I've seen some videos where people recommend the "finger rule" (as opposed to measuring) when figuring out how much water to put in a pot to boil rice. However, the explanations are pretty vague. Can someone explain this?
This rule is a good approximation for "usual" quantities of rice, and I've used it successfully a lot of times in the past few months after hearing about it.
As Jeroen explained, put your rice in the container you want to cook it in (pan or rice cooker with maybe 2-4 litres of total volume). I'd recommend washing your rice at least once, discarding the starchy water. Then, evenly spread the rice and cover it up with water until it hits a level where your fingertip touches the rice and the water level goes to your first knuckle. Note: afaik this method assumes you steam your rice, limiting the loss of water.
Now, some caveats: this doesn't work too well with very small or very large quantities of rice because the relation between "rice level" and water level skews more towards rice the more rice you cook; or if your hands are abnormally small or large ;) It's good enough for most uses in everyday cooking though.
Put your rice in a pan. Spread out the rice to it is evenly spread out in the pan. Put the tip of your index finger on top of the rice and then add water until the water level is at the first knuckle of your finger.
Personally I don't use that technique, if you don't pay enough attention the rice might burn, but this is the idea. I usually just throw in too much water and get rid of the left-over water afterwards.
This process works because of the space between the grains, the specific gravity of rice etc... In general, it only really works for regular white rice. If you look at the ratios of water to rice by volume, it starts with brown rice at 2.5 water to 1 rice. White rice at 2 to 1, and highly polished rice like sushi and Vietnamese broken rice at 1.75 to one.
Round rice such as Calrose needs a little less water than Basmati or Jasmin etc.
So, your mileage will vary depending on the rice you want to cook, and the size of your fingers.
I have been eating rice mixed with barley, quinoa and wheat to balance my protein and carb intake, and the finger rule has gone out the door.