How Many Calories Should You Eat? Working From BMR to a Real Target

The short answer for most adults is somewhere between 1,600 and 3,000 kcal a day, and the range is that wide because the question is really three questions stacked on top of each other. What does your body spend at rest? What does your movement add? And what are you trying to make happen to your weight?

A calculator answers the first two with a regression equation and a multiplier. The third one is yours. This article is about the second half of that process: taking a maintenance figure and turning it into a number you eat to, choosing how aggressive to be, recognising the floor below which the plan stops working, and knowing which signals mean the target needs changing. For how the underlying equations were derived and where their error comes from, we cover that separately in calorie needs, equations and calibration.

Maintenance first, target second

The most common mistake is going straight to a goal number. It produces targets that are either too aggressive to hold or so mild that nothing moves for two months, and in both cases you have no way of knowing which, because there was never a reference point.

Maintenance is that reference point: the intake at which your weight holds steady. Estimating it takes two steps. Basal metabolic rate comes first, the energy your body spends staying alive at complete rest, which for a sedentary adult is roughly sixty to seventy percent of the daily total. Then an activity multiplier scales it up to cover movement, digestion and everything else.

Our TDEE calculator uses Mifflin-St Jeor as the default basal equation, published by Mifflin and St Jeor in the American Journal of Clinical Nutrition (51:241-247, 1990) and fitted to indirect calorimetry measurements across several hundred healthy adults. Frankenfield, Roth-Yousey and Compher reviewed the competing equations in the Journal of the American Dietetic Association (2005) and found it the most reliable of the group, landing within ten percent of measured resting rate in roughly four out of five people.

That accuracy statement is worth reading twice. Within ten percent for four in five means one person in five sits further out than that, and ten percent of a basal rate is well over a hundred kilocalories before any multiplier touches it. The multiplier then scales the error along with the estimate.

Two worked examples

Abstract percentages are hard to reason about, so here are two real runs through the same engine the site uses.

A 34-year-old woman, 68 kg and 165 cm, training three or four times a week. Mifflin-St Jeor puts her basal rate at 1,380 kcal. The moderate activity multiplier of 1.55 gives a maintenance estimate of 2,139 kcal.

A 40-year-old man, 82 kg and 178 cm, exercising once or twice a week. His basal rate comes out at 1,738 kcal, and the light multiplier of 1.375 gives 2,389 kcal.

Now look at how much those figures move when you change one input. Run the same woman through the revised Harris-Benedict equation, as published by Roza and Shizgal in the American Journal of Clinical Nutrition (1984), and her basal rate reads 1,440 rather than 1,380. That works out to a maintenance estimate of 2,233 instead of 2,139. Nearly a hundred kilocalories of disagreement between two respectable equations describing the same person.

The activity multiplier moves it far more. Holding her basal rate constant and shifting from the sedentary multiplier to the moderate one takes maintenance from 1,656 to 2,139 kcal. Even one step, from lightly active to moderately active, is 241 kcal. That single dropdown choice carries more consequence than which basal equation you pick, which is why it deserves more honesty than it usually gets.

Choosing the multiplier honestly

People place themselves too high, consistently. Training three hours a week feels active, but measured against a hundred and twelve waking hours it does not shift the weekly average as far as intuition suggests, particularly when the rest of the week is spent sitting.

If you are genuinely torn between two levels, take the lower one. Discovering that you are losing weight faster than planned and adding food back is a pleasant problem. Spending eight weeks wondering why a deficit that was never a deficit produced nothing is not.

Applying the deficit or surplus

With a maintenance figure in hand, the target is a percentage adjustment. The TDEE calculator presents five, and the reasoning behind each is worth stating plainly.

Percentages rather than flat numbers, because a 500 kcal deficit is a different proposition for someone maintaining on 3,300 than for someone maintaining on 1,700. For the first it is a fifteen percent trim; for the second it is close to a third of their intake, with all the hunger and adherence problems that implies.

For most people wanting to lose fat, the fifteen percent figure is the better default. A deficit in the ten to twenty percent band is generally sustainable, preserves lean mass better than an aggressive cut when protein intake is adequate and resistance training is present, and leaves room to tighten later. The twenty-five percent option has a place earlier in a diet, at higher starting body fat, or when someone has a fixed deadline. But it comes with real costs in training quality, hunger and the risk of losing more lean tissue than necessary.

