Calorie calculator
Enter your age, sex, weight and height to estimate the calories that your body uses at rest, then pick how active you are to see the calories that keep your weight. The page shows the working and where the equation comes from.
This answer is for the example values in the boxes above. Woman of 35, 150 lb, 65 in, activity factor 1.375. Change the boxes to see your own answer.
This is an estimate. A review of the studies found that the equation of Mifflin and St Jeor came within 10 percent of the measured energy at rest for more people than the other common equations did, and warned of noteworthy errors when it is applied to one person.
It does not tell you what to eat or how much to lose or gain. Talk to a doctor or a dietitian before you change how you eat.
How it works
There are two steps. The first is the energy that the body uses at rest. The calculator takes it from the equation of Mifflin and St Jeor. It is 10 times the weight in kilograms, plus 6.25 times the height in centimeters, less 5 times the age in years. Then it adds 5 for a man and subtracts 161 for a woman. Pounds and inches are turned into kilograms and centimeters with the exact factors of NIST, 0.45359237 and 2.54.
The second step multiplies that figure by a factor for your activity: 1.2 for little or no exercise, then 1.375, 1.55, 1.725 and 1.9. The result is the calories a day that keep your weight, as an estimate.
A worked example
A woman of 35 who weighs 150 pounds and is 65 inches tall uses 1,376 calories a day at rest. With light exercise, a factor of 1.375, her estimate is 1,892 calories a day.
A man of 35 who weighs 80 kilograms and is 180 centimeters tall uses 1,755 calories at rest. With moderate exercise, a factor of 1.55, his estimate is 2,720.
Where the equation comes from
Mifflin and St Jeor measured the energy at rest of 498 healthy people, 247 women and 251 men aged 19 to 78, by indirect calorimetry, and fitted an equation to their weight, height, age and sex. In their study the older Harris-Benedict equations of 1919 overestimated the measured figure by 5 percent.
The activity factors are not from that study. They are a convention: a trial of 2024 multiplied the resting figure of its patients by the same five numbers, and it is among the sources of the page.
What it leaves out
The equation was made from healthy adults, so this page is not for children, and it says nothing about someone who is ill. A review of the studies warned of noteworthy errors when the equation is applied to one person, and found that older adults and some ethnic groups were underrepresented. Under 19 and over 78 the calculator says that the estimate is less certain.
It does not say what to eat or how much to lose or gain. A doctor or a dietitian can answer that for you.
Sources
- Mifflin et al., A new predictive equation for resting energy expenditure in healthy individuals (American Journal of Clinical Nutrition, 1990), read on 2026-11-02.
- Frankenfield et al., Comparison of predictive equations for resting metabolic rate in healthy nonobese and obese adults: a systematic review (Journal of the American Dietetic Association, 2005), read on 2026-11-02.
- A randomized controlled trial of a one-year weight loss program in patients with severe obesity and obstructive sleep apnea (Nutrients, 2024), whose methods apply the activity factors, read on 2026-11-02.
- NIST Special Publication 811, Appendix B.8: factors for units listed alphabetically, read on 2026-11-01.
Questions
How do you calculate calories a day?
Work out the energy at rest with an equation, then multiply it by a factor for how active you are. For a woman of 35 who weighs 150 pounds and is 65 inches tall, the equation gives 1,376 calories, and a factor of 1.375 for light exercise gives 1,892.
What is the Mifflin-St Jeor equation?
It comes from a study of 1990 that measured the energy at rest of 498 healthy people aged 19 to 78. For a man it is 10 times the weight in kilograms plus 6.25 times the height in centimeters, less 5 times the age, plus 5. For a woman the last term is minus 161.
How accurate is the equation?
A review of 2005 found it the most reliable of four common equations: it came within 10 percent of the measured energy at rest for more people than any other. The same review warned of noteworthy errors for individuals, and said that older adults and some ethnic groups were underrepresented in the studies.
What do the activity factors mean?
They are the multipliers 1.2, 1.375, 1.55, 1.725 and 1.9, from little exercise to hard exercise and a physical job. A trial of weight loss applied them to the resting figure of its patients. They are a convention, not a measurement of you.
Does the calculator say how many calories to eat to lose weight?
No. It estimates the calories that keep your weight and sets no target. A doctor or a dietitian can say how much to eat for a goal.
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