Rice is one of the most versatile and inexpensive staple foods in the world, which is a good reason to build a meal around it any time you're cooking for a group. That raises a difficulty in how recipes are written. You calculate your servings based on a given quantity of cooked rice, but uncooked rice is what you start with. Fortunately, you can make that conversion with an easy rule of thumb.
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Dry vs. Cooked Rice
Most of the volume of a grain of rice is made up of tightly packed granules of starch. When you cook the rice those granules absorb the cooking liquid, swelling up and increasing dramatically in size. Most varieties of rice absorb roughly twice their volume of water, so at the end of your cooking time they'll have tripled in volume. That means for every cup of uncooked rice you started with, you'll have approximately three cups of cooked rice. There is some variation, depending on the type of rice you've chosen.
Rice Selection and Volume
Broadly speaking, there are two types of rice. There are long-grain rices, including Thai scented rice and Indian basmati, which have a relatively cylindrical kernel. Short- and medium-grain rices are starchier and have a slightly oval shape, like tiny footballs. Because of their starchiness, shorter-grained rices cook up to a sticky consistency and hold together readily. This makes them compact when they're cooked, and closer to the three-to-one ratio. Long-grain rices are usually cooked in ways that make them light and fluffy, and can have a higher volume as a result.
Part of the appeal of basmati and other long-grain rices is that they have a light, chewy consistency as opposed to the softer texture of short-grain rices. To emphasize this, they're usually cooked in less water than other rices. When the rice is fully cooked, remove the lid and let steam escape for at least five minutes while the rice cools and then firms slightly. When you can lift it with a fork without the grains breaking, fluff it gently with the fork or turn out your rice into a bowl and toss it until the grains are loose and separate. This can increase your volume to as much as four cups of cooked rice for each cup of uncooked.
The other side of your recipe calculation is determining how much rice you'll actually need to serve. That depends largely on the type of meal you're planning. If rice is going to be one side dish among many, the usual allowance is a half-cup of rice per diner. If you're planning to serve a dish centered around rice, such as a stir-fry, it's prudent to allow up to one cup per diner. For a rice pilaf or similar dish, stretching the rice with vegetables or other ingredients, assume one cup of rice per person but serve at least 1 1/2 cups of the finished pilaf.