How Long To Cook Leg Of Lamb

Timing a leg of lamb correctly means accounting for weight, bone-in versus boneless, and the internal temperature you’re aiming for. So how long to cook leg of lamb? As a general rule, roast it at 325°F (160°C) for about 20 minutes per pound for medium-rare, though bone-in cuts cook a little faster per pound than boneless ones.

That single number is a good starting point, but it’s not the whole story. The real answer depends on a few things you can control: the size of your roast, whether the bone is still in, and how done you like your lamb.

Below you’ll find exact times, temperatures, and simple steps so your roast comes out right every time.

Use this chart as your baseline. All times assume an oven set to 325°F (160°C) and a roast that sat at room temperature for about 30 minutes before going in.

Bone-in leg of lamb, 5 pounds: 1 hour 40 minutes to 1 hour 55 minutes (medium-rare)
Bone-in leg of lamb, 7 pounds: 2 hours 20 minutes to 2 hours 40 minutes (medium-rare)
Boneless leg of lamb, 4 pounds: 1 hour 20 minutes to 1 hour 35 minutes (medium-rare)
Boneless leg of lamb, 6 pounds: 2 hours to 2 hours 20 minutes (medium-rare)

For medium, add roughly 5 to 10 minutes per pound. For well done, add 15 minutes per pound. Keep in mind these are estimates. Your oven, the shape of the roast, and whether it’s tied all affect the final time.

Bone-In Vs Boneless Timing

Bone-in lamb takes longer overall because it’s heavier. But per pound, it actually cooks a bit faster. The bone conducts heat into the center of the meat, so the roast cooks more evenly.

Boneless leg of lamb is often rolled and tied. That makes it denser, so heat takes longer to reach the middle. It also tends to cook less evenly around the edges.

Bone-in: about 20 minutes per pound at 325°F
Boneless: about 25 minutes per pound at 325°F

If you bought a boneless roast, check it earlier than you think you need to. It can go from perfect to overdone in 10 minutes.

The Right Internal Temperature

Time is a guide, but temperature is the truth. Always use a meat thermometer inserted into the thickest part, away from the bone and fat.

Rare: 120°F to 125°F (49°C to 52°C)
Medium-rare: 130°F to 135°F (54°C to 57°C)
Medium: 140°F to 145°F (60°C to 63°C)
Well done: 155°F to 160°F (68°C to 71°C)

Pull the lamb out of the oven when it’s 5°F below your target. Carryover cooking will raise the temperature while it rests. This is one of the most common mistakes home cooks make.

Step-By-Step Cooking Method

Follow these steps for a reliable roast every time.

Take the lamb out of the fridge 30 to 45 minutes before cooking. A room-temperature roast cooks more evenly.
Heat your oven to 325°F (160°C). Place a rack in the lower-middle position.
Pat the lamb dry with paper towels. Season generously with salt, pepper, garlic, and rosemary.
Set the lamb fat-side up on a rack in a roasting pan. Add a cup of water or broth to the bottom of the pan.
Roast uncovered. Start checking the temperature about 20 minutes before your estimated finish time.
Remove the lamb when it hits 5°F below your target temperature.
Cover loosely with foil and rest for 15 to 20 minutes. This lets the juices settle.
Carve against the grain into slices about half an inch thick.

Should You Sear First?

Searing at high heat before roasting adds color and flavor, but it doesn’t lock in juices. That’s a myth. If you want a browned crust, sear the lamb in a hot skillet for 2 to 3 minutes per side before moving it to the oven.

You can also roast at 450°F for the first 15 minutes, then drop to 325°F. This gives you a nice crust without extra pans to wash. Just watch the timing closely, since the higher heat speeds things up.

Resting And Carryover Cooking

Resting is not optional. Cut into a roast straight from the oven and the juices run out onto the board, leaving dry meat behind. Wait 15 to 20 minutes for a large leg, or 10 minutes for a small boneless one.

During that rest, the internal temperature climbs another 5°F to 10°F. That’s why you pull it early. If you wait until it reads 135°F in the oven, you’ll end up closer to 145°F on the plate.

Common Mistakes To Avoid

Trusting time alone instead of using a thermometer
Cooking cold lamb straight from the fridge
Skipping the rest period
Cutting the roast with the grain, which makes it tough
Opening the oven door too often, which drops the heat

One more tip: if your lamb is done early, tent it with foil and let it sit. It stays warm for 30 minutes or more, and the rest only improves the texture.

With the right weight-to-time ratio and a good thermometer, a leg of lamb is one of the easiest roasts you can make. Write down your times and temperatures each time you cook one. After two or three roasts, you’ll know your oven well enough to nail it without checking a chart at all.

How Long To Cook Leg Of Lamb