Prime rib serves a large group because it's a big roast. When cooking prime rib, the amount of time needed is based on both the size of the roast and the temperature you cook it to. Some enjoy their prime rib rare, leaving the centre relatively cool to the touch. Medium rare has a warmer temperature and is not as red in the centre as rare prime rib.
Determine the level of doneness that you would like to cook your prime rib to. This level will help you determine the temperature the amount of time you need to cook your prime rib.
Weigh the prime rib that you are preparing to cook. The store may have a weight listed on the package, but to ensure proper cooking you should verify this with a meat scale at home.
Multiply the weight of your prime rib by the number of minutes per pound necessary to cook your prime rib to the level of doneness you require. For rare prime rib you should cook it for 12 or 13 minutes per pound. Medium rare takes between 14 and 16 minutes per pound. Medium requires anywhere from 17 to 20 minutes per pound of cooking time.
Allow your prime rib to rest for 15 minutes after cooking, before carving if your prime rib is between 0.907 and 2.72kg. For a prime rib over 6lbs., allow approximately a half hour of resting time.
Calculate the entire cooking time for your prime rib by adding the amount of time in Steps 3 and 4.
Considering the amount of cooking time is an estimate based on the weight of the roast and the level of doneness you would like it cooked to, check the internal temperature with a meat thermometer. It should be at least 120 degrees for rare and between 125 and 130 degrees for medium rare.
Do not slice your prime rib without letting it rest. Slicing too early will let the juices flow out of the meat and the prime rib will be dry.