How this is worked out

Two steps, and no secret third one.

1. Your BMR, from the Mifflin-St Jeor equation

Basal metabolic rate is what you would burn lying still all day: breathing, circulating, staying warm. It is the great majority of most people's daily burn.


      BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age in years) + 5

      …and the same equation with − 161 in place of + 5 for women and for anyone who would rather not say.

Those two constants are all the equation says about sex; there is no third one, which is why the third option here takes the −161 branch rather than a number we would have had to invent.

2. Your activity factor, applied on top

BMR is multiplied by a physical activity level — the conventional PAL bands — to reach maintenance calories.

Activity level	Looks like	Factor
Sedentary	Desk job, little to no regular exercise.	× 1.2
Light	Light exercise or active hobbies 1-3 days/week.	× 1.375
Moderate	Standing or walking most of the day. Active lifestyle.	× 1.55
Active	Heavy training or physically demanding job.	× 1.725

What this number cannot know

It is an estimate, and it is worth being specific about why rather than hiding behind the word.

Mifflin-St Jeor is a prediction fitted to healthy adults. It does not measure your metabolism — only indirect calorimetry does that — and for any one person it can sit a couple of hundred calories either side of the truth.
The equation has no term for body composition. Two people at the same weight, height, age and sex get the same answer, even though the one carrying more muscle burns more.
The activity factor is a band you picked from four, describing a category of lifestyle rather than your week. Between two of them, the lower one is the safer starting point.
Your expenditure moves as your weight does, so a number worked out today is not the number in three months.

So treat it as a starting point rather than a verdict. Eat at it for two to three weeks, weigh yourself regularly, and let the trend on the scale correct the estimate — your own weight over time is better evidence about your expenditure than any equation. That is exactly what the app does with it: this figure is where a new account starts, and the weight trend is what moves it afterwards.

None of this is medical advice. If you are pregnant, under 18, or managing a health condition that affects your weight or appetite, work your targets out with a doctor or dietitian rather than with a calculator.

Sources

The same references the app cites on its Sources & References screen, because it is the same arithmetic.

Mifflin MD, St Jeor ST, Hill LA, Scott BJ, Daugherty SA, Koh YO. A new predictive equation for resting energy expenditure in healthy individuals. American Journal of Clinical Nutrition. 1990;51(2):241-247. doi.org/10.1093/ajcn/51.2.241
Frankenfield D, Roth-Yousey L, Compher C. Comparison of predictive equations for resting metabolic rate in healthy nonobese and obese adults: a systematic review. Journal of the American Dietetic Association. 2005;105(5):775-789. (Found Mifflin-St Jeor the most reliable of the common equations.) doi.org/10.1016/j.jada.2005.02.005
FAO/WHO/UNU. Human Energy Requirements: Report of a Joint FAO/WHO/UNU Expert Consultation. FAO Food and Nutrition Technical Report Series 1. Rome, 2004. (Chapter 5 defines the physical activity level bands these factors follow.) fao.org/4/y5686e/y5686e00.htm
Institute of Medicine. Dietary Reference Intakes for Energy, Carbohydrate, Fibre, Fat, Fatty Acids, Cholesterol, Protein, and Amino Acids. Washington DC: National Academies Press, 2005. (Chapter 12, physical activity coefficients.) doi.org/10.17226/10490

Does this calculator store anything?

No. Your height, weight, age and activity level are used to work out the answer on this request and are never written down anywhere — there is no account here and nothing to sign into. They do appear in the page address, which is what makes a result shareable, so treat the link the way you would treat the numbers in it. Privacy Policy

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