How accurate are calorie calculators?

The underlying equations predict measured energy expenditure within about ten per cent for most people, but the activity multiplier adds more uncertainty than the equation does. Treat the output as a starting point to test over two weeks, not a precise figure. Your actual weight trend is a better estimate of your expenditure than any formula.

Why have I stopped losing weight on the same calories?

A lighter body burns less at rest, unconscious daily movement tends to fall during a deficit, and logging accuracy drifts over time. All three shrink the deficit without anything appearing to change. Recalculate your target at your current weight and audit a few days of logging carefully before concluding your metabolism is broken.

Does exercise burn enough to matter for weight loss?

Less than most people assume. Deliberate exercise is often only five to fifteen per cent of total expenditure, and appetite frequently rises to meet it. Exercise is excellent for cardiovascular health, strength, mood and preserving muscle during a deficit. Just not efficient as the primary tool for changing weight.

Is 1,200 calories a day a safe target?

It is below the basal metabolic rate of most adults and is rarely appropriate without supervision. Very low intakes make adequate protein and micronutrients hard to achieve, cost more muscle, and are difficult to sustain. Which is why they so often end in regain. A moderate deficit held for longer beats an aggressive one abandoned.

How much protein should I eat while losing weight?

Roughly 1.6 to 2.2 grams per kilogram of body weight covers most people in a deficit. Higher protein preserves muscle when energy is restricted and is the most satiating macronutrient, which makes the target easier to hold. Spreading it across meals rather than concentrating it in one helps.

Calories, TDEE and Why the Numbers Only Get You Started

The four components of what you burn

Total daily energy expenditure is the sum of four things. Basal metabolic rate. What your body uses at complete rest to run organs, maintain temperature and repair tissue. Is the largest, typically sixty to seventy per cent of the total. The thermic effect of food, the energy used digesting and absorbing what you eat, is around ten per cent.

Deliberate exercise is usually the smallest component, often only five to fifteen per cent even for people who train regularly. The fourth, non-exercise activity thermogenesis, covers everything else: walking, fidgeting, standing, gesturing, household tasks. It varies between individuals by several hundred calories a day and is the least controllable.

The practical implication is that exercise is a poor primary lever for weight change and an excellent one for health, fitness and appetite regulation. Thirty minutes of moderate exercise burns roughly what is in a muffin.

How the equations work and how wrong they are

Basal metabolic rate is estimated from height, weight, age and sex using equations such as Mifflin-St Jeor or Harris-Benedict. The result is multiplied by an activity factor between about 1.2 for sedentary and 1.9 for very heavy training to arrive at total expenditure.

These equations predict measured expenditure within roughly ten per cent for most people, which on a 2,200 calorie estimate is a couple of hundred calories either way. Body composition is the main source of error: muscle is more metabolically active than fat, and the equations cannot see it, so they underestimate for muscular people and overestimate for the opposite.

The activity multiplier introduces more error than the equation itself, because people consistently overestimate how active they are. Choosing the multiplier one step below your instinct is a reasonable correction.

Deficits, and the adaptation nobody mentions

A pound of body fat stores roughly 3,500 calories, so a 500 calorie daily deficit predicts about a pound a week. This rule works reasonably at first and then progressively overstates the loss.

Three things change. A lighter body needs less energy simply to exist, so the deficit narrows as you lose weight. Non-exercise activity falls, often unconsciously. And measurement drift creeps in: portions grow, bites and tastes go uncounted, and the logged total diverges from the eaten one.

The consequence is that a target set once is wrong within a few months. Recalculating every five to ten kilograms, and adjusting based on actual weight trend rather than the equation, is what makes the process work over a year rather than six weeks.

Using the number properly

Treat the calculated target as a hypothesis, not a prescription. Eat at it for two weeks, weigh yourself at the same time under the same conditions, and take the average of each week rather than individual readings. Daily weight swings of one to two kilograms from water, food volume and hormones will otherwise drown the signal.

If the two-week average has not moved and you intended to lose, reduce the target by 100 to 200 calories rather than assuming the equation was badly wrong. If it is falling faster than about one per cent of body weight a week, add calories back: faster loss costs more muscle and is harder to sustain.

Protein is the single most useful thing to get right inside the target. It preserves muscle during a deficit and is the most satiating macronutrient, which makes the deficit easier to hold. Around 1.6 to 2.2 grams per kilogram of body weight covers most needs.

Where calorie counting is the wrong tool

For anyone with a history of disordered eating, tracking can be actively harmful, and the potential benefit does not justify the risk. Working with a practitioner on non-numeric approaches is the better path.

Athletes in heavy training, pregnant and breastfeeding people, children and adolescents, and anyone with a condition affecting metabolism or absorption all need individualised advice rather than a population equation. So does anyone on medication that affects appetite or weight.

For everyone else, the value of counting is mostly educational. A few weeks of accurate logging teaches you what portions actually contain, and most people can maintain the result afterwards without tracking at all.

Questions people ask

How accurate are calorie calculators?: The underlying equations predict measured energy expenditure within about ten per cent for most people, but the activity multiplier adds more uncertainty than the equation does. Treat the output as a starting point to test over two weeks, not a precise figure. Your actual weight trend is a better estimate of your expenditure than any formula.
Why have I stopped losing weight on the same calories?: A lighter body burns less at rest, unconscious daily movement tends to fall during a deficit, and logging accuracy drifts over time. All three shrink the deficit without anything appearing to change. Recalculate your target at your current weight and audit a few days of logging carefully before concluding your metabolism is broken.
Does exercise burn enough to matter for weight loss?: Less than most people assume. Deliberate exercise is often only five to fifteen per cent of total expenditure, and appetite frequently rises to meet it. Exercise is excellent for cardiovascular health, strength, mood and preserving muscle during a deficit. Just not efficient as the primary tool for changing weight.
Is 1,200 calories a day a safe target?: It is below the basal metabolic rate of most adults and is rarely appropriate without supervision. Very low intakes make adequate protein and micronutrients hard to achieve, cost more muscle, and are difficult to sustain. Which is why they so often end in regain. A moderate deficit held for longer beats an aggressive one abandoned.
How much protein should I eat while losing weight?: Roughly 1.6 to 2.2 grams per kilogram of body weight covers most people in a deficit. Higher protein preserves muscle when energy is restricted and is the most satiating macronutrient, which makes the target easier to hold. Spreading it across meals rather than concentrating it in one helps.

Sources

Australian Dietary Guidelines: National Health and Medical Research Council. Evidence-based Australian nutrition guidance