For a standard roasted 18-pound turkey in a 325°F (163°C) oven, plan on about 4¼ to 4½ hours if unstuffed and up to about 4¾ hours if stuffed, but always cook to a safe internal temperature of 165°F (74°C) instead of relying only on the clock.

Basic timing guide

  • Unstuffed 18 lb turkey at 325°F: about 4¼–4½ hours.
  • Stuffed 18 lb turkey at 325°F: about 4¼–4¾ hours.
  • General per‑pound estimate: around 15–20 minutes per pound at 325°F, but actual time varies by oven, pan, and turkey temperature.

Doneness and food safety

  • The turkey is done when the thickest part of the thigh and breast and the center of the stuffing (if using) reach 165°F (74°C) on a meat thermometer.
  • Check in multiple spots (inner thigh close to, but not touching, bone; thickest part of the breast; stuffing center) to be sure everything is at or above 165°F.

Step-by-step roasting outline

  1. Thaw completely in the fridge (about 24 hours per 4–5 pounds; an 18 lb bird usually needs 4–5 days).
  1. Pat dry, season, and optionally truss; bring closer to room temp for about 1 hour for more even cooking.
  1. Roast at 325°F on a rack in a roasting pan, breast side up, tented loosely with foil if browning too fast.
  1. Begin checking temperature around the 3¾–4 hour mark so you do not overcook.
  1. Rest the turkey, loosely tented with foil, for 20–30 minutes before carving so the juices redistribute.

If your situation is different

  • Convection oven: Many cooks reduce the temperature (for example to 300–310°F) and/or expect a somewhat shorter cook time; monitor the thermometer rather than fixed times.
  • Deep-frying: At 350°F oil, an 18 lb turkey typically cooks in roughly 59–65 minutes, but deep-frying this large a bird raises safety concerns and requires proper equipment and outdoor setup.

Forum-style quick scoop and “latest chatter”

  • Recent how‑to guides and Q&A threads around holidays echo the same core advice: focus on hitting 165°F, not an exact hour mark, and start checking early to avoid dry meat.
  • Many home cooks on forums report that the turkey often “finishes early” compared to charts, which is why planning some buffer time and keeping finished turkey warm (tented with foil, maybe in a warm oven) is a common strategy.

TL;DR

For “how long to cook an 18 pound turkey,” the best practical plan is: roast at 325°F for about 4¼–4½ hours if unstuffed (a bit longer if stuffed), start checking with a thermometer around 4 hours, and do not stop until every thick part reads 165°F.

Information gathered from public forums or data available on the internet and portrayed here.