Rule of thumb for standing rib roast/prime rib (still on the bone) is 2-3 people per rib. Easiest way to calculate size needed and has always worked for me all the years of cooking and catering I have done.
for a boneless prime rib pierino is right on, about a 1/2 lb per person but if you have big eaters you might want to up that by a pound or two, to say 14 lbs. If you are doing bone in obviously you will need to adjust probably to 16 to 18 lbs. because you will have lots of fat with the bone.
Considering the bone weight and hefty fat cap, I would consider doing TWO 8 pounders. Due to the mass in the oven, use a thermometer to make sure it comes out perfect. Remember it still is cooking while you rest it before carving. Pull out 5F before your desired doneness.
I usually think 1/2 pound per person so something in the 10 to 12 pound range should work. But it depends on what your butcher has to offer. Weight will affect the cooking time so be prepared for that.
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Due to the mass in the oven, use a thermometer to make sure it comes out perfect.
Remember it still is cooking while you rest it before carving. Pull out 5F before your desired doneness.