The FoodGPT calorie calculator: how it works and where it goes wrong
The calorie calculator works out basal metabolism with the Mifflin–St Jeor equation, multiplies it by an activity factor from 1.2 to 1.9, and shows the intake at which your weight stays the same, plus ranges for losing and gaining weight. For an individual the result can differ from real expenditure by several hundred kilocalories, so the number is checked against your weight.
What goes in and what comes out
You enter sex, age, weight, height and one of five activity levels. The calculator accepts ages from 18 to 100, weights from 30 to 250 kg and heights from 120 to 230 cm. Outside those limits there is no answer.
The answer has five numbers:
- maintenance: the intake at which your weight stays the same;
- basal metabolism, from which maintenance is derived;
- the activity factor basal metabolism is multiplied by;
- a range for losing weight: 80 to 90% of maintenance, that is, a 10–20% deficit;
- a range for gaining weight: 105 to 110% of maintenance, that is, a 5–10% surplus.
Every number is rounded to the nearest ten, because the equation is no more precise than that. The shares used for the ranges are settings of the calculator itself, not a standard.
The equation and where it comes from
Basal metabolism is calculated like this:
Men: 10 × weight (kg) + 6.25 × height (cm) − 5 × age (years) + 5
Women: 10 × weight (kg) + 6.25 × height (cm) − 5 × age (years) − 161
Mifflin, St Jeor and colleagues published it in 1990 in the American Journal of Clinical Nutrition. It was derived from measurements of 498 healthy people: 247 women and 251 men aged 19 to 78, 45 on average. Of them, 264 were of normal weight and 234 were obese. Resting energy expenditure was measured by indirect calorimetry. The equation explains 71% of the variation in those measurements (R² = 0.71). In the same study the 1919 Harris–Benedict equations overestimated measured expenditure by 5% on average. Mifflin et al., 1990.
The calculator uses the simplified version with coefficients of 10, 6.25 and 5. The authors showed that rounding the coefficients did not affect predictive value.
Basal metabolism is only what you burn at rest. Everything else, digesting food, everyday movement and exercise, the calculator adds with a single multiplier: 1.2, 1.375, 1.55, 1.725 or 1.9. The same arithmetic on paper is in the article on daily calorie intake, and how the equations differ from one another is covered in the article on which formulas calorie calculators use.
Example: one person, five activity levels
A woman aged 30, 70 kg, 170 cm. Basal metabolism: 10 × 70 + 6.25 × 170 − 5 × 30 − 161 = 1451.5, shown as 1450 kcal. From there everything depends on the row you pick:
| Factor | Maintenance, kcal | Loss range, kcal | Gain range, kcal |
|---|---|---|---|
| 1.2 | 1740 | 1390–1570 | 1830–1920 |
| 1.375 | 2000 | 1600–1800 | 2100–2200 |
| 1.55 | 2250 | 1800–2020 | 2360–2470 |
| 1.725 | 2500 | 2000–2250 | 2630–2750 |
| 1.9 | 2760 | 2210–2480 | 2900–3030 |
The body is the same, yet the first and last rows are more than a thousand kilocalories apart. Neighbouring steps differ by about 250 kcal.
Where the calculator goes wrong
The equation is off for individuals. A 2005 systematic review by the American Dietetic Association compared the four most commonly used resting-metabolism equations. Mifflin–St Jeor came out best: it predicted within 10% of measured in more people than the others, and its error range was the narrowest. The same authors write that noteworthy errors remain for an individual. Frankenfield et al., J Am Diet Assoc.
A small 2016 check shows the size of that error: 30 adults aged 18 to 65 with BMI 19 to 39, resting expenditure measured by indirect calorimetry. For every equation tested, the limits of agreement (two standard deviations) ran from 314 to 445 kcal a day. The mean of Mifflin–St Jeor differed significantly from the measured value. The authors conclude that at the individual level even the equations most accurate on average performed poorly, and that expenditure was underestimated more as fat-free mass increased. Flack et al., J Acad Nutr Diet, 2016. The sample is small and mostly white, so it shows the order of magnitude. The calculator asks for neither body fat nor muscle mass, so it cannot adjust for body composition.
Then the error is multiplied. The calculator multiplies basal metabolism by a factor, so an error of 380 kcal at rest becomes about 520 kcal a day at a factor of 1.375. Choosing the factor adds its own uncertainty: for this person neighbouring steps are about 250 kcal apart, as in the table above.
The ranges are narrower than the error. At maintenance of 2000 kcal a 10–20% deficit sits 200–400 kcal away, which is less than the possible error in maintenance itself. A range shows a proportion, not an exact target.
