How Many Calories Do You Need? BMR and TDEE Explained

The single number behind almost every diet plan is your daily calorie target, yet most people have no idea where that figure comes from. It starts with two ideas: the energy your body burns at complete rest, and the extra energy you spend moving through your day. Put them together and you get the calorie budget that decides whether you lose, gain, or hold your weight. This guide walks through how a calorie calculator builds that estimate, shows the exact formula it uses, and works through a full example you can follow line by line. To skip the arithmetic, run your own numbers in the Calorie Calculator.

BMR vs TDEE: two different numbers

Your basal metabolic rate, or BMR, is the energy your body needs just to stay alive while at complete rest. Breathing, pumping blood, keeping your brain and organs running, and maintaining body temperature all draw on this baseline. For most people the BMR accounts for the largest slice of daily energy use, roughly 60 to 70 percent, even if they never leave the sofa.

Your total daily energy expenditure, or TDEE, is the bigger picture: it is your BMR plus everything else you do. Walking to the bus, typing, fidgeting, exercising, and even digesting food all add to the total. TDEE is the number that actually matters for weight management, because it represents the calories you would need to eat to keep your weight perfectly stable. A calorie calculator estimates your BMR first, then scales it up to a TDEE using an activity factor.

The Mifflin-St Jeor formula

The Monu Calorie Calculator uses the Mifflin-St Jeor equation, published by Mifflin and colleagues in the American Journal of Clinical Nutrition in 1990. It is widely regarded as the most reliable BMR estimate for the general modern population, with a smaller average error than the older Harris-Benedict formula. It needs four inputs: sex, weight in kilograms, height in centimetres, and age in years.

Men:   BMR = 10 × weight(kg) + 6.25 × height(cm) − 5 × age + 5
Women: BMR = 10 × weight(kg) + 6.25 × height(cm) − 5 × age − 161

The only difference between the two is the constant
at the end: +5 for men, −161 for women.

The result is your resting calorie burn in kilocalories per day. On its own it is not a calorie target, because nobody spends all day lying down. To turn BMR into a usable daily figure, you multiply it by an activity factor.

Activity multipliers turn BMR into TDEE

The standard approach multiplies your BMR by a factor that reflects how active your typical week is. Pick the row that best matches your real lifestyle, not the one you wish were true. Overestimating activity is the most common reason a calorie target turns out too high.

TDEE is simply BMR multiplied by the chosen factor. Someone with a BMR of 1,780 kcal who trains a few times a week (1.55) has a TDEE of about 2,759 kcal, while the same person at a desk all week (1.2) sits closer to 2,136 kcal. Same body, very different budgets, which is why honest activity selection matters so much.

How a deficit or surplus moves the scale

Once you know your TDEE, weight change becomes a matter of energy balance. Eat fewer calories than you burn and you are in a deficit, so your body taps stored fat and you lose weight. Eat more than you burn and you are in a surplus, so you gain. One kilogram of body fat stores roughly 7,700 kcal of energy, which gives a handy rule of thumb.

A 500 kcal daily deficit:
500 kcal/day × 7 days = 3,500 kcal/week
3,500 ÷ 7,700 ≈ 0.45 kg of fat per week

A 500 kcal daily surplus does the reverse,
adding weight at a similar pace.

This is where the popular guideline of a roughly 500 kcal daily deficit for about half a kilogram of loss per week comes from. It is a steady, sustainable pace for most people. Larger deficits can speed things up but get harder to stick to and risk losing muscle along with fat. Eating enough protein and keeping some resistance training in your week helps protect lean mass while you lose. If your goal is fat loss, you can read more in our guide to protein for weight loss.

A worked example, step by step

Take a 30-year-old man who weighs 80 kg, stands 180 cm tall, and trains a few times a week (moderately active). Here is the full calculation a calorie calculator runs internally.

Step 1: BMR (Mifflin-St Jeor, men):
BMR = 10 × 80 + 6.25 × 180 − 5 × 30 + 5
    = 800 + 1,125 − 150 + 5
    = 1,780 kcal/day

Step 2: TDEE (moderately active, × 1.55):
TDEE = 1,780 × 1.55 = 2,759 kcal/day

Step 3: fat-loss target (500 kcal deficit):
2,759 − 500 = 2,259 kcal/day

So this person maintains weight near 2,759 kcal per day and would aim for roughly 2,259 kcal to lose about half a kilogram per week. To gain weight slowly instead, he would add around 500 kcal for a target near 3,259 kcal. The same machinery works for women by swapping the final constant: a 32-year-old woman at 68 kg and 165 cm has a BMR of about 1,390 kcal, and a lightly active TDEE near 1,912 kcal.

1. BMR: 1,780 kcal per day at complete rest
2. Maintenance TDEE: about 2,759 kcal per day
3. Fat-loss target: about 2,259 kcal per day
4. Weight-gain target: about 3,259 kcal per day

Why these are estimates, not exact numbers

Even the best equation is a population average applied to an individual. Two people of identical age, height, weight, and sex can have genuinely different metabolic rates because of differences in muscle mass, genetics, hormones, and how much they unconsciously move during the day. The Mifflin-St Jeor formula carries an average error of roughly five percent, and activity multipliers are broad brackets rather than precise measurements.

The smart way to use any calorie target is as a starting point, not a verdict. Eat at your estimated number for two to three weeks, track your weight trend, and adjust by 100 to 200 kcal if the scale is not moving the way you expect. Your own results over time are far more accurate than any formula. Bodyweight is also only one signal of health, so it helps to read it alongside what a healthy BMI means and your overall daily protein needs.

You now have the full picture: BMR is your resting baseline, an activity multiplier scales it into your TDEE, and a calorie deficit or surplus moves your weight from there. Plug in your own sex, weight, height, age, and activity level to get a personal starting number in seconds.