Daily calorie needs explained: BMR, activity, goals and macros

Guide · Updated 2026-11-10

How a daily calorie estimate is built from the Mifflin-St Jeor equation, an activity factor and a weight goal, then split into protein, fat and carbs, with a worked example in pounds and inches.

Open the Calorie calculator Free, in your browser, nothing uploaded.

The calorie calculator estimates how much energy your body uses in a day, then turns that into a daily target for keeping, losing or gaining weight, and splits the target into grams of protein, fat and carbohydrate. It is for people curious about where calorie numbers come from, students in nutrition or PE classes, and coaches who want a starting point to adjust. It is general education only, not personal advice: if you are pregnant, unwell, under 18 or have a history of disordered eating, talk to a doctor or dietitian before changing how you eat. To compare several resting energy equations side by side, try the energy needs calculator; to see where your weight sits first, use BMI calculator; and to turn daily walks into distance, use steps to distance.

What it is

The estimate is built in three layers.

1. Basal metabolic rate (BMR). This is the energy used just to stay alive at rest: breathing, heartbeat, keeping warm. The tool uses the Mifflin-St Jeor equation:

BMR = 10 × W + 6.25 × H − 5 × A + s

where W is weight in kg, H is height in cm, A is age in years, and s is +5 for males or −161 for females. The result is in kilocalories (kcal) per day, the unit food labels call “Calories”.

2. Total daily energy expenditure (TDEE). Moving, working and exercising burn more. The tool multiplies BMR by an activity factor from 1.2 (mostly sitting) to 1.9 (very hard training or a physical job). The result is the intake that would, on average, keep weight steady.

3. A goal adjustment. A common rule of thumb says 1 kg of body weight holds about 7,700 kcal. To change weight by r kg a week, the daily change is r × 7,700 ÷ 7.

Finally the target is split into macronutrients. Protein is set in grams per kg of body weight (or as a percent), fat as a percent of calories, and carbohydrate takes whatever is left. Protein and carbs give about 4 kcal per gram, fat about 9.

How to use it

1. Choose Units: metric (kg, cm) or imperial (lb, in). Switching converts the weight and height already typed.
2. Pick Sex, then type Age, Weight and Height.
3. Choose an Activity level. This fills in Activity factor, which you can fine-tune by typing your own number.
4. Pick a Goal: lose or gain 0.25 or 0.5 kg a week, or keep your weight. You can also change kcal in 1 kg of body weight and the Lowest daily target before a warning, which starts at 1,500 kcal for males and 1,200 for females.
5. Read BMR (at rest), To keep your weight and Daily target, plus the sentence that shows the sum.
6. In Macro split, choose whether protein is set by grams per kg or percent of calories, type the protein and Fat (% of calories) values, and read the table of grams, kcal and percent. Reset restores the example.

A worked example

A made-up woman, aged 42, weighs 150 lb and is 66 in (5 ft 6 in) tall. She walks or cycles 1 to 3 days a week, so she picks Light (1.375), and her goal is to lose 0.25 kg a week.

Converted: 150 lb × 0.45359 = 68.0 kg, and 66 in × 2.54 = 167.6 cm.

BMR = 10 × 68.04 + 6.25 × 167.64 − 5 × 42 − 161 = 680.4 + 1,047.8 − 210 − 161 = 1,357 kcal/day.

To keep her weight: 1,357 × 1.375 = 1,866 kcal/day.

The goal: 0.25 × 7,700 ÷ 7 = 275 kcal/day less, so the daily target is 1,591 kcal/day. That is above the 1,200 kcal warning line, so no warning shows.

Macros, with protein at 1.6 g/kg and fat at 30 %:

• Protein: 1.6 × 68.04 = 109 g, or 435 kcal (27 %).
• Fat: 30 % of 1,591 = 477 kcal, or 53 g.
• Carbs: 1,591 − 435 − 477 = 678 kcal, or 170 g (43 %).

If she had picked Little or no exercise and lose 0.5 kg a week, the target would be 1,629 − 550 = 1,079 kcal/day, and the tool would warn that it is below 1,200. That is a cue to choose a gentler goal or talk to a professional.

Where it is used

• Sports coaching: a coach sets a starting intake for an athlete, then adjusts it from weekly weight and performance.
• Nutrition classes: students see how each input moves the answer, and why age lowers BMR.
• Meal planning: the macro grams give a shopping and cooking target for the week.

Common mistakes

• Overstating activity. Most people pick a level higher than their real week. If unsure, start one level lower and watch what happens over a few weeks.
• Treating the target as exact. Equations can miss a real person by 10 % or more. Use it as a starting point and adjust.
• Expecting steady weight loss forever. The 7,700 kcal rule works only at first; as weight drops, so does energy use.
• Setting protein and fat too high. If the carbs row goes negative, the tool warns you; lower one of the other two.
• Using it for children. Mifflin-St Jeor was fitted on adults, and the tool warns under 18.

Questions people ask

Why does the tool ask for sex? Bodies of the same size differ on average in muscle and fat, and the equation has a separate constant for each. It is a statistical shortcut, not a statement about any one person.

Should I set protein by grams per kg or by percent? Grams per kg ties protein to body size, which is how most sports guidance is written. Percent is handy when copying a plan written that way.

Why do my results differ from a fitness watch? Watches estimate activity from movement and heart rate, then use their own formulas. Neither is exact; look at trends over weeks rather than one day.