Guide

How many calories should I eat?

Most adult women need between 1,600 and 2,400 calories a day, and most adult men between 2,000 and 3,000. Your position in that range depends on your size, your age, how much you move, and whether you want your weight to fall, hold or rise. A calculator narrows it in about thirty seconds.

Written by the Calybite team · Last updated 24 August 2026

Those ranges come from population dietary guidelines, and they are wide for a reason: a sedentary 60-year-old and a manual labourer in their twenties are both “adults” and can differ by well over a thousand calories a day. The useful question is not what adults need, but what you need.

The three-step version

Every credible answer to this question follows the same three steps.

1. Estimate what you burn at rest        → BMR
2. Multiply by how much you move         → TDEE
3. Add or subtract for your goal         → daily target

You can run all three at once with the calorie calculator, or take them separately: see what you burn at rest, then find your maintenance calories, then size your calorie deficit.

What actually moves the number

In rough order of influence: body weight, then activity level, then height, then age, then sex. Weight dominates because it appears in the equation with the largest coefficient and because a heavier body costs more to move. Age is gentler than people expect — the Mifflin-St Jeor equation subtracts five calories per year, so a decade costs about fifty calories a day, not the collapse that “metabolism slows after thirty” implies.

The activity multiplier is where most errors enter. It ranges from 1.2 to 1.9, so choosing one level too high inflates your target by roughly ten percent — for many people, two to three hundred calories a day. That is enough to turn an intended deficit into maintenance, and it is the single most common reason a calorie target quietly fails to work.

When in doubt, go one level down. Self-reported activity skews high almost universally. If you are between “lightly active” and “moderately active”, pick lightly active and let two weeks of weight data correct you upward if you were wrong.

Treat the output as a hypothesis

Predictive equations land within about ten percent of measured expenditure for most healthy adults. That is genuinely useful for choosing a starting point and useless for arguing about a single day. The right way to use your number is as a two-week experiment: eat at it, weigh yourself regularly enough to see a trend rather than daily water-weight noise, and adjust by 100 to 200 calories if the trend is not going where you intended.

After a month, your own data outranks any equation on this site. The calculator’s job was only ever to stop you starting from nothing.

One thing that never counts against the number: whether water has calories — plain water is zero, though tonic and sweetened waters are not.

Where it stops applying

These estimates assume a generally healthy adult. They fit poorly for people who are very muscular or very lean, for the elderly, during pregnancy or breastfeeding, and for anyone with a thyroid condition or on medication affecting metabolism. If you are recovering from an eating disorder, calorie targets from a web calculator are the wrong tool entirely — that belongs with a clinician.

The part nobody tells you

Getting the number right is maybe a fifth of the problem. The other four fifths is knowing what you actually ate, and that is where most attempts fall apart — not through lack of willpower, but through the sheer tedium of searching a database for every meal and guessing which of forty “chicken curry” entries matches your plate.

Reducing that friction is the entire premise of Calybite, the AI food-tracking app: it names the components from one photo, estimates the portions, and shows you the result to confirm before anything is saved. It will not be as precise as a kitchen scale. It will, for most people, be the difference between logging and not logging — and over a month, that gap matters far more than a hundred calories of precision.

For the mechanics of that, what actually happens when you scan a plate breaks the process down, and how accurate photo calorie counting really is covers what the independent research says about it.

Sources

Mifflin MD, St Jeor ST, et al. “A new predictive equation for resting energy expenditure in healthy individuals.” Am J Clin Nutr, 1990.
U.S. Department of Agriculture & U.S. Department of Health and Human Services. Dietary Guidelines for Americans, 2020–2025 — source of the population calorie ranges quoted above.
Frankenfield D, et al. “Comparison of predictive equations for resting metabolic rate.” J Am Diet Assoc, 2005 — on the ~10% accuracy band.