Prime Rib Calculator
Plan a standing rib roast: how big to buy for your guests, about how long it takes at 325 °F, and the exact temperature to pull it from the oven for the doneness you like.
- Based on USDA food-safety data
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How much prime rib per person
- Bone-in: plan on about 1 pound per person, which works out to roughly one rib for every two guests.
- Boneless: about ½ to ¾ pound per person.
If you have hearty eaters or want leftovers, add a rib. A 3-rib roast (about 6 to 7 pounds) comfortably serves 6, and a 4-rib roast about 8.
Doneness temperatures
Large roasts keep cooking after they leave the oven. The internal temperature typically climbs another 5 to 10 °F while the roast rests, so pull it out before it reaches your target.
| Doneness | Pull from oven at | After resting | Looks like |
|---|---|---|---|
| Rare | 115–120 °F | about 125 °F | Cool red center |
| Medium-rare | 125–130 °F | about 135 °F | Warm red-pink center |
| Medium | 135–140 °F | about 145 °F | Pink center |
| Medium-well | 140–145 °F | about 150 °F | Slightly pink |
| Well done | 150–155 °F | 160 °F | No pink |
The USDA recommends cooking beef roasts to a minimum internal temperature of 145 °F, then resting them for at least 3 minutes. Rare and medium-rare are common preferences but fall below that recommendation. The restaurant-style doneness ranges above are included so you can make an informed choice.
How long to cook prime rib
The USDA's holiday roasting chart gives these approximate times in a 325 °F oven to reach 145 °F:
| Roast | Weight | Time at 325 °F |
|---|---|---|
| Rib roast, bone-in | 4–8 lb | 23–30 min per lb |
| Rib roast, boneless | 4 lb | 39–43 min per lb |
Source: USDA Food Safety and Inspection Service, "Roasting Those 'Other' Holiday Meats." Boneless roasts take longer per pound because heat has to travel through solid meat.
For rare or medium-rare, the roast gets there sooner. Many cooks start checking at around 15 minutes per pound. Roasts vary a lot with shape, starting temperature and oven, so a probe thermometer left in the thickest part is the only reliable guide.
Tips for a perfect rib roast
- Season early. Salting the roast a day ahead and leaving it uncovered in the fridge seasons it deeply and helps the crust brown.
- Bones down. Stand the roast on its rib bones, fat side up. The bones act as a natural rack.
- For a browner crust, finish with 10 minutes at 450–500 °F once the roast is at temperature, or start it hot and then lower the oven.
- Rest it loosely tented with foil for 20–30 minutes before carving, so the juices settle and carryover cooking finishes.
- Carve by cutting along the bones to remove them in one piece, then slice the roast across the grain.
Frequently asked questions
How big a prime rib for 10 people?
About a 10-pound bone-in roast (5 ribs), or 5 to 8 pounds boneless.
How long to cook a 6-pound prime rib?
At 325 °F the USDA estimates 23–30 minutes per pound to reach 145 °F, about 2¼ to 3 hours. For medium-rare, start checking around the 1½-hour mark.
Is prime rib the same as a standing rib roast?
Yes. "Prime rib" is the popular name for a beef rib roast, whether or not the beef is USDA Prime grade. "Standing" means it's roasted standing on its bones.