One-quarter cup of dry lentils makes about one-half to five-eighths cup cooked lentils. As a general rule of thumb, dry lentils approximately double in volume when cooked.
According to Peggy Trowbridge Filippone for About.com, the best way to cook lentils is to cover them in water or broth and boil for two to three minutes. The next step is to reduce the heat and simmer until the lentils are tender. The cooking time may take from 10 minutes to an hour, depending on the variety and age of the lentils. Older lentils take longer to cook. Filippone notes that lentils should be liberally seasoned, but cooks should not add salt until the lentils are completely cooked. Salt toughens the lentils when added during cooking.