How Roast Timing Works, and Why the Thermometer Decides
Say this part out loud before you read anything else: the number this calculator produces is a planning estimate, and the probe is what tells you the joint is done. Roasting time depends on weight and cut, but also on the starting temperature of the meat, the shape of the joint, whether it is bone-in, how full your oven is, how often you open the door, and how much your thermostat lies. Those variables move the answer by more than the calculation itself. Treat every figure below as the centre of a spread that runs about ±15%.
What the estimate is genuinely good for is planning. It tells you when to put the joint in, and whether a recipe is asking for something plausible. It cannot tell you when the meat is ready. For that you need an instant-read thermometer in the deepest part of the meat, and a rest before carving.
How the Estimate Is Built
The arithmetic has two parts: a rate and a fixed offset. The rate is minutes per kilogram, and it accounts for how long heat takes to travel to the centre. The offset is a fixed number of minutes added on top, and it accounts for the fact that a roast must come up to temperature from cold before the rate starts to matter. A whole chicken is 45 minutes per kilogram plus 20 minutes, so a 2 kg bird is 90 + 20 = 110 minutes at 180 °C conventional or 160 °C fan, uncovered.
The offset is why a small bird is proportionally slower than a large one. At 1.5 kg it is 25% of the total; at 3 kg it is only 14%. Both cook at the same rate per kilogram, but the small one spends more of its time just getting started.
Reference Times by Cut
These are the numbers the calculator works from, all at 180 °C conventional or 160 °C fan, uncovered, with the joint at room temperature at the start. Read the last column as well as the second, because the rest is not optional and it belongs in your plan.
| Cut | Rate | Internal Target | Rest |
|---|---|---|---|
| Whole chicken | 45 min/kg + 20 min | 74 °C minimum | 15 minutes |
| Whole turkey | 40 min/kg + 25 min | 74 °C in the thigh | 30 minutes |
| Beef rib roast, bone-in | 35 min/kg (medium-rare), 45 min/kg (medium) | See temperature table | 20–30 minutes |
| Beef boneless roast | 50 min/kg (medium-rare), 60 min/kg (medium) | See temperature table | 20–30 minutes |
| Leg of lamb, bone-in | 45 min/kg for medium | See temperature table | 15–20 minutes |
| Pork loin, boneless | 45 min/kg | 63 °C plus a 3-minute rest | 10–15 minutes |
| Pork shoulder | About 90 min/kg | 90 °C and above | 30 minutes |
| Cooked ham | 25 min/kg | 60 °C | 10–15 minutes |
| Whole duck | 40 min/kg | 74 °C in the thigh | 15–20 minutes |
Note the pork shoulder line. At roughly 90 minutes per kilogram it is not really a roast at all; it is a braise that happens to be uncovered. It needs 90 °C or above before the collagen dissolves into gelatine and the meat shreds. Pulled at 63 °C it is safe but tough, so the safe minimum and the useful target are two different numbers.
Pull Temperature, and Which Number Is a Safety Figure
Beef and lamb are where the confusion lives, so be precise. The USDA minimum for whole cuts of beef, veal, lamb and pork is 63 °C followed by a three-minute rest. That is the safety figure, and it is the one that matters if you are cooking for anyone in a vulnerable group. Rare at 52–54 °C, medium-rare at 54–57 °C and medium at 60–63 °C are preference figures, and they sit at or below that minimum. It is a choice you make knowingly.
Poultry and ground meat have no such range. Whole chicken, turkey and duck must reach 74 °C in the deepest part of the meat, measured in the thickest part of the thigh for a whole bird. Ground or minced meat of any kind must reach 71 °C. These are not targets to negotiate with, and there is no version of rare poultry that is safe to serve.
| Protein and Doneness | Internal Temperature | Status |
|---|---|---|
| Poultry, all of it | 74 °C | Safety minimum, no range |
| Ground or minced meat | 71 °C | Safety minimum, no range |
| Beef, lamb and pork, whole cuts | 63 °C plus a 3-minute rest | Safety minimum |
| Beef or lamb, rare | 52–54 °C | Preference, below the safety figure |
| Beef or lamb, medium-rare | 54–57 °C | Preference, below the safety figure |
| Beef or lamb, medium | 60–63 °C | Preference, at the safety figure |
| Pork shoulder, for pulling | 90 °C and above | Texture target, above safety |
Carry-Over Cooking: Why You Pull Early
A joint does not stop cooking when it leaves the oven. The outside is hotter than the centre, and heat keeps flowing inward until the two equalise, so the internal temperature climbs another 3–8 °C during the first part of the rest. A large roast carries over more than a small one, because it stores more heat. A foil tent increases the effect by trapping escaping warmth; leaving the joint open on the rack reduces it.
This is why you pull the meat early rather than at target. To finish a beef roast at 55 °C, take it out at around 50–51 °C and let the carry-over finish the job. A large rib roast can be pulled 5 °C or more below target; a small chicken may need only 2–3 °C. On a fast, hot roast the gradient is steeper and the rise is larger; on a long, low roast it is modest. There is no single correction that fits everything, which is why you check twice: once when you pull it, and again halfway through the rest.
