How the calorie calculator works
Your daily calorie needs come from two parts. The first is your basal metabolic rate (BMR): the energy your body uses at rest to breathe, pump blood and keep warm. The second is everything else you do in a day, from walking to the bus to a hard gym session. The calculator estimates the first with an equation and the second with an activity factor.
By default it uses the Mifflin–St Jeor equation, published in 1990. A 2005 review for the American Dietetic Association compared the common equations and found Mifflin–St Jeor predicted resting energy within 10% of the measured value for more healthy adults than any other.
Women: BMR = 10 × weight (kg) + 6.25 × height (cm) − 5 × age − 161
If you enter US units, the calculator converts them first (1 lb = 0.45359237 kg, 1 inch = 2.54 cm). You can also pick the revised Harris–Benedict equation, or Katch–McArdle if you know your body fat percentage. Katch–McArdle works from lean mass, so it can suit very muscular people better.
To lose or gain weight, the calculator then adds or removes a fixed number of calories a day. It uses the long-standing rule of thumb that about 3,500 kcal equals 1 lb of body weight (about 7,700 kcal per kg).
So losing 1 lb a week needs a deficit of about 500 kcal a day, and losing 0.5 kg a week needs about 550 kcal a day.
Activity levels
Pick the level that matches a normal week, not your best one. Most people overestimate how active they are.
| Activity level | Factor | What it looks like |
|---|---|---|
| Sedentary | 1.2 | Desk job, little or no planned exercise |
| Lightly active | 1.375 | Light exercise or sport 1–3 days a week |
| Moderately active | 1.55 | Moderate exercise 3–5 days a week |
| Very active | 1.725 | Hard exercise 6–7 days a week |
| Extra active | 1.9 | Physical job plus training, or training twice a day |
If you walk a lot for work but never exercise, “lightly active” is usually closer than “sedentary”. If you train hard three times a week but sit the rest of the time, “moderately active” is a fair choice.
Worked examples
Example 1 (US units). A 35-year-old man, 5 ft 10 in and 180 lb, lightly active.
- Height 70 in × 2.54 = 177.8 cm. Weight 180 × 0.45359 = 81.6 kg.
- BMR = 10 × 81.6 + 6.25 × 177.8 − 5 × 35 + 5 = 816 + 1,111 − 175 + 5 = 1,758 kcal.
- Maintenance = 1,758 × 1.375 = 2,417 kcal a day.
- To lose 1 lb a week: 2,417 − 500 = 1,917 kcal a day.
- To lose 2 lb a week: 2,417 − 1,000 = 1,417 kcal. That is below 1,500 kcal, so the calculator flags it and suggests a slower rate.
Example 2 (metric, kilojoules). A 28-year-old woman, 165 cm and 68 kg, moderately active.
- BMR = 10 × 68 + 6.25 × 165 − 5 × 28 − 161 = 680 + 1,031 − 140 − 161 = 1,410 kcal.
- Maintenance = 1,410 × 1.55 = 2,186 kcal, or 2,186 × 4.184 = 9,146 kJ a day.
- To lose 0.5 kg a week: 2,186 − 550 = 1,636 kcal (6,845 kJ) a day.
Example 3 (calorie cycling). The man in example 1 wants a 1,917 kcal target but finds weekends hard. The zigzag week keeps his weekly total the same (13,419 kcal) with Saturday and Sunday at 10% more, about 2,109 kcal, and Monday to Friday at about 1,840 kcal. The weekly deficit, and so the expected weight loss, does not change.
How fast should you lose weight?
