Prime Rib Size & Time Calculator
Quick answer: Plan one rib for every two guests — about 1 lb of bone-in prime rib per person — and roast at 325°F for about 15 minutes per pound for medium-rare, pulling at 125°F and resting 20–30 minutes.
Prime rib is the most expensive mistake in the meat case, so size it right. Enter your guest count and doneness, and we'll give you the roast weight, the rib count, the time at 325°F, and the temperature to pull it.
Prime rib size by guest count
| Guests | Bone-in roast | Ribs |
|---|
| 4 people | 4 lb | 2 ribs |
| 6 people | 6 lb | 3 ribs |
| 8 people | 8 lb | 4 ribs |
| 10 people | 10 lb | 5 ribs |
| 12 people | 12 lb | 6 ribs |
How it works
Sizing starts from the rib: a whole standing rib roast has seven, each section weighing about 2 pounds and feeding two people — hence the butcher's rule of one rib per couple. From your headcount the tool works out the roast weight and rib count, then applies minutes-per-pound at 325°F for your doneness (13 rare, 15 medium-rare, 17 medium). The clock is only a forecast, though: pull the roast by thermometer — 120/125/135°F for rare/medium-rare/medium — and rest it 20–30 minutes, during which it coasts up another 5–10 degrees.
Frequently asked
How much prime rib per person?
Plan about 1 lb of bone-in roast per person — which works out to one rib for every two guests, since each rib section weighs about 2 lb. Boneless, plan ¾ lb per person.
How long does prime rib take to cook?
At 325°F, roughly 13 minutes per pound for rare, 15 for medium-rare and 17 for medium. A 3-rib (6 lb) roast runs about 1½ hours for medium-rare — but always go by the thermometer, not the clock.
What temperature do I pull prime rib?
Pull at 120°F for rare, 125°F for medium-rare, 135°F for medium — the temperature climbs another 5–10 degrees while it rests. Rest at least 20–30 minutes before carving.
Bone-in or boneless prime rib?
Bone-in looks the part and the bones insulate the meat during the roast; boneless is easier to carve and there's no bone weight to buy. Cooking behaves the same — go by the thermometer either way.
Should I sear prime rib first?
A hot start (15 minutes at 450°F, then down to 325°F) builds a better crust, or you can reverse-sear at the end. Either way, the minutes-per-pound here cover the 325°F portion — the thermometer is still the finish line.
Prime rib vs ribeye — what's the difference?
Same muscle. Prime rib is the whole roast cooked slowly and carved; ribeyes are the same section cut into steaks and cooked hot and fast.
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