Planning a roast turkey — whether it’s Christmas, Thanksgiving, Easter or just a Sunday showpiece — comes down to one anxious question: when do I put it in? This calculator gives you the precise cooking time, rest time and a backwards-calculated oven start time so you can serve at exactly the moment you want.
How the formula works
The calculation follows the official guidelines published by the UK Food Standards Agency (FSA), British Turkey, and the USDA Food Safety and Inspection Service (FSIS):
Cook time (minutes) = 20 × weight (kg) + 90
The 20 minutes-per-kilogram factor covers the time heat takes to penetrate a dense muscle mass; the fixed 90-minute base accounts for the time needed to safely heat through the centre of even a small bird regardless of weight. This formula applies to a whole, unstuffed turkey roasted at 180 °C (350 °F, Gas Mark 4) in a conventional oven.
Three adjustments are then applied as needed:
- Crown (no cavity): multiply by 0.70 — a crown has no hollow to conduct heat through and substantially less total mass.
- Stuffed bird: add 40 minutes — the stuffing inside the cavity must also reach a safe temperature, which keeps the oven on longer.
- Fan / convection oven: multiply by 0.83 (a ~17 % time reduction) — circulated hot air transfers heat more efficiently than still air.
Finally, the calculator always adds 45 minutes of resting time to the total, because resting is not optional — it is part of the cooking process.
Worked example — the Christmas centrepiece
A 6 kg whole turkey, unstuffed, in a conventional oven at 180 °C:
| Step | Calculation | Result |
|---|---|---|
| Base cook time | 20 × 6 + 90 | 210 min (3 h 30 min) |
| Rest time | fixed | 45 min |
| Total | 210 + 45 | 255 min (4 h 15 min) |
| Start time to eat at 14:00 | 14:00 − 4 h 15 min | Put in at 09:45 |
If the same bird is stuffed, add 40 minutes: cook time becomes 250 min (4 h 10 min) and you need to start at 09:05 to eat at 14:00.
What about very large turkeys?
Birds above 7 kg take longer in absolute terms but the formula scales well. The key practical concern is that a very large bird (above 7 kg) may need to be positioned low in the oven, and you should add a foil tent over the breast for the first two-thirds of cooking to prevent the skin burning before the thigh is cooked through. Remove the foil for the last 30–40 minutes to brown the skin.
Always verify with a probe thermometer: 74 °C (165 °F) in the thickest part of the thigh, not touching bone. This is the standard endorsed by the FSA, the NHS, and the USDA. Visible juice clarity and colour are secondary checks only.
The resting stage
Resting is the most skipped step in home turkey cooking and the main reason carved turkey is dry. During roasting, proteins contract and push moisture towards the centre of the joint. When you remove the bird from the heat, those proteins gradually relax. If you rest the turkey for 45 minutes, tented loosely in foil (not tightly sealed — you want steam to escape, not to continue steaming the skin), the fibres reabsorb most of the expelled liquid. Every slice you carve will be significantly juicier.
The foil tent keeps the surface warm enough to serve comfortably; an internal temperature of 74 °C will only drop by a few degrees over 45 minutes, well within the safe zone.