Here is a simple guideline:  Peeling the skin on a potato will drop your yields by 20% over leaving the peels on. To come up with enough servings for 25 people you’ll need to start with 6 pounds of fresh unpeeled or 5 pounds of peeled Idaho® potatoes.  If you bought a five pound bag of potatoes and peeled them, you would need to adjust the ratios of the milk and margarine or butter down slightly and would end up with about 22.5 servings.Here is a chart to help you with the exact amount of ingredients you will need for the perfect mashed potatoes: