Hong Life CalculatorsUnderstanding the roast cooking time calculator — concept and principle
How long should a roast stay in the oven? Traditional British guidance answers with a simple rule: a set number of minutes per 500 g, plus a fixed extra. This calculator applies those rules for beef, lamb, pork, chicken and turkey at a moderate oven of about 180–190 °C (fan 160–170 °C) so you can plan when to start cooking and when dinner will be ready.
How it is calculated
For most meats: time = (weight in kg × 2) × minutes per 500 g + the same number of minutes extra.
- Beef medium: 20 min per 500 g + 20 min.
- Beef well done and lamb: 25 min per 500 g + 25 min.
- Pork: 30 min per 500 g + 30 min.
- Chicken: 20 min per 500 g + 20 min.
- Turkey: about 40 min per kg + 20 min.
Weight is in kilograms; the result is in minutes and excludes resting.
The principle
Heat travels from the hot oven air into the meat by conduction, and the centre is the last part to reach the target temperature. Larger roasts need longer, so a per-weight rule is a simple approximation; the fixed extra covers the initial period while the surface and oven recover. In reality time grows more slowly than weight: heating time depends roughly on the square of thickness, and thickness grows with only about the cube root of weight. As a result, per-weight rules tend to overestimate for very large roasts and can underestimate for long, thin or very cold ones.
Worked example
Defaults: 2 kg, beef medium.
- 2 kg = 4 portions of 500 g.
- 4 × 20 min = 80 min.
- Add the extra 20 min: 80 + 20 = 100 minutes.
Then rest the roast, loosely covered, before carving.
| Weight (kg) | Approx. time (min) |
|---|---|
| 0.5 | 40 |
| 1 | 60 |
| 1.5 | 80 |
| 2 | 100 |
| 2.5 | 120 |
| 3 | 140 |
| 4 | 180 |
How to read the result
Treat the time as a plan, and start checking the internal temperature 15–20 minutes before it ends. Doneness is decided by the thermometer, not the clock. As a safety floor, whole cuts of beef, lamb and pork should reach at least 63 °C followed by a rest, and poultry 74 °C in the thickest part. A resting period of around 15–30 minutes lets juices settle and the temperature even out; the centre usually rises a few degrees more.
Common mistakes
- Roasting meat straight from the fridge and expecting the stated time to hold.
- Using a fan oven at the conventional temperature, which cooks faster.
- Opening the oven door often, which lets heat escape.
- Forgetting that a stuffed bird takes longer.
Frequently asked questions
Does bone-in meat cook differently?
Bone conducts heat differently and changes shape, so check temperature near, but not touching, the bone.
Should I cover the roast with foil?
Covering slows browning; many cooks uncover for the final part to crisp the outside.
How long can I rest a large roast?
Large roasts and turkeys can rest 30 minutes or more, loosely covered, without getting cold.
Sources: USDA Food Safety and Inspection Service — Safe Minimum Internal Temperature Chart. · Harold McGee, On Food and Cooking (Scribner) — heat transfer in meat.