To boil lobsters humanely, first freeze the lobster.
The number of lobsters to be boiled determines the minimum kettle size. The water should fill the pot one-half to not more than two-thirds full. Bring water to a rolling boil over high heat. Place lobsters, head first, into the pot, completely submerging them.
Cover the pot tightly and return to a boil as quickly as possible. When water boils, begin counting the time. Regulate the heat to prevent water from boiling over, but be sure to keep the liquid boiling throughout the cooking time.
Boil a lobster for 10 minutes per pound, for the first pound. Add 3 minutes per pound for each additional pound thereafter. For example, a 2-pound lobster should boil for 13 minutes and a 1 1/2 pound lobster should boil for 11 1/2 minutes.
These times are for hard shell lobsters; if cooking new shell lobsters, reduce boiling or steaming time by 3 minutes. When the antennae pull out easily, the lobsters are done.
Source: Maine Lobster Promotion Council