How to Calculate Cooking Time for Prime Rib

Roast beef always makes a meal feel a bit more special than usual, but some roasts are more special than others. A lean eye of round makes a good roast for sandwiches, and a chuck roast is perfect for comfort-food pot roasts, but the queen of them all is the prime rib. It's one of the costliest cuts of beef, so if you're going to cook it, you want to do it right.

First, Let's Define Prime Rib

There's a lot of debate over what prime rib actually is and how it's best cooked. One thing most agree on is that prime rib is a really superior cut of beef for roasting. It's an extension of the loin muscle that runs through the rib section of a beef carcass, specifically from the sixth rib to the 12th. This makes perfect sense: A prime rib combines loin-like tenderness with the fatty richness that makes ribs so tasty. A good prime rib is well-marbled with fat through the muscle itself as well as a few larger seams of fat between the muscles.

The name prime rib just means it's the best of the rib cuts. "Prime" in this sense has nothing to do with beef grading, and you'll seldom see a prime rib at your butcher that's actually graded USDA Prime. They do exist, and you can order one if you really want to splash out some cash, but usually they only go to restaurants. Choice is still plenty good for most home cooks' purposes, and you can even find a passable prime rib that's graded Select if you're prepared to dig through the meat counter and find the best-marbled one.

Should You Use a Roast Cooking Calculator?

It's not hard to find a prime rib cooking time-per-pound chart if you look around online, or you can even find an app or online calculator that lets you enter the weight of your roast that tells you how long to cook it. Unfortunately, those aren't especially useful. Your real-world cooking time will depend on whether the roast is boneless or bone-in, the shape and size of the roast, the temperature and cooking method you choose, and the degree of doneness you're looking for.

If you're driving to an unfamiliar place, you'd probably rather have detailed guidance from a GPS than a stranger waving a vague arm, saying "it's that-a-way." Just like directions from a local, time and temperature charts are still helpful and they get you to the right neighborhood. However, you need more detailed help, depending on your cooking method – you need different directions in different neighborhoods, right? So choose your roast and cooking method first, and then work out your times from there.

Choosing Your Roast

Prime rib roasts are often measured by the number of rib bones you get in your cut. The full roast, usually called a "standing rib roast" when it's sold that way, has seven ribs. Usually, they're cut into smaller, more practical sizes for retail. A two-rib roast will weigh 4 to 6 pounds, while a four- or five-rib roast can weigh 8 to 10 pounds. As a rule, you can allow two people per rib, which works out to one serving for every 1 to 1 1/2 pounds of uncooked roast.

You'll lose some of that weight in cooking and some of it is inedible bone, so that's not as much as it might seem. You can scale that number up or down, depending on who you're feeding. If it's a roomful of light-eating elderly relatives, you might need less. If it's a gang of your bachelor college friends, you may need to increase the portion size substantially.

If you'd rather go boneless, the roasts automatically get smaller. A 3- to 4-pound boneless prime rib roast, sometimes called a rib-eye roast, is the same amount of beef you'd get with a 4- to 6-pound two-rib roast. At 6 to 8 pounds, you're getting the equivalent of a bone-in roast that weighs 8 to 10 pounds. Search online to find a prime rib calculator that'll work out the weight of beef you need to serve your guests.

Preparing Your Roast

Roasts don't come from the steer in perfectly uniform sizes and shapes. They are bigger and smaller, flatter or more cylindrical each time you buy one. The more evenly cylindrical you can make your roast, the more evenly it will cook, so tie it with butcher's twine to keep the shape regular. If you're doing a boneless roast, cut off the bones first and tie up the roast back to the bones for roasting. If you aren't confident in your skills, your butcher can do this for you.

Prime rib is as flavorful as beef gets, so you don't need to go crazy with seasonings. It helps to rub the surface generously with salt and leave it overnight so the salt can penetrate, but even an hour or two is helpful. If you choose, use other seasonings, such as black pepper, garlic and rosemary. Include liquid flavor enhancers such as Worcestershire or soy sauce if you like.

Some recipes encourage you to leave your roast out for a couple of hours so it comes to room temperature. However, this doesn't really happen, because even after a couple of hours, your roast won't be anywhere close to room temperature on the inside. Other recipes go the opposite route and call for chilling your roast in the freezer for a little while before roasting. The outer area is the part that starts to freeze, according to this logic, and it's also the part that's prone to overcooking. If you chill it first, you reduce the band of gray, well-one beef around the outer edges of your roast. That actually works, but it's up to you to take the extra step.

