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Slow Roasted Prime Rib on a cutting board
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5 from 1 vote

Prime Rib Roast

This Slow Roasted Prime Rib Recipe will make you look like you spent hours preparing in the kitchen. But we both know that isn't so. How to cook prime rib in a roaster is easier than you think and you might just impress some people along the way.  
Prep Time10 minutes
Cook Time1 hour 40 minutes
Resting Time30 minutes
Total Time2 hours 20 minutes
Course: Main Entrées
Cuisine: American
Keyword: easy dinner, family meals, prime rib roast
Servings: 12
Calories: 720kcal
Author: AmyG


  • 6 lbs prime rib 1 rib per 2 people (6 ribs)
  • 1 tbsp salt
  • 1 tsp pepper
  • ¼ cups olive oil


  • 1 cups water plus 2 tbsps or more
  • 1 Beef Bouillon
  • 1 tbsp corn starch you can substitute potato starch for Whole30/Paleo
  • pan drippings


  • Calculate the cooking time using the chart in the notes section.
  • Preheat oven to 500°℉.
  • Cut the bones if you prefer. Season with salt, pepper, olive oil and any herbs.
  • Place the roast on top of the bones in the roasting pan and place in oven to cook for 15 minutes at 500°℉. (uncovered)
  • After 15 minutes reduce oven to 325 degrees. Using a thermometer check the roast in half way through cooking time and then every half hour thereafter.
  • Once it has reached the internal temperature you desire, remove from oven and let rest for 30 minutes.


  • While the roast is resting, warm up one cup of water in a small saucepan, add a beef bouillon cube and some pan drippings. Bring to just about boiling. In a small cup add the starch and a little bit of water just to create a runny paste. Add starch mixture and bring to boil until mixture thickens to your liking.
  • Serve gravy with roast.


Cooking Time

Initial Cooking
Well Done
Resting Time
15 mins 15 mins per 1 lb 17 mins per 1 lb 19 mins per 1 lb 30 mins
15 mins 15 mins per .454 Kg 17 mins per .454 Kg 19 mins per .454 Kg 30 mins
Prime Rib Roast Cooking Time Calculation
Example: 5 lb prime rib roast 
Initial cooking: 15 mins at 500°F/260°C
Continue cooking for: 5 lbs x 17 (for medium) = 85 mins or 1 hour and 25 minutes.
Resting: 30 minutes (tented)
Total time: 130 minutes or 2 hours and 10 minutes

Beef Internal Temperature Reference

Rare Medium-Rare Medium Well Done
120ºF/49ºC 130ºF/54ºC 140ºF/60ºC 165ºF/74ºC
Prime Rib Internal Temperature Reference


Serving: 1Slice | Calories: 720kcal | Carbohydrates: 1g | Protein: 31g | Fat: 65g | Saturated Fat: 26g | Polyunsaturated Fat: 3g | Monounsaturated Fat: 30g | Cholesterol: 137mg | Sodium: 764mg | Potassium: 505mg | Fiber: 0.05g | Sugar: 0.05g | Vitamin A: 1IU | Calcium: 19mg | Iron: 3mg