Why Your Calorie Count Is Wrong, and by How Much
A label reading 200 kcal per 50 g makes an 80 g portion 320 kcal: 200 × 80 ÷ 50. Read a two-serving package as one and that single mistake outweighs every rounding decision on the label combined. Your calorie log stacks three estimates on top of each other, and the TDEE calculator adds a fourth: the food’s energy value, the portion you ate, your record of the meal, and the activity band you graded yourself. Those four errors point in different directions, which is why each person’s total is off by a different amount in a different direction.
Start with the serving size
Check whether the database entry describes your food raw or cooked, drained or sitting in its liquid, in grams or in cups. The FDA guide to Nutrition Facts labels explains serving and per-package information; label tolerances vary by rule and by jurisdiction, and the one thing they never licence is a blanket 20% added to everything you eat.
What a food record leaves out
Cooking oil goes into the pan and out of the log. So do dressings, drinks, the bites you take while cooking, and the gap between the portion you planned and the portion you served. Nobody is accusing you of dishonesty here: these are measurement gaps, and the size of your own is yours to find.
A 1992 study of self-reported intake found substantial discrepancies in a small, selected group; read it as evidence that the problem is real, and leave it there. It cannot tell you that you underreport by 47%, and it cannot explain why your weight sat still last month.
Your watch is estimating too
A 2017 study of seven wrist-worn devices found their energy-expenditure figures less accurate than their heart-rate readings in its test conditions. That was seven devices under one set of conditions, so your watch on your workout sits outside it.
If your TDEE estimate already includes your usual exercise, adding the workout on top counts it twice, so check the activity multiplier guide before you combine numbers from two tools.
Make the record useful
- Pick entries that match the food and the way you cooked it.
- Check the units.
- Log the oil, the dressings and the drinks that move the total.
- Read several weeks at once, because one day’s number is a report on one day and a fortnight is the shortest stretch that shows you a direction.
Consistency shrinks some of that uncertainty and leaves the rest exactly where it was. A flat week can still be water. A watch and a scale will disagree. Cutting your food every time they do is a fast route to a diet you give up on.
Detailed tracking is optional. If it brings anxiety, compulsive checking or restriction, go looking for support instead of better numbers. Persistent unexplained changes deserve professional assessment, and the plateau guide covers the causes a calculator will always miss.
Scope and correction note, 13 September 2026: This guide provides general adult education. It has been revised to distinguish estimates from measurements and to remove unsupported guarantees. It has not received independent clinical review. See the editorial policy and medical disclaimer.