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You don't need to log every calorie

A Moderly guide · October 2026

Nobody's food log is complete. The oil in the pan, a handful of nuts, the milk in a coffee, the bites of a child's leftovers: they rarely make it in. Most apps answer by asking you to log more. There's a better answer, and it's arithmetic.

The short version. If your calorie budget comes from a formula, every calorie you don't log is a calorie you eat over it. If your budget is learned from your own log and your own scale, what you leave out is left out of both, and it cancels. Then a rough log is enough, as long as it's rough the same way most days.

Every food log is short

When researchers measure what people really burn (with doubly labelled water, the most accurate method there is) and compare it with what they reported eating, the reports come up short, and individual gaps of half are not unusual. Apps don't fix this. In early results from a test led by the US National Institutes of Health this year, four photo-logging apps underestimated real meals by 250 to 345 kcal each, mostly missing fat: butter, oil, cheese, sauces.

That isn't a character flaw. Hidden calories are hidden.

Why it breaks a formula budget

A formula guesses how much you burn from your age, height, weight and activity, then subtracts a deficit. Say it guesses 2,500 kcal a day and gives you a budget of 2,000. If your log misses 400 kcal a day, you're really eating 2,400: almost no deficit, while the app says you're on target. Weeks later the scale hasn't moved, and the usual advice is to weigh everything and log harder.

Why it doesn't break a learned budget

Your scale doesn't care what you logged. Over a few weeks, the trend of your weight tells you whether you ate more or less than you burned. Put that next to what you logged, and you can work out what you maintain on as you log it.

Say you really eat 2,500 kcal a day and log 2,100 of it. Your trend is flat.

So, in your log's own units, you maintain on 2,100. A budget a few hundred under that, say 1,800 logged, means you really eat about 2,200: a real deficit of about 300 kcal a day, and the 400 you never log is in both numbers, so it cancels.

This works with any steady habit of missing things, which is everyone's. It only needs two things: you weigh yourself most mornings, and you log the same way most days. If you suddenly start logging the oil, the learned number adjusts within a couple of weeks, the same way.

What it doesn't fix

A log that's half-empty some weeks and complete in others can't be learned from, because there's no steady habit to learn. That's why Moderly learns from whole weeks, moves its number gently from one Sunday to the next, and never changes your budget by more than one step at a time. And it can't see what you never log at all, like a holiday week; it just picks the trend back up afterwards.

Where the idea comes from

Learning maintenance from intake and weight change isn't new: it's how researchers estimate energy balance, and paid apps such as MacroFactor are built on it. Moderly does it for free, keeps the budget in your log's own units, and says so on the Food screen, so you never have to wonder why the number isn't a textbook one.

Moderly is free, open source and has no ads. Log the way you already do; your budget is learned from your scale and your log, and it never asks you to log more.
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