Calorie math
How many calories should I eat a day?
How is a daily calorie number calculated?
A daily calorie target is resting metabolic rate times an activity multiplier. Resting metabolic rate is the energy your body burns at complete rest, and it is the largest piece of the day for almost everyone. Activity, digestion and exercise sit on top of it.
The most widely used formula is the Mifflin-St Jeor equation, published in the American Journal of Clinical Nutrition in 1990. It uses weight in kilograms, height in centimetres and age in years:
- Men: 10 × weight + 6.25 × height − 5 × age + 5
- Women: 10 × weight + 6.25 × height − 5 × age − 161
That gives resting calories. A 2005 systematic review in the Journal of the American Dietetic Association compared the common predictive equations and found Mifflin-St Jeor was the most reliable of them, predicting within 10 percent of the measured value in more people than any other equation. A 2013 follow-up in Clinical Nutrition measured 337 adults and found it landed within 10 percent of the true figure in 82 percent of them: 87 percent of non-obese adults and 75 percent of obese adults. That is why it is the equation behind most calculators, including the free NutraCompass calculator.
Then you multiply by how active you actually are across a whole week, not on your best day.
What are the activity multipliers?
Activity multipliers scale resting calories up to a full day. The commonly used set runs from 1.2 for a desk job with no training to 1.9 for hard physical work plus daily training.
| Level | Multiply by | What it looks like |
|---|---|---|
| Sedentary | 1.2 | Desk job, little or no exercise |
| Light | 1.375 | Exercise 1 to 3 days a week |
| Moderate | 1.55 | Exercise 3 to 5 days a week |
| Very active | 1.725 | Hard exercise 6 to 7 days a week |
| Extra active | 1.9 | Physical job plus daily training |
Most people pick one level too high. If you sit for work and train three times a week, you are "light", not "moderate". Steps count too: a person who walks 10,000 steps a day at a desk job burns more than a person who walks 3,000, and the multiplier is the only place that shows up.
Two worked examples
A 35-year-old woman, 5 ft 5 in (165 cm), 160 lb (72.6 kg), desk job, gym twice a week.
- Resting: 10 × 72.6 + 6.25 × 165 − 5 × 35 − 161 = 1,421 calories
- Times 1.375 (light activity) = 1,954 calories to maintain
- Minus 15 percent for fat loss = about 1,660 calories a day
A 30-year-old man, 5 ft 10 in (178 cm), 190 lb (86.2 kg), on his feet at work, lifts four days a week.
- Resting: 10 × 86.2 + 6.25 × 178 − 5 × 30 + 5 = 1,830 calories
- Times 1.55 (moderate activity) = 2,836 calories to maintain
- Minus 15 percent for fat loss = about 2,410 calories a day
Notice how far apart those two numbers are. Generic advice like "eat 1,200 calories" or "eat 2,000 calories" ignores the two things that matter most, body size and movement. The 2,000-calorie figure on nutrition labels is a reference value for reading labels, not a recommendation for you.
How much should I cut for fat loss?
Cut 10 to 20 percent below maintenance, which for most people is a deficit of 250 to 500 calories a day. A 500-calorie daily deficit is roughly a pound of fat a week on paper, though real-world loss is slower and bumpier than that because water, food in the gut and daily activity all move.
Going lower than 20 percent is where plans fall apart. Hunger goes up, training quality goes down, and the weekend becomes a rebound. A smaller deficit you can hold for four months beats a large one you abandon in three weeks. If you want the deeper reasoning, read what a cheat day actually does to your week.
To gain muscle, go the other way: 5 to 10 percent above maintenance, with protein set first. The macro guide covers how to split the calories once you have the total.
Why can the calculator be off by 200 calories?
Every formula is an average, and you are not average. Muscle mass, genetics, sleep, how much you fidget, and how honestly you rated your activity all push the real number around. In the 2013 Clinical Nutrition study, the best equation still missed the measured value by more than 10 percent in about one adult in five, and one in four among obese adults.
That is not a reason to skip the math. It is a reason to treat the number as a first guess and then let the scale correct it.
How do I check my number in two weeks?
Eat at your target, log what you eat, and weigh yourself every morning under the same conditions. After 14 days, compare the average of week two with the average of week one, not single days against each other.
- Losing 0.5 to 1 percent of body weight a week: the number is right. Keep going.
- Weight flat: the number is your maintenance. Drop 100 to 150 calories.
- Losing faster than 1 percent a week: add 100 to 200 calories, especially if you train.
This works because the average wipes out water swings. It also means you never have to guess your activity multiplier perfectly. The trend tells you.
The reason logging matters here is not the logging itself, it is the feedback. A 2011 systematic review in the Journal of the American Dietetic Association found a consistent association between self-monitoring and weight loss across the 22 studies it examined, while cautioning that many of those studies were methodologically weak. The evidence on tracking is worth ten minutes if you have ever quit a diet out of frustration.
What does NutraCompass do with the number?
NutraCompass calculates your target with the same formula, then does the part that calculators cannot. You point the camera at a plate, the meal is read and logged, and the rest of the day moves to absorb it. Log a 780-calorie dinner and the remaining number updates on the spot. Eat big on Saturday and the days around it ease instead of the plan resetting.
If you only want the number, the calculator is free and does not ask for an email. If you want the number to survive contact with real life, that is what the app is for.
Questions people ask
Is 1,200 calories a day too low?
For most adults, yes. 1,200 calories sits below the resting needs of almost every adult man and many women, so it usually cannot be sustained and often comes with muscle loss. Calculate maintenance first and cut 10 to 20 percent from that.
Should I eat back the calories I burn exercising?
Not if your activity multiplier already includes your training. Counting workouts twice, once in the multiplier and once as exercise calories, is the most common way people accidentally erase their deficit. Watch-based burn estimates also tend to run high.
How many calories do I need to maintain my weight?
Resting metabolic rate times your activity factor. For a typical adult that is 1,800 to 2,800 calories. The two-week weigh-in test above will confirm it for you.
Do calories from protein, carbs and fat count the same?
For fat loss, the total is what moves the scale, but protein is more filling and costs more energy to digest, so a higher-protein diet at the same calories is easier to stick to. That is why every plan in the macro guide sets protein first.
How often should I recalculate?
Every 10 to 15 pounds of change, or any time your activity changes for more than a couple of weeks. A lighter body needs fewer calories, so a target that worked in month one can stall by month four.
Sources
- Mifflin MD, St Jeor ST, Hill LA, Scott BJ, Daugherty SA, Koh YO. A new predictive equation for resting energy expenditure in healthy individuals. Am J Clin Nutr. 1990;51(2):241-247. https://pubmed.ncbi.nlm.nih.gov/2305711/
- Frankenfield D, Roth-Yousey L, Compher C. Comparison of predictive equations for resting metabolic rate in healthy nonobese and obese adults: a systematic review. J Am Diet Assoc. 2005;105(5):775-789. https://pubmed.ncbi.nlm.nih.gov/15883556/
- Frankenfield DC. Bias and accuracy of resting metabolic rate equations in non-obese and obese adults. Clin Nutr. 2013;32(6):976-982. https://pubmed.ncbi.nlm.nih.gov/23631843/
- Burke LE, Wang J, Sevick MA. Self-monitoring in weight loss: a systematic review of the literature. J Am Diet Assoc. 2011;111(1):92-102. https://pubmed.ncbi.nlm.nih.gov/21185970/
This guide is general nutrition education, not medical advice. If you are pregnant, under 18, or managing a medical condition, talk to a clinician or registered dietitian before changing how you eat. Prices and menus mentioned were checked on September 19, 2026 and can change.