The age limits are wider than the sample. The calculator accepts ages 18 to 100, while the equation was derived in people aged 19 to 78. The authors of the 2005 review note that older adults were underrepresented both in deriving the equations and in validating them, and advise a high level of suspicion about accuracy for such groups.
At the lowest maintenance levels the range collapses. The calculator does not take the loss range below 1200 kcal for women or 1500 kcal for men. A 60-year-old woman, 50 kg, 155 cm, factor 1.2, gets maintenance of 1210 kcal, and a 10–20% deficit would give 970–1090. The calculator shows the floor instead, and an intake that low is for a doctor to decide, not a formula.
How to use the result
- Treat maintenance as a hypothesis. It is an estimate from averages.
- Eat at roughly that intake for two to three weeks without changing your habits and track your weight as a weekly average. Look at the average for the week rather than single weigh-ins; why weight swings during a day is explained in a separate article.
- If the average holds, you have found your own maintenance, and it matters more than the formula's number. If it drifts one way, that drift is the correction the number needs.
- Recalculate after a noticeable change in weight or lifestyle: expenditure changes with them. How it changes during weight loss is covered in the article on metabolic adaptation.
The loss range is a span to choose an intake you can hold for weeks from, not a lower edge to pick by default. Constant hunger that does not pass after eating, poor sleep and thoughts of food that crowd out everything else are a reason to reduce the deficit, and if they persist, to talk to a doctor. How a deficit is calculated, and why the arithmetic overshoots reality, is in the articles on calorie deficit and on calories in a kilogram of fat.
The same calculation shown as daily expenditure is in the TDEE calculator; how to pick its factor and check the result is in the article on TDEE.
When the number is not for you
- Pregnancy and breastfeeding. The calculator does not apply to these states.
- Under 18. The calculator does not accept that age.
- Medical conditions and regular medication, including thyroid disease: in these cases a doctor sets calorie intake. On the thyroid and weight, see this article.
- Anxiety around counting. If the numbers bring guilt or fear of food, the question is no longer about the formula and is best discussed with a specialist. When calorie counting becomes a problem.
Weight that goes down without any change in food or activity is a separate case. The UK's NHS advises seeing a GP if you keep losing weight without changing your diet or exercise routine, and as soon as possible if other symptoms come with it. NHS page on unintentional weight loss.
You can work out your own maintenance in the calorie calculator.
Frequently asked questions
- Which formula does the FoodGPT calorie calculator use?
- The Mifflin–St Jeor equation (1990): basal metabolism = 10 × weight + 6.25 × height − 5 × age, plus 5 for men and minus 161 for women. The result is multiplied by an activity factor from 1.2 to 1.9.
- How accurate is the calorie calculator?
- For an individual the gap can be several hundred kilocalories. In a 2016 check on 30 adults the limits of agreement between equations and measurement ran from 314 to 445 kcal a day for basal metabolism alone, and the activity factor adds its own uncertainty.
- Why does the loss range not go below 1200 or 1500 kcal?
- The calculator does not take the loss range below 1200 kcal for women or 1500 kcal for men. An intake that low is for a doctor to decide, not a calculator.
- How do I check the number from the calculator?
- Eat at roughly that intake for two to three weeks and compare your average weight week by week. If it holds, that is your maintenance; if it drifts one way, the drift is the correction.
- Who is the calculator not suitable for?
- People who are pregnant or breastfeeding, people under 18, and anyone with a medical condition or on regular medication: in these cases a doctor sets calorie intake.
Read next
- Daily calorie intake — There is no universal norm: it depends on weight, height, age, sex, and activity level.
- Calorie calculator: what formulas are inside and why numbers differ — Two calculators using the same height, weight, age, and gender can provide numbers that differ by two or three hundred kilocalories.
- TDEE: how the calculator works out daily expenditure and how to check your number — TDEE (total daily energy expenditure) is everything the body burns in a day: basal metabolism, digesting food and all movement.
- Calorie deficit: how to calculate it and how to hold it — A calorie deficit is the one thing without which weight does not come down.
This article is for general information. It is not medical advice, a diagnosis, or a prescription for treatment or a diet, and it does not replace a consultation with your doctor. If you have a health condition, are pregnant, take medication, or follow a diet prescribed to you, decisions about food belong with your doctor.
Figures from regulations, guidelines and studies are given as they stood when this article was prepared and may since have changed; check them against the primary sources. This article is not advertising, an offer, or individual advice, and neither the author nor the site owner is responsible for decisions taken on the basis of it.