Calculating the cooking time for a lamb roast depends on a number of factors, including the type of cut, the desired doneness and the oven temperature. The easiest way to find the proper cooking time is to use a well-respected and detailed time and temperature chart.
For oven roasting at 325 degrees Fahrenheit, cooking times range from 15 minutes per pound for whole leg of lamb cooked to medium-rare to 45 minutes for a well-done boneless shoulder roast. A good digital thermometer is key, as all cook times are estimates. Actual times vary from oven to oven, so it is essential to measure the temperature of the roast early and often. The roast continues cooking even after leaving the oven, so it should be removed at about 5 to 10 degrees below the target temperature. Target temperatures for medium-rare, medium and well-done are 145, 160 and 170 degrees, respectively.
For the most popular desired doneness, medium, a typical bone-in shoulder roast should be roasted for 25 minutes per pound, a rib roast or rack of lamb for 35 minutes per pound at 375 degrees, a crown roast at 30 minutes per pound and a boneless leg roast for 25 minutes per pound.