How this calculator works
A sandwich is mostly bread by weight and mostly spread by energy. Two teaspoons is two thirds of a tablespoon, and a tablespoon of peanut butter is about 16 g and 94 kcal, so a slice carries around 63 kcal of spread before the bread. Chocolate hazelnut spread is denser still, with more sugar and fat, while hummus and cottage cheese are much lighter per spoon. The calculator converts teaspoons to tablespoons, then to grams and energy using the typical values for each spread, and compares that with the 80 kcal in a slice of white bread. The servings per jar is the number of slices one jar will cover at your spoon size, which is usually the point of the exercise.
Formula
tablespoons per slice = teaspoons / 3. spread per slice = tablespoons x grams per tablespoon. total spread = spread per slice x slices x people. calories from spread = tablespoons x kcal per tablespoon x slices x people. calories per person = (80 kcal bread + tablespoons x kcal per tablespoon) x slices. slices per jar = jar weight / spread per slice.
Worked example
Two slices of bread with two teaspoons of peanut butter on each is 2 / 3 = 0.667 tablespoons per slice, or 0.667 x 16 = 10.7 g. Two slices carry 1.33 tablespoons, or 1.33 x 94 = 125 kcal of peanut butter, and with bread the total is 2 x 80 + 125 = 285 kcal. A 350 g jar gives 350 / 10.7 = 32.7 slices, about 16 sandwiches. At $5 a jar the spread costs $5 / 32.7 = $0.15 a slice.
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Frequently asked questions
How much spread is a serving?
A serving is usually one tablespoon, about 16 g for peanut butter, which is three teaspoons. A sandwich with a thin scrape on each slice uses close to one serving, while a generous spread uses two or more. The label figures are per serving, so the spoon size decides the calorie count.
Why is chocolate hazelnut spread higher in calories than peanut butter?
It has more sugar and added fat, so a tablespoon weighs more and carries more energy per gram. Peanut butter is also dense but brings more protein and less added sugar. Both are easy to over serve because a tablespoon is a small amount.
Is the bread counted?
Yes, at 80 kcal a slice for a standard white slice. Whole grain, sourdough and thick cut slices differ, so check the label if you want a closer figure. The spread calories are shown on their own as well.
How long does an open jar keep?
Follow the label. Peanut butter and jam keep for weeks or months at room temperature, while hummus, ricotta, cottage cheese and cream cheese are perishable and should be kept cold and used within a few days of opening. Never double dip a knife that has touched bread.