Yes, 2/3 cup is exactly twice as much as 1/3 cup. If you picture a cup divided into three equal parts, 1/3 cup fills one of those parts while 2/3 cup fills two. In practical kitchen terms, 1/3 cup is about 79 milliliters and 2/3 cup is about 158 milliliters.
How the Two Measurements Relate
The simplest way to think about it: 2/3 cup is just two 1/3 cups combined. That means 2/3 cup contains double the volume of 1/3 cup. In tablespoons, 1/3 cup equals 5 tablespoons plus 1 teaspoon, while 2/3 cup equals 10 tablespoons plus 2 teaspoons. In fluid ounces, 1/3 cup is roughly 2.67 ounces and 2/3 cup is roughly 5.33 ounces.
This relationship is useful when you’re adjusting recipes. If a recipe calls for 2/3 cup of something and you want to cut it in half, you need exactly 1/3 cup. Going the other direction, doubling 1/3 cup gives you 2/3 cup.
How to Measure Without a 2/3 Cup
Not every measuring cup set includes a dedicated 2/3 cup. If yours doesn’t, fill a 1/3 cup twice. This gives you exactly 2/3 cup with no math required.
For dry ingredients like flour or sugar, spoon the ingredient into the measuring cup until it overflows, then level off the top with the flat edge of a butter knife. This prevents packing, which can add significantly more than the intended amount. For coarser ingredients like chocolate chips or chopped nuts, give the cup a shake so everything settles evenly before you level it off.
Why Precision Matters in Cooking
Confusing 1/3 cup for 2/3 cup (or vice versa) means you’re either doubling or halving the intended amount of an ingredient. For something like broth in a soup, that might not ruin the dish. For flour in a cake, it almost certainly will. Baking is especially unforgiving because the ratio of dry to wet ingredients controls the texture and structure of the final product.
It also matters which type of measuring cup you use. Dry measuring cups are designed to be filled and leveled off, while liquid measuring cups have a pour spout and markings on the side. Using a liquid measuring cup for flour makes it nearly impossible to get an accurate reading, since you can’t level the surface properly. For liquids, a dry measuring cup forces you to fill it to the very brim, which leads to spills and inaccurate amounts. Matching the right cup to the right ingredient keeps your 1/3 and 2/3 measurements reliable.