Alexandra Cristina Negoita/Demand Media

Cooking for one or two people can be a pain sometimes, but it's not especially difficult. Cooking for three or four is even easier, because a lot of foods – a chicken, a small roast, a fish – are naturally the right size for a small group. It's when you start doing big holiday meals and group get-togethers that things get complicated. Especially with popular side dishes like mashed potatoes, you'll need to know how many potatoes to buy and prepare.

Mashed Potato Serving Size

The USDA's National Nutrient Database assumes a portion size of 1 cup for mashed potatoes, which checks in at 210 grams or just under a 1/2 pound. That's not at all an unreasonable amount, and a lot of cookbooks and online resources suggest that you should allow 1/3 to 1/2 pound of mashed potatoes per person. The catch is that a lot of people really, really like mashed potatoes and will cheerfully eat more than a standard portion if they're given the opportunity.

The bottom line is that you'll need to be flexible with your math. If your guests are light-eating seniors, the USDA portion size probably works. If it's your kid brother's high school football team, you might need to bump that up to 3/4 pound per person. Also, since people will eat more if that's an option, you have to decide if you'll give them that opportunity.

Buffet Style Requires Bigger Portions

One way to do that is to simply bump up the quantity you make, so you can offer seconds if anybody wants them. Not everybody will, and the second portion doesn't have to be as big as the first, so you can probably get away with adding another 10 to 20 percent to your math. If you're making mashed potatoes for 20 people, for example, you might bump that up to 24 or 25 portions just in case.

That's also a good rule if you're serving buffet style. Guests serving themselves will take a larger first serving and still come back for more, so it's best to start with the larger 3/4-pound portion size and then allow an extra 10 to 20 percent on top of that. You can control portion size a little bit by keeping your serving spoons small, but you'll still need lots of potatoes. You might have leftovers, but that's better than running out.

The Math for the Mashed

If you go searching on the internet you can probably find a mashed potato calculator to tell you how many mashed potatoes per person. If the calculator lets you set your portion size, go ahead and use it. Otherwise, you're probably better off doing the math yourself. If you're allowing a 1/2 pound per person, for example, it's pretty straightforward: You'll need a pound of potatoes for every two people.

It's not much more difficult to work out the potatoes for a 3/4-pound serving. Take the number of people you're cooking for, multiply that by .75, and that's how many pounds you'll need. If you're cooking for 20 people, that means 15 pounds of potatoes. For 100 people, you'd need 75 pounds of potatoes.

Use a Recipe and Prepare Ahead

If you're making a huge batch of mashed potatoes it's best to use a recipe instead of winging it. If you've never cooked in a restaurant and don't usually prepare foods in large batches, it's difficult to get the proportions right. For one thing, it takes way more salt than you'd ever imagine. Some recipes are also fine-tuned to give you potatoes that do well when they're made in advance and reheated. That can save you a lot of time and stress, especially with really large get-togethers.

You can reheat make-ahead mashed potatoes in a Crock-Pot, for example, which is fuss-free and leaves your stovetop and oven free for the rest of the meal. A normal sized 6- or 7-quart slow cooker can hold 10 pounds of potatoes, enough to serve up to 20 people, and will reheat them in about three hours on the low setting. It's best to start an hour ahead of time, just to be safe, and to stir the potatoes two or three times so they're evenly heated. You can always turn your cooker to the keep-warm setting if the potatoes are ready too soon.