Time and Temperature for Conventional Roasting

Most of us first learn to roast beef by heating the oven to 350 degrees Fahrenheit, because that's a good general-purpose temperature that works well. If you're using this traditional method, allow roughly 30 minutes per pound for medium-rare, and about another 15 to 20 minutes for any degree of doneness past that. To take a 4-pound roast to medium-rare, then you'd allow 2 hours. Bone-in roasts, or roasts with a thicker-than-usual layer of fat at the surface, may take a few minutes longer.

Time and Temperature for Slow-Roasting

The problem with conventional roasting methods is that even if your roast is perfectly medium-rare in the middle, the outer edges will be well-done. A couple of other techniques use a lower, slower roasting method to keep more of your high-priced beef pink and perfect when it's done.

The standard version of that technique calls for roasting your beef at a very low temperature, typically 225F or 250F until it reaches the right temperature for medium-rare. A temperature that low won't give the beef a brown and savory outer crust, so you usually sear it first in a screaming-hot pan to create browning, and then transfer it to a roasting pan and on to the oven. Alternatively, some recipes suggest starting the roast at a very high temperature to brown it, and then lower it to 225F or 250F to finish cooking.

For this method of low-temperature cooking, allow about 40 to 50 minutes per pound for your roast to reach medium-rare. The lower your temperature and the thicker the roast, the more time per pound you should allow.

The "Reverse Sear" for Prime Rib

Most recipes call for searing the roast first and then finishing it at low temperature. In recent years, experiments by a few science-geek kitchen mavericks have demonstrated a strong case for doing it the other way around. It's still low-temperature cooking, but the searing happens at the end, which is why it's often referred to as the "reverse sear" method.

It's all about basic physics. Your beautiful beef can't start to brown until the moisture at its surface turns to steam and boils away, which takes time and a whole lot of thermal energy if you do it first. If you wait until your beef is mostly cooked and then sear it at the end, you've already evaporated the surface moisture during the course of your roasting time. The beef sears up beautifully, and there's even less risk of a well-cooked outer layer.

Your cooking time for this technique stays at 40 to 50 minutes per pound, but you'll pull it out before it's completely done – we'll get to temperatures in a minute – and let it rest for several minutes. Then, you put it back into a 500F oven for another 5 to 8 minutes, which sears it beautifully, and it's ready to serve.

Why Low and Slow Is Tenderer

Whether you sear your roast at the beginning or the end, it stays more tender and juicier if you cook it at low temperature. There are a few reasons for this. One is that heat causes the protein strands in muscle tissue to contract and tighten, making them denser and squeezing out moisture. The more-done outer layer, then, will always be drier and chewier.

Another important point is that the meat contains natural enzymes that help tenderize it, and those become more active at warm temperatures. That's why meats were traditionally hung for a few days, or even weeks, to tenderize them. Low-temperature cooking lets those enzymes stay active for a longer part of the cooking process, leaving the meat tenderer.

Doneness and Carryover Cooking

Most diners consider medium-rare the perfect degree of doneness for a prime rib roast. If you look at a prime rib temperature chart, you'll see that medium-rare falls at 130F to 134F. Medium, which some diners prefer, ranges from 135F to 144F. Even if the standing rib roast cooking times chart you found on the internet used the correct cooking temperature, it's not going to tell you exactly when your roast is at the right temperature. For that, you need a thermometer.

Use a leave-in meat thermometer or probe thermometer inserted into the thickest part of the roast, or test it periodically with an instant-read thermometer as you get close to your expected finishing time. In fact, it's good to double-check your leave-in thermometer with an instant-read because the metal probe itself conducts heat into the meat, so the spot surrounding your thermometer will actually be slightly more cooked than the rest.

Another complication is that your roast will keep cooking after you take it out of the oven. Some prime rib temperature charts tell you to pull it out of the oven at your finished temperature of 135F or higher, and that's flatly wrong. Instead, pull it out when it's still 5 to 10 degrees short of the final temperature you want. The heat that's trapped in the roast will continue to cook the beef, something professional cooks call "carryover cooking." If you don't allow for that, your roast will cook more than you want.

Resting Time

Before you cut into your roast, it's always best to let it sit for 15 to 30 minutes. First, this gives time for carryover cooking to do its thing and finish your roast. Second, and more importantly, it means your roast will stay juicier once it's carved. Your roast will still be hot, if not as hot as it would be when it's oven-fresh. One benefit of the reverse-sear method is that you can let your roast rest first, then sear it immediately before serving. It won't need to rest again, and you and your guests can enjoy perfectly hot beef.