Leg of Lamb Cooking Time Calculator: Per Kg & Temperature Guide
Quick answer: for a classic roast leg of lamb, start at 220°C (200°C fan) for 20 minutes, then reduce to 180°C (160°C fan). As a starting guide, allow about 40 minutes per kg for rare, 50 minutes per kg for medium, or 60 minutes per kg for well done, with the initial 20 minutes included in the total calculation.
Use the table below as a quick leg of lamb cooking time calculator, then confirm doneness with a meat thermometer because ovens, joint shape and bone-in vs boneless cuts can all affect the final time.
Leg of Lamb Cooking Time Calculator
| Weight | Rare | Medium | Well done |
|---|---|---|---|
| 1kg | 1 hr | 1 hr 10 min | 1 hr 20 min |
| 1.5kg | 1 hr 20 min | 1 hr 35 min | 1 hr 50 min |
| 2kg | 1 hr 40 min | 2 hrs | 2 hrs 20 min |
| 2.5kg | 2 hrs | 2 hrs 25 min | 2 hrs 50 min |
| 3kg | 2 hrs 20 min | 2 hrs 50 min | 3 hrs 20 min |
Formula: after the initial 20 minutes at the higher oven temperature, use roughly 40 minutes per kg for rare, 50 minutes per kg for medium, or 60 minutes per kg for well done.
Example: a 2kg leg of lamb cooked medium is around 20 minutes at 220°C, followed by 100 minutes at 180°C, giving a total roasting time of approximately 2 hours.
Boneless Leg of Lamb Cooking Time
A boneless rolled leg can cook a little differently from a bone-in joint because its shape is usually more even. Use the same calculation as a starting point, but begin checking the internal temperature around 10–15 minutes before the calculated finish time.
Looking for the joint itself? Shop our Welsh Leg of Lamb, available bone-in or boneless, or our Minted Boneless Leg of Lamb.
What Temperature Should Lamb Be Cooked To?
For a whole leg of lamb, use a meat thermometer in the thickest part of the joint and avoid touching the bone.
- Rare: around 50–52°C before resting
- Medium: around 58–60°C before resting
- Well done: around 65–70°C+
The temperature will usually rise slightly while the meat rests. These are doneness targets for a whole lamb joint. Whole cuts of lamb can be served pink as long as the outside surfaces are thoroughly cooked.
How Much Leg of Lamb Do I Need?
- Boneless: allow around 200–250g per person
- Bone-in: allow around 250–300g per person
Allow a little more if you want generous portions or leftovers.
How to Season a Leg of Lamb
Lamb works brilliantly with simple flavours. A classic combination is:
- Salt and black pepper
- 2–4 cloves of crushed garlic
- Fresh rosemary
- Olive oil
- Optional lemon zest or a squeeze of lemon
Butcher tip: lightly score the fat without cutting deeply into the meat, then push a few slivers of garlic and rosemary into the cuts.
How to Roast a Leg of Lamb
- Remove the lamb from the fridge before cooking so it loses some of its chill.
- Preheat the oven to 220°C / 200°C fan.
- Season the lamb and place it in a roasting tin.
- Roast for 20 minutes at the higher temperature.
- Reduce the oven to 180°C / 160°C fan.
- Continue cooking using the time-per-kg guide above.
- Start checking the internal temperature before the calculated finish time.
- Rest for 20–30 minutes before carving.
How Long Should Lamb Rest?
Rest a leg of lamb for around 20–30 minutes. Transfer it to a board and cover loosely with foil. Resting allows the juices to redistribute and makes the meat easier to carve.
How to Carve a Leg of Lamb
- Identify the direction of the grain.
- Slice across the grain for a more tender bite.
- For a bone-in leg, carve along the bone first, then slice the larger muscles.
Leg of Lamb Cooking FAQs
How long do I cook a leg of lamb per kg?
After the initial 20 minutes at 220°C, allow roughly 40 minutes per kg for rare, 50 minutes per kg for medium, or 60 minutes per kg for well done at 180°C.
How long does a 2kg leg of lamb take to cook?
As a guide, a 2kg leg takes around 1 hour 40 minutes for rare, 2 hours for medium, or 2 hours 20 minutes for well done.
How long does a 1kg leg of lamb take to cook?
As a guide, allow around 1 hour for rare, 1 hour 10 minutes for medium, or 1 hour 20 minutes for well done.
Do bone-in and boneless lamb legs take the same time?
They can be close, but boneless rolled joints may cook a little differently because the shape is more uniform. Use the weight-based calculation as a starting point and rely on the internal temperature for the final check.
Should I cover leg of lamb with foil while roasting?
Usually not for a classic roast. Leaving it uncovered helps with browning. Cover it loosely with foil while it rests after cooking.
Why is my roast lamb tough?
Common causes include overcooking, carving with the grain, or skipping the resting period. A meat thermometer and a 20–30 minute rest make a big difference.
Shop Welsh Leg of Lamb Online
Choose from our Welsh Leg of Lamb, available bone-in or boneless, our Minted Boneless Leg of Lamb, or browse the full Welsh Lamb collection. Available delivery dates are shown at checkout.
Cooking times are a guide only. Ovens and joint shapes vary, so use a thermometer and check the joint before serving.