The CDC notes that people who lose weight gradually, about 1 to 2 lb (0.5 to 1 kg) a week, are more likely to keep it off. The table shows what each rate means in daily calories.
| Weekly goal | Daily change | Typical use |
|---|---|---|
| Lose 0.5 lb (0.25 kg) | −250 kcal | Close to goal weight, or very active people who want to keep performance |
| Lose 1 lb (0.5 kg) | −500 to −550 kcal | A sensible default for most adults |
| Lose 1.5 lb (0.75 kg) | −750 to −825 kcal | Larger bodies with more to lose |
| Lose 2 lb (1 kg) | −1,000 to −1,100 kcal | Usually only for people with a high maintenance level |
| Gain 0.5 lb (0.25 kg) | +250 kcal | Slow muscle gain with strength training |
| Gain 1 lb (0.5 kg) | +500 to +550 kcal | Faster gain; more of it will be fat |
A common safety floor is about 1,200 kcal a day for women and 1,500 for men. Going lower makes it hard to get enough protein, vitamins and minerals, and should only happen under medical supervision.
Typical calorie needs by age
The US Dietary Guidelines for Americans 2020–2025 give estimated needs for a reference woman (5 ft 4 in, 126 lb) and man (5 ft 10 in, 154 lb). Larger or smaller people need more or less, which is why a personal estimate is more useful.
| Age | Women: sedentary to active | Men: sedentary to active |
|---|---|---|
| 19–30 | 1,800–2,400 kcal | 2,400–3,000 kcal |
| 31–50 | 1,800–2,200 kcal | 2,200–3,000 kcal |
| 51 and over | 1,600–2,200 kcal | 2,000–2,800 kcal |
Needs fall with age mainly because people tend to lose muscle and move less. Staying active and doing strength training helps slow that drop.
Where calories come from
Every gram of food energy comes from four sources:
| Nutrient | Energy per gram |
|---|---|
| Carbohydrate | 4 kcal (17 kJ) |
| Protein | 4 kcal (17 kJ) |
| Fat | 9 kcal (37 kJ) |
| Alcohol | 7 kcal (29 kJ) |
Fat is more than twice as energy-dense as carbohydrate or protein, and alcohol adds calories with no nutrients. Once you have a calorie target, the macro calculator turns it into grams of protein, carbohydrate and fat, and the protein calculator sets a protein target for your goal.
Why the estimate can be off
Every calorie formula is an average. Two people with the same age, height and weight can differ by a few hundred calories a day because of muscle mass, genetics, hormones and how much they fidget. Weight change is also not perfectly linear. A 2008 paper by Kevin Hall at the US National Institutes of Health showed that the 3,500-calorie rule overstates long-term loss, because your energy needs fall as you get lighter. The NIH Body Weight Planner models that slowdown.
In practice that means:
- Use the number as a starting point. Eat to your target for 2 to 3 weeks and weigh yourself several times a week at the same time of day.
- Look at the trend, not single days. Water, salt and food in the gut can move the scale by 1 to 2 kg (2 to 4 lb) overnight.
- Adjust in small steps. If the weekly average does not move as planned, change your intake by 100 to 200 kcal and try again.
- Recalculate as you go. Run the calculator again after every 5 kg (10 lb) of change.
Tips for hitting your target
- Prioritise protein and fibre. Both help you feel full on fewer calories, and protein helps you keep muscle while losing fat.
- Measure for a week or two. Portion sizes are easy to underestimate, especially oils, nuts, sauces and drinks.
- Do not double-count exercise. Your activity level already includes your usual workouts. Adding fitness-tracker calories on top usually means eating too much.
- Keep a little flexibility. Calorie cycling, a planned higher-calorie day or a slower rate can make a plan easier to stick to, and consistency matters more than the exact number.
Who should not rely on this calculator
The equations are designed for healthy adults. They are not suitable for children and teenagers, during pregnancy or breastfeeding, for people with an eating disorder, or for anyone with a medical condition that affects weight or metabolism. In those cases speak to a doctor or a registered dietitian.
Related calculators
For a deeper look at each part of the estimate, the BMR calculator compares the three resting equations side by side, and the TDEE calculator adds a macro split. To plan a goal weight by a date, use the calorie deficit calculator, and check where your weight sits with the BMI calculator.