Spinach is one of the more delicate culinary greens, suitable for eating raw even when fully mature. It can be used cooked or uncooked in a wide range of recipes from the world's major cuisines, from a French omelet to an Indian curry. Like other greens, spinach tends to shrink dramatically when it is cooked. This can make it difficult to estimate how much to buy for a given recipe. The crucial detail is how the spinach is to be measured.
Follow the recipe. Some recipes call for raw spinach in cups or pounds, others call for cooked spinach in cups. Use the appropriate measurement.
Strip the stems from mature spinach, and measure it in volume by packing the leaves tightly into a measuring cup. It is not usually necessary to remove the stems from baby spinach.
Purchase spinach in a bag of known weight, then estimate the portions by eye as you remove them from the bag. Alternatively, weigh the spinach on a kitchen scale for accuracy and better portion control.
Measure cooked spinach or thawed frozen spinach by squeezing out any excess water, then tightly packing the spinach into a measuring cup. If the recipe calls for a given weight of cooked spinach, follow the same procedure and then weigh it on your kitchen scale.
Use the rule of thumb that one pound of raw spinach equals one cup of cooked, which in turn equals two portions as a side dish. If you are working without a recipe or using fresh spinach to generate a given volume of cooked spinach for a recipe, this will usually be suitable.