What it is#
Maintenance is the daily intake at which your weight would hold steady. Eat above it and you gain, below it and you lose. Everything else in calorie tracking is downstream of this one number: a deficit is defined against it, a budget is it plus a delta, a goal date is arithmetic on the gap.
It is also the number nobody can look up for you.
What it is made of#
Four parts, in descending order of size for most people:
- Basal rate — what you burn at complete rest. Typically 60–70% of the total.
- Everyday movement — walking, standing, fidgeting, carrying things. The most variable part between two similar people, and the reason two office workers can differ by several hundred calories.
- Digestion — processing food itself costs energy, around 10% of intake, and more for protein than for fat or carbohydrate.
- Deliberate exercise — usually the smallest of the four, which surprises people.
A formula can predict the first with reasonable accuracy. The second is the one it cannot see, and it is the one that varies most.
Why two calculators disagree#
Because they are predicting, and predictions have error bars.
The common equations — Mifflin-St Jeor, Harris-Benedict, Katch-McArdle — are fitted to groups of people. For an individual, the true resting rate scatters around the prediction by roughly 10% in either direction. Then an activity multiplier chosen from a dropdown is applied on top, and that dropdown is standing in for everything you do between waking and sleeping.
Two people who give identical answers to every question can genuinely need several hundred calories a day apart. That is not a flaw in any particular calculator. It is what a population average is.
Why yours changes#
Maintenance is not a constant you discover once.
- It falls as you get lighter. A smaller body costs less to run, and moving it costs less too.
- It falls further during a sustained deficit than size alone explains — metabolic adaptation. Less spontaneous movement, lower resting expenditure, the same activity feeling harder.
- It moves with your life. A new job, a new season, an injury, a house move.
- It rises with muscle, slowly and by less than fitness marketing suggests.
Which means a maintenance figure calculated once, in the first minute of using an app, and never revisited, is wrong by a growing amount as time passes — and nothing in it is measured, so nothing in it can update itself.
How it can be measured instead#
Energy balance is arithmetic, and it can be run backwards. If you know roughly what you ate over a window of days, and which way your weight actually moved over the same window, you can solve for the maintenance figure that would explain both:
maintenance = average intake − (weight trend in kg/day × 7700)
The 7700 is the energy in a kilogram of body mass. The weight trend is the slope of a line through your weigh-ins, not the difference between two mornings.
This is what SmartCrunch does, and the whole method is published — including the four conditions it insists on before it will answer, and the reason the coverage threshold is 85% rather than something more permissive.
The catch, stated plainly#
The method inherits whatever bias your diary has and calls it your metabolism.
If you consistently under-record portions by a tenth, the solved figure comes out about a tenth low. That answer is correct for the diary you keep — the budget it produces is self-consistent and will move your weight the way you asked — but it is not the same thing as your physiology, and no arithmetic inside the app can tell the two apart.
That is why there is a plausibility band around the answer, why it is deliberately tighter on the low side than the high side, and why the app would rather fall back to a formula and tell you it has done so than print a number it cannot support.
What to do with all this#
If you are starting out, take a calculator’s number, use it for a fortnight, and watch the trend rather than the mornings. If the line moves the way you intended, the estimate was close enough. If it does not, the estimate was wrong — and by then you have exactly the data needed to work out what it should have been.
Read next
- How many calories should I eat to lose weight? The ten-second number is an average of people shaped like you. Your own logged weeks give a better one — and this explains how, and when not to trust either.
- What is TDEE, and why do calculators disagree? TDEE is maintenance under another name. The disagreement between calculators is almost entirely the activity multiplier, and it is the part no form can know.