A food thermometer should be used to ensure a safe minimum internal temperature of 165 °F has been reached to destroy bacteria and prevent food borne illness. 

Many variables can affect the roasting time of a whole turkey:

• A partially frozen turkey requires longer cooking; if you have to buy a frozen turkey, get it far enough in advance to allow it to defrost in the refrigerator for at least four days; five for a larger bird. If you find yourself with a partially frozen bird the day of the Big Cook, thoroughly sanitize your sink and cover the bird in room temperature water, never warm. Change the water several times as it cools down.
• A stuffed turkey takes longer to cook.
• The oven may heat food unevenly.
• Temperature of the oven may be inaccurate.
• Dark roasting pans cook faster than shiny metals.
• The depth and size of the pan can reduce heat circulation to all areas of the turkey.
• The use of a foil tent for the entire time can slow cooking.
• Use of the roasting pan's lid speeds cooking.
• An oven cooking bag can accelerate cooking time.
• The rack position can have an affect on even cooking and heat circulation.
• A turkey or its pan may be too large for the oven, thus blocking heat circulation.

ROASTING INSTRUCTIONS 

1. Set the oven temperature no lower than 325°F.  
2. Be sure the turkey is completely thawed. Times are based on fresh or thawed birds at a refrigerator temperature of around 40°F. 
3. Place turkey breast-side up on a flat wire rack in a shallow roasting pan 2 to 2 1/2 inches deep.

• Optional steps:
• Tuck wing tips back under shoulders of bird (called "akimbo").
• Add one-half cup water to the bottom of the pan.
• For larger birds, aluminum foil may be tucked over the breast of the turkey for all but the last 1 to 1½ hours, then removed for browning. This has two benefits- the foil traps steam in the breast, keeping it moist; it prevents the breast from over-browning. Baking the turkey to the desired color, then tenting it for the remainder of the roasting time to preven over-browning is not recommended, because of the aforementioned steam; you'll have soggy browned skin instead of the crispy skin we all crave.

4. For optimum safety, cook stuffing in a casserole. If stuffing your turkey, mix ingredients just before stuffing it; stuff loosely. Additional time is required for the turkey and stuffing to reach a safe minimum internal temperature (see chart); stuffing should be removed from the bird within 30 minutes of it leaving the oven. 
5. For safety and doneness, the internal temperature should be checked with a food thermometer. The temperature of the turkey and the center of the stuffing must reach a safe minimum internal temperature of 165 °F. Check the temperature in the innermost part of the thigh and wing and the thickest part of the breast. 
6. Let the bird stand 20 minutes before removing stuffing and carving. 

For estimated turkey cooking times for alternative cooking methods, click here

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Turkey Basics - Safe Cooking | Turkey Basics - Safe Thawing | Turkey Basics - Safe Stuffing | Turkey Basics - Storing Leftover Turkey