how long to cook roast lamb

For a roast leg of lamb , a good rule is about 20 minutes per 500g for medium-rare , 25 minutes per 500g for medium , and 30 minutes per 500g for well done at around 200°C ; if you’re roasting a shoulder or bone- in cut , plan on a longer, slower cook, often around 19–31 minutes per 500g at 180°C depending on doneness.

Quick guide

• Leg of lamb: about 20 min per 500g for medium-rare, then rest it well.

• Boned and rolled leg or shoulder: about 19–30 min per 500g at 180°C , depending on how done you want it.

• Butterflied leg: about 12–18 min per 500g at 180°C.

• Rack of lamb: usually 20–25 minutes total for medium-rare.

Best practice

Use a meat thermometer if you can, because oven performance and roast shape can change the timing. One recipe source recommends roasting a leg hot for 20 minutes, then lowering the oven and cooking until the center reaches about 53°C for medium-rare after resting.

Slow-roast option

If you want the lamb very tender and fall-apart soft, shoulder and shanks are better for slow roasting than leg. A slow-roast method typically uses a low oven, around 140°C , for several hours until the meat becomes meltingly tender.

Doneness targets

• Medium-rare: about 58–60°C internal temperature.

• Medium: about 65°C internal temperature.

• Well done: about 70°C+ internal temperature.

A useful example: a 1.5 kg leg of lamb cooked for medium-rare at 20 minutes per 500g would take about 60 minutes , plus resting time.

TL;DR

For most roast lamb, start with 20 minutes per 500g for medium-rare and adjust by cut, oven temperature, and whether you want a quicker roast or a slow, tender one.