Luckily, personal trainer Max Weber, ACE, NASM, shared a helpful tip on his Instagram for how to eyeball your portion sizes for each meal — all you need is your hand.

A serving size is not meant to tell you how much food to eat. It lets you calculate how many calories and nutrients you are getting based on the amount you eat.

To do it accurately, you have to precisely calculate serving sizes, which sometimes requires a food scale or other measuring tools.