Subtract the ending weight
If a jar weighs 500 g before you serve the food and 482 g afterward, you used 18 g. For a label serving of 32 g, that is 18 ÷ 32 = 0.5625 servings. A 190-Calorie serving becomes about 107 Calories.
Food used = container weight before - container weight after
The calculator applies the same serving factor to the printed protein, carbohydrate, and fat values. It does not reconstruct label calories from those macro grams.
You can skip the subtraction with tare
Put the jar on the scale, remove the lid, press tare, and take out the food. When the display reads -18 g, the magnitude is the amount removed: 18 g. You can enter that amount in the calculator by using 18 g as the before weight and 0 g as the after weight.
This works well for nut butter, cream cheese, mayonnaise, sauce, oil, honey, and other foods that stick to a spoon or are added directly while eating. Read the reverse-weighing guide for both workflows and a full label example.
The scale supplies weight, not nutrition
The FDA explains that Nutrition Facts values usually describe one labeled serving. The serving line includes a household measure followed by a metric gram amount. Use that gram amount and values from the same label column.
If a label gives nutrition per 100 g, use the nutrition per 100g calculator. If you already know the portion grams and do not need the before-and-after subtraction, the serving size calculator is the shorter route.
Four mistakes that change the result
- Putting the lid on for one measurement but not the other.
- Switching between grams and ounces.
- Letting the scale turn off or auto-tare between readings without recording the number.
- Using nutrition for a different food, recipe, or preparation state.
A home kitchen scale also has a display increment and a maximum capacity. A one-gram scale cannot reliably distinguish every fraction of a gram, and a heavy container may exceed its limit. Use a stable surface and follow the scale manual.
What the sources support
NIST kitchen-scale guidance describes zeroing or taring a scale to remove container weight before a new measurement. The FDA serving-size guide explains the gram amount and nutrition basis printed on US labels.
Recent community discussions show that the method is useful but not obvious to every new scale owner. Those discussions support the usability problem, not a claim that reverse weighing causes weight loss or that every food must be measured.
Common questions
Do I include the lid?
Either approach can work if the setup is identical both times. Leaving the lid off is usually simpler because you do not need to replace it before the ending measurement.
What if the ending weight is higher?
Recheck the units, container setup, and scale. The calculator treats a higher ending weight as an input error because food removed from an unchanged container should reduce its weight.
Does a negative reading mean negative calories?
No. The minus sign only shows that weight left the tared container. Use the positive magnitude as the grams eaten or added elsewhere.