Free Meat Cooking Time calculator with clear step-by-step results.
Roasting time is a per-kilo rate plus a fixed allowance for bringing the joint up to temperature, adjusted for two things people routinely ignore: bone conducts heat differently and slows the overall time slightly, and a joint straight from the fridge needs about ten percent longer than one brought toward room temperature.
Roasting time
Minutes = (weight kg x minutes per kg + fixed allowance) x bone factor x fridge factor
Resting time
Rest = 8 minutes per kilo, between 5 and 45 minutes
Food safety guidance: poultry must reach 74 C throughout, measured in the thickest part away from bone. Stuffed birds need longer, and any joint served to vulnerable people should be cooked through rather than pink.
That is the standard instantaneous target. Lower temperatures can be safe if held long enough, but 74 C in the thickest part of the thigh is the practical domestic rule.
Resting lets the temperature even out and the muscle fibres relax, so the juices redistribute instead of running out when carved. It also allows for carry-over cooking.