ensures a comfortable and precise chest measurement useful for clothing or fitness tracking.