On the gaining side, the asymmetry surprises people. A ten percent surplus is enough to support muscle gain in most trained lifters, and going higher mostly buys fat. Rate of gain is limited by how fast you can actually build tissue, not by how much you eat above maintenance.

The floor, and why it exists

Below roughly 1,200 kcal for women and 1,500 for men, a self-directed diet starts running into problems that have nothing to do with willpower.

The first is nutritional. Meeting requirements for iron, calcium, essential fatty acids and the fat-soluble vitamins from a very small amount of food is difficult, and it gets harder as the food choices narrow the way they tend to under restriction. The second is that protein and fat both have floors of their own: protein to defend lean tissue, fat because two fatty acids are genuinely essential. At a low enough total there is nothing left for carbohydrate to fuel training with. Our macro calculator raises a warning when fat falls below 0.6 g/kg for exactly this reason; on an aggressive cut at higher body weights, that threshold gets crossed.

The third is hormonal and it is well documented. Loucks and Thuma, in the Journal of Clinical Endocrinology and Metabolism (2003), showed that luteinising hormone pulsatility in exercising women was disrupted once energy availability fell below a threshold, independently of body composition. The relative energy deficiency in sport framework, set out by Mountjoy and colleagues in the British Journal of Sports Medicine (2014), extended that picture across bone health, immune function, metabolic rate and performance in both sexes.

Very low calorie diets do exist as a legitimate clinical intervention, and they work. They are run with medical supervision, formulated products and monitoring. That is a different thing from setting a calculator to its most aggressive option and hoping.

Why the number expires

Energy expenditure is not a constant you discover once. It falls as you lose weight, partly because a smaller body costs less to run and partly through adaptive thermogenesis, a reduction beyond what the change in mass alone predicts. Rosenbaum and Leibel reviewed this in the International Journal of Obesity (2010) and described the downward adjustment persisting well past the period of active loss.

The practical consequence is that a deficit which worked in week two will not still be the same deficit in month four. Hall and colleagues, writing in the Lancet (2011), made the quantitative version of this point: the familiar rule that 3,500 kcal equals a pound of fat overstates long-run loss precisely because expenditure declines as body mass does. Over a fortnight it is a serviceable approximation. Over six months it is not.

Plan for recalibration rather than treating it as failure. Every four to six weeks, or whenever progress stalls for a fortnight against consistent intake, the figure needs revisiting. We go into the mechanism in more detail in metabolic adaptation explained, and the same reasoning is why a stall is usually a signal rather than a defeat.

Reading the feedback

The target is a hypothesis. Your weight trend tests it, and the test needs to be set up properly to mean anything.

Weigh daily, under consistent conditions, and average across seven days. Day-to-day weight moves a kilogram or more on water balance, glycogen and gut contents, so comparing Tuesday to Wednesday tells you nothing. Comparing this week's average to last week's tells you a great deal. A weekly change of about half a pound implies your intake sits roughly 250 to 300 kcal away from maintenance.

One caution about the input side. Lichtman and colleagues, in the New England Journal of Medicine (1992), studied people who reported being unable to lose weight on low intakes and found they under-reported food by close to half while over-reporting activity by around a quarter. The participants were not being dishonest; portion estimation is genuinely hard, and cooking oils, drinks and food eaten standing up are easy to miss. When your tracked intake and your weight trend disagree sharply, suspect the tracking first. We cover the size of that error in calorie tracking accuracy.

Where a calculator is the wrong instrument

Predictive equations were fitted to healthy adults, and they carry assumptions that do not hold everywhere. Pregnancy and breastfeeding change requirements substantially. Thyroid disorders, some medications and a number of metabolic conditions alter expenditure in ways no dropdown captures. Growth means children and adolescents need clinical guidance rather than an adult regression.

And if counting has become distressing, or if the number on the scale governs how the day goes, that is worth raising with a clinician. The evidence base for calorie targets says nothing about whether tracking is the right tool for a particular person, and for some people it is not.

What the number gives you is a defensible starting point and a way to be wrong usefully. Set it, hold it steady for a fortnight, then let your own data correct it. That loop matters more than the precision of the first estimate.