Maintenance calories: the number everything else hangs on

Before you can eat in a deficit or a surplus, you need to know what "zero" is. Maintenance calories are that zero: the amount you can consume on an ordinary day and weigh the same next month. Every target you will ever set is that number plus or minus something, which makes it the most important figure in nutrition and, strangely, the one almost nobody knows.

People know their bench. They know their five-kilometre time to the second. Ask them what their body burns on a Tuesday and you get a shrug, or a number they read off a calculator two years ago and never checked. This post is about that number: what it is made of, why the calculators are usually a few hundred calories off, and how to find yours without a lab.

What the number is made of

Your daily burn has three big parts and one small one. The biggest is your resting metabolism: what it costs to keep a body alive, warm and thinking, even if you never got out of bed. For most people that is well over half of the whole day. Then comes everything you do that is not training - walking to the bus, standing at the counter, fidgeting through a meeting. It is boring, and for many people it outweighs their actual workouts. The workouts themselves are the third part, and they are smaller than the fitness industry would like you to believe: an hour of hard lifting is real, but it is not the thousand calories a machine's display promises. The small part is digestion, which burns a little of what you consume just to process it, protein more than the rest.

Add those up and you have maintenance. Eat that and you hold. Eat less, over weeks, and you lose. Eat more, over weeks, and you gain. Everything else in this field is detail on top of that sentence.

Why the calculators get it wrong

Every online calculator does the same two steps. First it estimates your resting metabolism from your sex, age, height and weight, usually with the Mifflin-St Jeor equation, which was fitted to a few hundred people in 1990 and holds up surprisingly well as a population average. Then it multiplies that by an "activity factor" you pick from a list: sedentary, lightly active, moderately active, and so on up.

The first step is fine. The second is where the number goes wrong, and it goes wrong by a lot. The factors spread from roughly 1.2 to 1.9, so the difference between the band you picked and the band you actually live in is easily three or four hundred calories a day. And people are bad at picking. Almost everyone reads "moderately active" as a compliment and clicks it. A desk worker who trains hard four evenings a week and sits the rest of the time is a genuinely difficult case, and the list does not have a row for them.

The population average has its own problem too. The equation describes a typical body for your height and weight. If you carry more muscle than typical, it undershoots; more fat than typical, it overshoots. The equation cannot see that, because it was never told.

Still want the starting guess? Our maintenance calorie calculator does exactly those two steps in your browser, and shows how far the answer moves between one activity level and the next.

The only measurement that counts

Here is the thing the calculators are standing in for: the scale. If you eat a consistent amount for a few weeks and your weight trend is flat, that amount was your maintenance, whatever a formula says. If the trend drifts down, you were under it, and the rate of the drift tells you by roughly how much. That is not an approximation of the number. That is the number, measured on the one instrument that actually matters.

Doing it by hand takes some patience and two habits. Weigh in most mornings, because a single weigh-in is noise (water, salt, what time you ate last night) and only the trend over a couple of weeks means anything. And log what you consume honestly for the same stretch, because "roughly two thousand" is not data. Then compare: the average you consumed, against what the trend did. A pound of body weight is about three and a half thousand calories, so a trend that loses half a pound a week says you were around two hundred and fifty a day under maintenance.

People find this unglamorous and then find it works. The calculator gave you a starting guess. The scale and the log correct it, and unlike the calculator, they keep correcting it as you change - because a body that has lost ten kilos burns less than it did, and a body that has spent a winter building muscle burns more.

What to do with it

Once you have the number, targets become simple arithmetic. Cutting is maintenance minus a moderate amount, small enough that training does not fall apart. Building is maintenance plus a smaller amount than most people think, because muscle grows slowly and the surplus beyond what it can use just becomes the other thing. Maintaining is the number itself, which is harder than it sounds and worth practising: most people have never spent a month deliberately eating at zero.

And check it again in a few months. Not because the first measurement was wrong, but because you will not be the same person. That is the whole point of measuring instead of calculating.

Sources

• Mifflin MD et al., "A new predictive equation for resting energy expenditure in healthy individuals", The American Journal of Clinical Nutrition, 1990. pubmed.ncbi.nlm.nih.gov/2305711
• Hall KD et al., "Quantification of the effect of energy imbalance on bodyweight", The Lancet, 2011 - why the old "a pound is three and a half thousand calories" rule is a useful approximation and not a law. pubmed.ncbi.nlm.nih.gov/21872751