Daily Calorie Needs, Explained
How many calories you need a day: BMR vs TDEE, the Mifflin–St Jeor formula, activity multipliers, and what a 250 or 500 calorie deficit means in practice.
By Udit Agarwal · Updated October 17, 2026
The number of calories you need each day is your total daily energy expenditure (TDEE): the calories your body burns at rest, called your basal metabolic rate (BMR), multiplied by a factor for how active you are. For a 30-year-old man who weighs 75 kg, is 175 cm tall and exercises moderately, the widely used Mifflin–St Jeor equation puts that at about 2,630 calories a day. Eating less than your TDEE creates a calorie deficit, and eating more creates a surplus.
Basal metabolic rate (BMR) is the energy your body uses at complete rest just to keep running: breathing, circulation, keeping warm and powering the organs. Total daily energy expenditure (TDEE) adds everything else, which means digesting food and every kind of movement, from workouts to walking around the house. For most people the resting part is the biggest share, often around 60 to 70 percent of the total; digesting food takes roughly another 10 percent, and movement accounts for the rest.
TDEE is the figure that answers "how many calories do I need?" Eating about that much on average tends to keep weight roughly stable. The TDEE Calculator shows both numbers side by side.
Strictly, the Mifflin–St Jeor equation below predicts resting energy expenditure, also called resting metabolic rate (RMR), which is measured under slightly less strict conditions than BMR and comes out a little higher. Calculators, including ArrayKit's, use the two terms interchangeably.
The ArrayKit Calorie Calculator, like its BMR and TDEE calculators, uses the Mifflin–St Jeor equation, published in 1990 in the American Journal of Clinical Nutrition. With weight in kilograms, height in centimetres and age in years:
| Sex | BMR formula (calories per day) |
|---|---|
| Men | 10 × weight + 6.25 × height − 5 × age + 5 |
| Women | 10 × weight + 6.25 × height − 5 × age − 161 |
The two versions differ only in the constant at the end, which puts them 166 calories apart for the same weight, height and age. A 2005 systematic review for the American Dietetic Association compared the common equations with measured resting energy in healthy adults and found Mifflin–St Jeor the most reliable, although it still missed by more than 10 percent for some people.
Other equations are still in use. The Harris–Benedict equation dates from the late 1910s and was revised in 1984. The Katch–McArdle equation, 370 + 21.6 × lean body mass in kilograms, starts from lean mass instead of total weight, so it needs a body-fat estimate first; the Body Fat Calculator reports lean mass from tape measurements when you add your weight.
TDEE = BMR × activity multiplier. The multipliers below are a widely used set, and they are the ones the ArrayKit calculator applies. The last column shows the result for the man from the worked example in the next section.
| Activity level | Multiplier | Description in the tool | Example TDEE |
|---|---|---|---|
| Sedentary | 1.2 | Little or no exercise | 2,039 |
| Lightly active | 1.375 | Light exercise 1–3 days a week | 2,336 |
| Moderately active | 1.55 | Moderate exercise 3–5 days a week | 2,633 |
| Active | 1.725 | Hard exercise 6–7 days a week | 2,930 |
| Very active | 1.9 | Hard daily exercise or a physical job | 3,228 |
The steps are large. For this man each level adds about 300 calories a day, so picking the right activity level matters more than small changes to weight or age.
Take a 30-year-old man who weighs 75 kg, is 175 cm tall and does moderate exercise 3 to 5 days a week. These are also the calculator's starting values.
A second example: a 35-year-old woman who weighs 65 kg, is 165 cm tall and is lightly active. Her BMR is 650 + 1,031.25 − 175 − 161 = 1,345.25, and her TDEE is 1,345.25 × 1.375 = 1,850 calories a day.
In US units the calculator converts to metric first. A 40-year-old woman who weighs 150 lb (68.04 kg) and is 5 ft 5 in (165.1 cm) tall has a BMR of about 1,351; if she is sedentary, her TDEE is about 1,622.
