Hard-boiling an egg doesn't require boiling it for any length of time. However, it should sit in hot water for nine to 15 minutes after the water comes to a boil and is removed from the heat.
Place the egg in a large saucepan and cover it with 1 inch of cold water. Bring the water to a boil. Turn off the heat after the water comes to a boil.
The size of the egg determines how long it needs to sit in the hot water. Medium eggs require about 9 minutes of standing time to become fully cooked. Large eggs require about 12 minutes and extra-large eggs require about 15 minutes of standing time in the hot water.