There are many food scales out there that have a million different functions from weighing food to breaking down the nutritional components (ie. number of carbohydrates, etc). While the extra functions are worth while, the top reason to use a food scale is to be able to figure calories, especially if you are looking for weight loss. Even though the extra functions may be beneficial to some, we teach our patients to utilize a food scale for most foods to determining calories.
Take a banana for example. If you look a banana up in any calorie book it will say size: medium calories: x. Someone please tell me what a medium sized banana is! My medium is different from your medium and the book may say 6-7" but not all bananas are the same girth. The way to solve this problem is to weigh the banana and calculate the number of calories per ounce or gram and then the number of calories for the entire banana. A much more accurate way to take a measurement. We know estimating is key, but we are not always good estimators if we don't have reference for which we are estimating. Bananas being easy to travel with and one of the more popular fruits are also one of the most calorie-dense fruits. If we underestimate and consume a lot of them, we could be making a very inaccurate estimate. This inaccuracy becomes a real problem when it happens over and over again or with foods that can be higher in calories like meat.
We recommend a standard digital scale that you can get at any department store for around $20-$30 like this one below. However, if you look HERE, there are a million different types and styles.
Use the National Nutrient Database for Standard Reference to assist you in your quest for accurately measuring calories. The example in the link is the banana. The gram measurements are much more accurate than size measurements.