A calorie deficit is the gap between what you burn and what you eat. The common rule of thumb says a pound of body fat holds about 3,500 calories, which gives these round numbers:
| Daily change | Weekly change | Weight change by the 3,500 rule |
|---|---|---|
| −500 calories | −3,500 calories | about −1 lb (−0.45 kg) a week |
| −250 calories | −1,750 calories | about −0.5 lb (−0.23 kg) a week |
| +250 calories | +1,750 calories | about +0.5 lb (+0.23 kg) a week |
| +500 calories | +3,500 calories | about +1 lb (+0.45 kg) a week |
In metric, the same rule works out to roughly 7,700 calories per kilogram.
The 3,500 figure comes from a 1958 estimate of the energy stored in a pound of body fat, and it is an approximation. It treats your calorie needs as fixed, but they change: as weight falls, a smaller body burns fewer calories, so the same daily deficit produces slower losses over the following months. Early changes on the scale also include water, so the first week or two rarely match the arithmetic. For a projection that changes over time, the NIH's Body Weight Planner models how weight changes week by week instead of applying a fixed rule.
Run the same person through three calculators and you may get three numbers. The reasons are ordinary:
The Calorie Calculator asks for your sex (male or female), age, weight and height in Metric or US units, one of the five activity levels above, and a goal: Lose, Lose mildly, Maintain, Gain mildly or Gain. It shows three results: your maintenance calories (TDEE), your target calories after the goal adjustment (250 or 500 calories a day, or none for Maintain), and the estimated weekly change in pounds, worked out with the 3,500-calorie rule. Calories are rounded to whole numbers, and a copy button gives you a plain-text summary.
Its limits are those of the method. It uses Mifflin–St Jeor only and does not display the BMR itself (the BMR Calculator does). The weekly change is shown in pounds even in metric mode. It does not set a minimum target or warn when a target is very low. The equation was developed from measurements of adults, so it does not apply to children, and it does not allow for pregnancy or medical conditions. Everything is calculated in your browser, and the figures you enter are not sent anywhere.
These numbers are estimates from a standard formula and not medical or diet advice, so talk to a doctor or registered dietitian before making decisions about your diet or weight.
How many calories do I need a day? It depends on your size, age, sex and activity. Estimate your BMR, then multiply it by an activity factor between 1.2 and 1.9. A moderately active 30-year-old man of 75 kg and 175 cm comes out at about 2,630 calories a day, and a lightly active 35-year-old woman of 65 kg and 165 cm at about 1,850.
What is the difference between BMR and TDEE? BMR is what your body burns at complete rest. TDEE adds digestion and all your movement, so it is always higher, between 1.2 and 1.9 times BMR in most calculators. TDEE is the figure to compare with what you eat.
Is a TDEE calculator accurate? It gives a reasonable starting estimate for most healthy adults, but it predicts an average for people like you. The equation alone can miss your resting energy by 10 percent or more, and choosing the wrong activity level can add an error of a similar size or larger.
Does a 500-calorie deficit mean losing a pound a week? By the 3,500-calorie rule it works out to about a pound a week, but the rule is an approximation. At the same intake, losses slow over time because a lighter body burns less, and water shifts make the first weeks hard to predict.
Which formula is best for estimating calories? For healthy adults, a 2005 review for the American Dietetic Association found Mifflin–St Jeor the most reliable of the common equations, and many calculators use it. If you know your body-fat percentage, an equation based on lean mass, such as Katch–McArdle, is an alternative.
Why does my calorie target differ between calculators? Calculators use different equations, different activity multipliers and different goal adjustments. For the example man in this guide, switching from Mifflin–St Jeor to the revised Harris–Benedict equation alone adds about 100 calories a day.
Are food calories the same as kilocalories? Yes. The calorie on a food label is a kilocalorie (kcal), and one kilocalorie equals 4.184 kilojoules. A day of 2,000 kcal is about 8,368 kJ.
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