The Rest, and What It Is Actually For
Resting is not a formality. While the joint sits, the muscle fibres relax and the juices pushed towards the centre by the heat redistribute back through the meat. Cut immediately and those juices run out onto the board, which is why an unrested roast looks generous on the carving board and dry on the plate. Resting also lets the carry-over finish the cooking evenly rather than leaving a hot rim and a cool centre.
Plan around these: 15 minutes for a chicken, 30 for a turkey, 20–30 for a large beef roast. Tent loosely with foil if you like, but leave a gap for steam to escape, or you will soften the skin you worked to crisp.
Your Oven Is Not the Temperature on the Dial
Domestic oven thermostats are commonly 10–15 °C out from the setting, and they cycle around the target rather than holding it exactly. A dial set to 180 °C may be running at 165 °C or 195 °C, which is enough to move your timings by twenty minutes on a large joint. An inexpensive oven thermometer hung from the middle shelf tells you what you are actually cooking at. Check it once a year, because the calibration drifts.
Fan ovens are a separate issue. Moving air transfers heat far more efficiently than still air, so a fan oven set to the same temperature will cook faster and brown harder. You have two ways to fix that, and you use one of them, never both: drop the temperature by about 20 °C, or keep the temperature and take roughly 25% off the time. Doing both gives you a joint that is underdone in the middle and overcooked outside.
One more variable worth planning for. A joint straight from the refrigerator needs longer than one that has sat on the counter, so letting a large roast come up for 30–60 minutes before it goes in makes the estimate more reliable. Do not leave meat out for hours on end; the danger zone rules apply here as everywhere.
Planning Backwards from the Serving Time
The start-time planner runs the same arithmetic in reverse. Take a 2 kg chicken and a target carving time of 19:00. The rate is 45 minutes per kilogram, so 2 × 45 = 90 minutes, plus the 20-minute offset = 110 minutes in the oven. Add the 15-minute rest, because the bird comes out before it is carved, and the total is 125 minutes. The chicken goes in at 16:55.
Remember the ±15% spread. On 110 minutes of oven time the window is roughly 94 to 126 minutes, so a 16:55 start could see the bird ready any time between about 18:30 and 19:00. The probe resolves it: start checking at the early end, around 18:35, and trust a reading of 74 °C in the thigh over any clock. If it is done early, rest it longer under foil. Never stuff a bird you are roasting for a crowd: the stuffing must also reach 74 °C, and the only reliable way to guarantee that is to cook it in a separate dish.
Frequently Asked Questions
45 minutes per kilogram plus 20 minutes, at 180 °C conventional or 160 °C fan, uncovered. A 2 kg bird is 90 + 20 = 110 minutes. Treat that as a planning estimate with a spread of about ±15%, so anywhere from roughly 94 to 126 minutes. The bird is done when the deepest part of the thigh reads 74 °C, and that reading overrides the clock.
Usually one of four things. Your oven runs hot, which is common. You are using a fan oven at the conventional temperature, which cooks faster. The joint is flatter than the weight suggested, or boneless where the recipe assumed bone-in. Or it came out of the fridge early and started warmer. Check at the early end of the range rather than the middle, and let the probe decide.
Covering traps steam and slows browning, so the crust stays pale. Leave it uncovered for the roasting itself and tent it only during the rest. If the top is browning too fast, a loose foil cap over the crown works better than wrapping the whole joint, and remove it for the last 15 minutes so the surface can crisp.
Typically 3–8 °C, with the larger figure applying to big roasts and tightly tented ones. A small chicken might climb only 2–3 °C, while a three-rib beef roast under foil can gain 6 °C or more. The rise is fastest in the first few minutes and then flattens. To hit 55 °C on the board, pull a large roast at around 50 °C.
Medium-rare is 54–57 °C, below the USDA minimum of 63 °C with a three-minute rest for whole cuts of beef. That safety figure applies to an intact whole cut, because any bacteria are on the surface and are killed by the oven heat. If the meat has been pierced, rolled or mechanically tenderised, bacteria can be inside, and 63 °C is the figure that applies.
Into the thickest part of the thigh, aiming towards the body without touching bone, which conducts heat and gives a false high reading. The breast is the other place to check, at its thickest point away from the cavity. Both should read 74 °C. If the bird is stuffed, the centre of the stuffing must also reach 74 °C.
Yes, and you have two options. Set the fan oven about 20 °C lower than the conventional figure, or keep the temperature the same and take roughly 25% off the cooking time. Use one or the other, never both, or the joint will be underdone at the centre and overcooked at the edges. Most recipes quote the conventional figure, so converting down by 20 °C is the simpler habit.
Slower, which is why the reference figures differ so much between a bone-in rib roast at 35–45 minutes per kilogram and a boneless roast at 50–60. Bone is a poor heat conductor and it sits in the middle of the joint, so it insulates the centre. A boneless roast is also usually rolled and tied into a more even shape. Go by the internal temperature rather than assuming the two behave alike.