“How many calories should I eat?” is one of the most searched health questions there is, and the honest answer is: it depends. It depends on your size, your age, your sex and, above all, how active you are. The good news is that there is a well-tested method for turning those factors into a reliable estimate. This guide walks through it step by step — from the energy your body burns at rest to a daily target for losing, maintaining or gaining weight — and explains how to adjust the numbers once real life gets involved.
What a calorie actually is
A calorie is a unit of energy. The “calories” on food labels are really kilocalories (kcal): the energy needed to warm one kilogram of water by one degree Celsius. Some countries also label food in kilojoules (kJ); one kilocalorie is about 4.184 kJ. Your body gets its energy from three main nutrients, plus alcohol:
- Carbohydrate — about 4 kcal per gram.
- Protein — about 4 kcal per gram.
- Fat — about 9 kcal per gram.
- Alcohol — about 7 kcal per gram.
Your weight changes when the energy you take in differs from the energy you use. Eat more than you burn and the surplus is stored; eat less and the body draws on its stores. Working out how much you burn is therefore the starting point.
Step 1: your basal metabolic rate (BMR)
Your basal metabolic rate is the energy your body uses at complete rest just to stay alive: breathing, circulating blood, keeping warm and repairing cells. For most people it is the largest part of daily energy use — often 60 to 70%.
The equation most dietitians use today is the Mifflin–St Jeor equation, published in 1990 and found in later reviews to be the most accurate of the common formulas for most adults:
The two versions differ only in the final constant, which reflects the average difference in body composition between men and women. The BMR calculator works this out in metric or imperial units.
Step-by-step example
Take a 35-year-old man who weighs 80 kg and is 180 cm tall.
- Weight: 10 × 80 = 800.
- Height: 6.25 × 180 = 1,125.
- Age: 5 × 35 = 175, which is subtracted.
- BMR = 800 + 1,125 − 175 + 5 = 1,755 kcal a day.
His body burns about 1,755 calories a day at complete rest.
For a 30-year-old woman weighing 65 kg and 165 cm tall, the same steps give 650 + 1,031.25 − 150 − 161 = about 1,370 calories a day.
Step 2: add your activity level (TDEE)
Nobody lies still all day. Your total daily energy expenditure (TDEE) adds the energy used in everything else: walking, working, exercising and even digesting food. It is estimated by multiplying your BMR by an activity factor:
| Activity level | Typical day | Multiplier |
|---|---|---|
| Sedentary | Desk job, little exercise | 1.2 |
| Lightly active | Light exercise 1–3 days a week | 1.375 |
| Moderately active | Moderate exercise 3–5 days a week | 1.55 |
| Very active | Hard exercise 6–7 days a week | 1.725 |
| Extra active | Physical job plus hard training | 1.9 |
Our moderately active man has a TDEE of 1,755 × 1.55 = about 2,720 calories a day. The lightly active woman’s is 1,370 × 1.375 = about 1,884. These are the amounts that would keep their weight roughly stable. The TDEE calculator combines both steps.
Step 3: set a target for your goal
Once you know your maintenance calories, adjusting for a goal is straightforward.
- To lose weight, eat less than your TDEE. A deficit of about 500 calories a day leads to a loss of roughly 0.45 kg (1 lb) a week, based on the common estimate that a kilogram of body fat stores about 7,700 kcal.
- To maintain weight, eat around your TDEE.
- To gain weight or build muscle, eat a modest surplus of 250–500 calories a day alongside strength training.
For our example man, a 500-calorie deficit means a target of about 2,220 calories a day. The calorie calculator works out targets for several rates of loss or gain, and the calorie deficit calculator estimates how long reaching a goal weight might take.
Why weight loss slows down over time
The “500 calories a day equals half a kilo a week” rule is a good starting point, but it is not linear forever. As you lose weight, your BMR falls — a smaller body needs less energy — so the same intake produces a smaller deficit. The body also adapts a little by reducing unconscious movement and becoming more efficient. That is why progress often slows after the first few months, and why it helps to recalculate your needs every 5 kg or so.
Early weight loss is also exaggerated by water. In the first week or two of a lower-calorie diet, the body uses stored carbohydrate (glycogen), which is held with water, so the scales can drop quickly before settling into a slower, steadier pace.
Safe limits
Faster is not better. Very large deficits make it hard to get enough protein, vitamins and minerals, increase muscle loss, and are difficult to stick to. As a general guide:
- Aim to lose no more than about 0.5–1 kg (1–2 lb) a week.
- Many health services suggest not going below roughly 1,200 calories a day for women or 1,500 for men without medical supervision.
- Children, teenagers, pregnant or breastfeeding women, and anyone with a medical condition should get personal advice before changing their intake.
Where your calories come from matters
Calories set the total, but the balance of nutrients affects how full you feel, how much muscle you keep and how well you perform. Protein is the most important to get right when losing weight: around 1.2–1.6 g per kg of body weight helps preserve muscle and keeps hunger in check. Fibre from vegetables, fruit, pulses and whole grains adds bulk with few calories. The macro calculator splits a calorie target into grams of protein, carbohydrate and fat.
Is a calorie always a calorie?
For energy balance, broadly yes: a surplus of calories from any source is stored, and a deficit from any source is drawn from the body’s reserves. But foods differ in how satisfying they are and how much energy the body spends digesting them. Protein costs more energy to digest than fat or carbohydrate, and it keeps you fuller for longer. Highly processed foods tend to be easy to overeat because they pack many calories into a small, quickly eaten portion. In practice, the same calorie target is far easier to stick to when most of it comes from minimally processed foods with plenty of protein and fibre.
Tracking and adjusting
Every formula gives an estimate, and individuals vary by 10% or more either side. The practical way to find your real number is to test it:
- Eat at your calculated target consistently for two to three weeks.
- Weigh yourself several times a week, at the same time of day, and look at the weekly average rather than daily swings.
- If your weight is not moving the way you expected, adjust your intake by 100–200 calories and try again.
Your own results are always more accurate than any equation.
Common mistakes
- Overestimating activity. Pick the lower level if in doubt.
- Counting exercise twice. If your activity factor already includes your workouts, don’t add the calories burned in the gym on top.
- Forgetting drinks, oils and snacks. Cooking oil, sauces, coffee drinks and alcohol add up quickly.
- Cutting too hard. Big deficits are hard to sustain and cost muscle.
- Never recalculating. Your needs fall as your weight does.
Frequently asked questions
How many calories do I need a day?
Work out your BMR with the Mifflin–St Jeor equation, then multiply it by your activity factor. For a moderately active 35-year-old man of 80 kg and 180 cm, that is about 2,720 calories a day to maintain weight.
How many calories should I cut to lose weight?
A deficit of about 500 calories a day typically leads to losing around 0.45 kg (1 lb) a week at first. Avoid going below roughly 1,200 calories for women or 1,500 for men without medical advice.
Why am I not losing weight on my calculated calories?
The estimate may be too high for you, portions may be larger than you think, or water retention may be hiding fat loss. Track carefully for two to three weeks and adjust by 100–200 calories if needed.
Final thoughts
Knowing your calorie needs turns weight management from guesswork into a plan. Start with your BMR, add an honest activity level, choose a sensible target, then let your own results fine-tune it. The calorie calculator brings all three steps together and shows the working, so you can see exactly where your number comes from.