Methods for cooking a common staple are less commonly shared than you might think.
Billions of people around the world eat rice every day—and with all those people come an almost equal number of rice-cooking strategies. Most methods involve heating rice and water in a pot until it boils, then cooking them at low heat; the confusion comes from deciding on the right amount of water for a given amount of rice. Plenty of people use fixed ratios of water to rice. But then there’s the “knuckle method” popular in East Asia, which doesn’t use a fixed ratio (or measuring cups!) and instead uses a tiny bit of the human body to ensure perfectly hydrated rice—regardless of how much you’re making. How does it work and why does it make great rice at any quantity? Let’s find out.
First, let’s save the discussion of rice varieties for another time and assume we’re talking about a medium-grain, starchy rice, such as jasmine. To use the knuckle method, rinse the rice thoroughly with water, then keep adding water until it covers the rice to the depth of the knuckle closest to your fingertip (AKA the distal interphalangeal joint). You can measure the right depth by gently placing the tip of your pointer finger on top of the rice and measuring from there (see photo below).
In the knuckle method, you add one first-knuckle’s worth of water regardless of the amount of rice being cooked. Now, you probably have two questions: 1) how can this work, since people have different hands? and 2) shouldn’t the amount of water you’re using be dictated by how much rice you’re cooking? First, let’s look at finger variation. People have all different hand sizes, but it turns out that differences in the distance from the fingertip to the first knuckle are small. You can see in the photo below that even in two people with differently-sized hands, the length of the first segments of their pointer fingers are just about the same.
Two differently-sized hands have a similar-length first segment of their pointer fingers.
Don’t take our word for it. Compare your hands with folks around you (ask them first), and see for yourself. Not everyone will be absolutely the same—there’s a certain amount of human variation—but by and large, we’ve found that people are typically within 10% of each other.
So the real question is, how can the knuckle method work for any amount of rice? The answer is all about two things that happen to the water you add to the rice pot: absorption and evaporation. The water that’s absorbed is the water that enters each rice grain and helps cook its starches. Because each rice grain needs about the same amount of water, if you double the amount of rice, then you need to double the amount of water that will be absorbed. The water that’s evaporated escapes in the form of steam as the rice cooks. The exact amount of loss through evaporation depends on the surface area of the pot, the tightness of the lid, and the total cooking time. These variables are all independent of the quantity of rice and instead depend on your cooking equipment. If you use the same pot, the total amount of that evaporates will be about the same, regardless of how much rice you’re cooking. Therefore, this amount should not change when you cook a double batch of rice. So why does the knuckle method work? Assume you’re using your favorite rice-cooking pot. The volume of water between the top of the rice and your first knuckle is always the same. This is roughly the amount that evaporates during cooking if your pot has a diameter around 8 inches (20 cm). Water also fills the spaces between the rice grains, and this amount increases linearly with the quantity of rice as shown in the graph below.
Now consider using a fixed ratio of water to rice, such as 2 parts water to 1 part rice. Doubling the water would end up doubling the amount for both absorption and evaporation. The absorption part is fine—the rice needs it to cook—but the extra water will not evaporate and will instead be absorbed, leaving you with a gummy mess. You can see in the graph above that for larger amounts of rice, the ratio method uses far more water than the knuckle method. So the fixed water-to-rice ratio that works for the size of your family will not scale correctly when you’re cooking for a crowd. Your finger segment is not only handy; it also stays comfortably the same.