How do I estimate the calories I’m eating if I didn’t make my food?

Use a Calorie Counter’s “Meal” function

Many free sites such as MyFitnessPal or NutritionData have pre-entered estimates from other users already in their respective databases. Use this until you feel comfortable with foods you know. Familiarity with sizes/numbers is the best way to learn how to estimate what you’re eating.

Use “rule of thumb” charts

SparkPeople has the most well-known of these charts, but they are everywhere on the internet. The same basic concept is here as with using an actual calculator, but it doesn’t require as much repetition to learn. You can even carry cheat sheets with you, or just leave them at work.

Use “real-life” objects as portion size estimators

It’s much easier to estimate sizes if you have something to relate it to that you deal with every day. Whether that be an unrelated set of objects that you can visualize, or a set of objects you deal with every day, knowing that the meal the waiter just dropped in front of you is roughly 2 cups of potato and a pound of steak makes it much easier to either use your online calorie counter, or your rule of thumb.