For many, chowing down on a slice of pizza is one of the simple pleasures in life. Yet it isn't always easy finding out the nutritional value of that slice of pepperoni or ham and pineapple treat because the exact "serving size" that slice represents often varies among the popular pizza chains.

So here is Good Housekeeping's guide to show you how to avoid making mistakes when trying to watch what you eat.

Portion distortion

When you order a "Pizza for One" or a burger, it's fair to assume you'll get a meal intended for one person. But when Good Housekeeping scoured the websites of 48 popular national restaurants, we found four that listed dishes a customer might order just for herself that actually serve two or more. To see what a diner's in-restaurant experience might be, we sent volunteers to order those dishes at three locations of Uno Chicago Grill, Pizza Hut, Domino's and Papa John's. None of the waitstaff, and few of the menus, alerted our volunteers to the true serving sizes.

High-calorie traps

Uno Chicago Grill: According to a careful reading of this eatery's online nutritional info, single burgers, main-course salads and many other entrees are meant for two. The individual Deep Dish pizzas come in at 500 calories to 770 calories per serving, instead of an astonishing 1,500 to 2,310 for the whole seven-inch pie.

Pizza Hut: The takeout menu was clear regarding the pasta dishes, which serve two or four people, but not on the P'Zone. That calzone-like dish is meant for two, per the online calorie counts. Our volunteers noted that the entree came with one side of dipping sauce, not two, which might have clued them in on the real portion size.

Domino's Pizza: A single serving of any variety of the Pasta in a Bread Bowl is half the bowl, yet in each of the eateries our volunteers visited, they were given just one set of cutlery. We called attention to this in 2009. Though Domino's now lists calorie counts on some of its takeout menus, we didn't see them on menu boards.

Papa John's: The biggest surprise — the Pizza for One weekday lunch special is meant for four people. Your portion, it turns out, is a 180-calorie slice. Eating the whole eight-inch pie would load you up with 720 calories.

Tips before ordering

Check restaurant websites for nutrition stats and serving sizes first because you may not find them in a restaurant. If a dish serves multiple people, you'll have to calculate the nutrition stats to see what you'll actually eat if you order it for one.