Source: I got it from Cindy5 NY, who got it from whatscookingamerica.net
adapted by http://theresalwaysthyme.blogspot.com/
For a generous serving of standing rib, figure on two people per rib. That means if you plan to serve six you should be able to do so with a three rib roast; eight people, four ribs. Don't even bother with less than a three-rib roast, any less than that is not a roast but rather a thick steak and would be better treated as such.
NOTE: This chart is only a guide. You must rely on an accurate meat thermometer and start taking temperatures half an hour before the end of the estimated roast time.
Rib Count Approximate Weight Oven Temperature Total Estimated Time Meat Thermometer Reading (Rare)
2 ribs 4 to 5 pounds 450°/325° F 60 to 70 minutes 120° F
3 ribs 7 to 8.5 pounds 450°/325° F 1 1/2 to 1 3/4 hours 120° F
4 ribs 9 to 10.5 pounds 450°/325° F 1 3/4 to 2 1/4 hours 120° F
5 ribs 11 to 13.5 pounds 450°/325° F 2 1/4 to 2 3/4 hours 120° F
6 ribs 14 to 16 pounds 450°/325° F 3 to 3 1/4 hours 120° F
7 ribs 16 to 18.5 pounds 450°/325° F 3 1/4 to 4 hours 120° F
Bring Prime Rib Roast to room temperature (very important). If your rib roast is frozen, let it thaw completely in the refrigerator. Remove the roast from the refrigerator 2 1/2 to 4 hours before cooking, the longer time for the largest roast (if you don't let come to room temperature, if will take longer to cook your roast). Pat the rib roast dry with a towel or napkin.
Preheat oven to 450°F. Smear the cut ends of the roast with softened butter (I usually don't bother - Cindy's note). Rub with or insert garlic all over. Place the roast (ribs down) on the rack in the roasting pan.
Sear the rib roast for 15 minutes at the higher oven temperature (450°F), then turn the oven to the lower temperature (325° F) for the rest of the cooking time. Every 1/2 hour, baste the cut ends of the roast with the fat accumulated in the roast pan.
About 1/2 hour before the estimated end of the roasting time, begin checking the internal temperature (use a good digital thermometer). Cook until rib roast reaches an internal temperature of 120°F. Remove from oven, cover with aluminum foil, and let sit approximately 20 to 30 minutes. Remember, the roast will continue to cook as it sets. The temperature will rise to 125 to 130° F internal temperature (medium rare).