The most important ingredient for a successful Thanksgiving? Math.

That's because crunching the numbers is the best way to make sure that you have enough turkey for all your guests, that the turkey you serve them won't be raw, and that it comes out of the oven on time.

The following estimates are rounded up to ensure plenty of leftovers. Keep in mind that the bigger the bird, the higher the meat-to-bone ratio. That means that with smaller turkeys, you need more total weight per serving than for larger turkeys.

If you're thawing a frozen turkey in cold water, change the water every 30 minutes. Otherwise, thaw in the refrigerator; never let it thaw at room temperature.

Roasting times are estimates for stuffed birds (subtract 20 to 40 minutes if not stuffed); actual cooking time will vary by stove and turkey. Federal health officials say a turkey should be cooked until an instant-read thermometer inserted at the innermost part of the thigh (without touching bone) reaches 165 degrees, though some cooks say thigh meat tastes better cooked to 170 degrees.

DINERS: 6 TO 7

Turkey: 9 to 10 pounds

Thawing: In the refrigerator for 48 hours, or in cold water for 41/2 hours

Roasting: 325 degrees for 3 to 4 hours

Gravy: 21/2 cups

Stuffing: 5 cups

DINERS: 8 TO 9

Turkey: 13 to 14 pounds

Thawing: In the refrigerator for 3 days, or in cold water for 7 hours

Roasting: 325 degrees for 4 to 41/2 hours

Gravy: 3 cups

Stuffing: 7 cups

DINERS: 10 TO 11

Turkey: 17 to 18 pounds

Thawing: In the refrigerator for 4 days, or in cold water for 9 hours

Roasting: 325 degrees for 41/2 to 5 hours

Gravy: 31/2 cups

Stuffing: 9 cups

DINERS: 12 TO 13

Turkey: 20 pounds

Thawing: In the refrigerator for 4 days or in cold water for 10 hours

Roasting: 325 degrees for 5 to 6 hours

Gravy: 41/2 cups

Stuffing: 10 cups

DINERS: 14 TO 15

Turkey: 23 pounds

Thawing: In the refrigerator for 5 days, or in cold water for 111/2 hours

Roasting: 325 degrees for 5 to 6 hours

Gravy: 5 cups

Stuffing: 111/2 cups