If you cook, bake, brew, anything with food and drink.

Common volume conversions: 3 teaspoons in a tablespoon, 4 tablespoons/2 fluid ounces in a quarter cup, 8 fluid ounces in a cup, 2 cups in a pint, 2 pints in a quart, 4 quarts in a gallon.

Common weight conversions: 28 grams in an ounce, 16 ounces in a pound, 2 pounds and 3(ish) ounces in a kilo.

And common volume to weight conversions based on ingredients you use. For me: 200 grams in a cup of sugar, 125 grams in a cup of AP flour, 6 grams in a tablespoon of cocoa powder.

Makes it a lot easier to halve/double recipes, or use a scale for a volume written recipe.