The following cooking chart is only to be used as a guide for estimating cooking time. Your most important tool for knowing when your prime rib is done to perfection is with a good quality meat thermometer. The time to start monitoring the internal temperature of your prime rib is at least 45 minutes before you expect it to be done.
You must insert the thermometer each time you check the temperature because instant read thermometers cannot remain in the meat while cooking. Also, make sure your thermometer probe is not touching bone or resting in fat. It should be in the thickest part of meat.
Instant Read Thermometer
The Thermapen – Instant Read Thermometer pictured on the left is the thermometer most used for cooking, baking and grilling meat. According to reviews, it is the best and most accurate available. It was originally designed for professionals, and now chefs from all over the world use it. To learn more about it and get one for yourself (if you want), click on the picture or the link above.
What constitutes rare and medium rare meat?
According to the Wyoming Beef Council, a prime rib roast will be medium rare when it reaches an internal temperature of 135 degrees F and medium when it reaches 150 degrees F. They say to remove the roast from the oven 5 to 10 degrees before it reaches the desired temperature. So, according to them you would take it out of the oven at 125 degrees F for a medium rare roast.
However, meat lovers who like “rosy meat” say that rare begins at 120 degrees F and becomes medium rare at around 125 to 130 degrees F. The chart below depicts more of what rare meat lovers agree with, so if you want your prime rib to be rare, according to the chart you would take it out of the oven at 110 degrees F because it will continue to rise in temperature about 10 more degrees during it’s 20 to 30 minute resting period.
Estimated Cooking Time Chart
The oven temperatures in this chart depict 450 deg for the first 15 minutes and 325 deg for the remainder of cooking time.
Personal Note: Most of my family like their meat well done and this past Christmas my Dad cooked a full 7 bone prime rib roast that reached an internal temperature of 145 degrees F before removing it from the oven. There was no pink at all throughout the entire roast and it was still juicy and delicious.
Check Your Oven Temperature
Many ovens do not get to the exact temperature that you read on the dial. In fact, it is not unusual for an oven to be off by as much as 25 to 50 degrees F. That much of a discrepancy can seriously affect the time needed for baking your prime rib to perfection. It is always a good idea to check your oven temp before cooking something as expensive as a prime rib roast. To do that you will need to get an oven thermometer and place it on the rack in the center of the oven. Heat the oven for at least 15 minutes at 350 degrees F. If the oven temperature reads differently than 350 degrees F then you will need to adjust your dial until you get it right.
The Doneness Chart
This next chart shows the final temperatures for getting the meat to your desired doneness. As mentioned above, the meat should be removed from the oven 10 degrees before reaching the temperature shown in the chart.
Baking At High Altitudes
The time it takes to cook your prime rib may vary slightly at higher altitudes but here again, the time is not the big concern in getting perfect meat. The only way to get your roast perfect is with a thermometer. Using the “Estimated Cooking Time Chart” as a guide for cooking time will work at any altitude because you are going to start checking the temperature within 45 minutes to an hour before you expect it to be done. If your oven’s temperature is correct then the chart above will be very close no matter what altitude you are at.
According to O-Chef the biggest concern when cooking at high altitudes are with baked goods that have leavening to make them rise and with things that are cooked in water on top of the stove such as rice and pasta, because water boils at 212 degrees F at sea level and at 203 degrees F at 5000 feet.