What is a calorie?
A calorie is a unit of energy. In the context of food and nutrition, when we talk about “calories” we technically mean kilocalories (kcal) — the amount of energy needed to raise one kilogram of water by one degree Celsius. The terms are used interchangeably on food labels and in everyday conversation.
Your body uses calories to power everything it does — breathing, circulating blood, regulating temperature, digesting food, thinking, moving. It is essentially a biological engine that runs on the energy extracted from food.
When you consume more calories than you use, the surplus is stored, primarily as body fat. When you consume fewer than you use, your body draws on those stores. This energy balance is the fundamental mechanism behind weight change — though as we’ll explore, the reality is more nuanced than a simple maths equation.
Why 2,000 calories is rarely right for you
The “2,000 kcal a day” figure you see on UK food packaging is a reference value set by UK and EU regulators as a standardised benchmark for nutrition labels. It is not a recommendation based on your individual needs — it is a convenient average chosen to make food label percentages consistent across products.
In reality, daily calorie needs vary enormously between individuals based on:
- Sex — men generally need more calories than women due to greater average muscle mass
- Age — metabolic rate declines gradually with age, roughly 1–2% per decade after 30
- Height and weight — a larger body requires more energy to maintain
- Activity level — by far the most variable factor, with a 400–1,000+ kcal difference between sedentary and very active lifestyles
- Body composition — muscle burns more calories at rest than fat does
- Health conditions — thyroid disorders, medications, and other factors affect metabolism
A sedentary woman in her 60s might genuinely need 1,500 kcal a day. A male construction worker in his 30s might need 3,200. The single number on the cereal box tells neither of them anything useful about their own needs.
Step 1: Your BMR — calories at rest
The starting point for working out your calorie needs is your Basal Metabolic Rate (BMR) — the number of calories your body burns if you did nothing but lie completely still for 24 hours. It represents the energy cost of keeping you alive: heart beating, lungs breathing, kidneys filtering, brain ticking over.
BMR typically accounts for 60–70% of total daily calorie expenditure for most people. Even if you never moved, you’d still burn the majority of your daily calories just existing.
The Mifflin-St Jeor equation
The most widely validated formula for estimating BMR is the Mifflin-St Jeor equation, published in 1990 and consistently shown in research to outperform older methods like the Harris-Benedict equation. It is what we use in our calorie calculator.
Women: (10 × kg) + (6.25 × cm) − (5 × age) − 161
Example: a 35-year-old woman, 165 cm, 68 kg → BMR = 1,418 kcal/day
BMR is an estimate, not a measurement. A DEXA scan or metabolic chamber test can measure it precisely, but Mifflin-St Jeor typically comes within 10% for most people — accurate enough to be genuinely useful for planning.
Step 2: Your TDEE — total daily needs
BMR is your baseline, but you are not lying still all day. Your Total Daily Energy Expenditure (TDEE) accounts for all the other ways you burn calories beyond basic survival — moving around, exercising, digesting food, and the small involuntary movements you make throughout the day.
To get from BMR to TDEE, you multiply by an activity multiplier that reflects your lifestyle:
| Activity level | Description | Multiplier | Example TDEE (BMR 1,500) |
|---|---|---|---|
| Sedentary | Desk job, little or no exercise, mostly sitting | × 1.2 | 1,800 kcal |
| Lightly active | Light exercise or sport 1–3 days per week | × 1.375 | 2,063 kcal |
| Moderately active | Moderate exercise 3–5 days per week | × 1.55 | 2,325 kcal |
| Very active | Hard exercise or sport 6–7 days per week | × 1.725 | 2,588 kcal |
| Extra active | Physical job plus exercise, or twice-daily training | × 1.9 | 2,850 kcal |
Notice the range: a sedentary person with a BMR of 1,500 kcal needs 1,800 kcal to maintain weight. The same person, if they were doing hard daily exercise, would need nearly 2,850 kcal — a difference of over 1,000 calories. This is why activity level matters so much more than most people realise.
Real-world calorie examples
To make this concrete, here are estimated TDEE figures for three different people, all calculated using the Mifflin-St Jeor method:
None of these people should be aiming for 2,000 kcal. The 28-year-old gym-goer needs more than 50% above that figure just to maintain his current weight. The 65-year-old retiree needs significantly less. This is why treating 2,000 as a personal target rather than a label convention can lead people in completely the wrong direction.
Adjusting for your goal
Once you know your TDEE, adjusting for a specific goal is straightforward:
To lose weight
Create a calorie deficit by eating less than your TDEE. A deficit of 250–500 kcal per day is generally recommended as a sustainable range. Larger deficits tend to cause muscle loss, fatigue, and are harder to maintain long-term.
- Mild deficit (−250 kcal/day) → roughly 0.25 kg per week
- Moderate deficit (−500 kcal/day) → roughly 0.5 kg per week
To gain weight or build muscle
Eat slightly above your TDEE — a surplus of 250–500 kcal per day. A smaller surplus minimises fat gain while supporting muscle growth. Larger surpluses lead to faster weight gain but a higher proportion of that gain will be fat rather than muscle.
To maintain your current weight
Eat at your TDEE. This sounds simple, but most people find that in practice they are either consistently above or below without realising it — which is exactly why weight gradually changes over months and years without any deliberate decision.
What about macros?
Total calories determine whether you gain, lose, or maintain weight. But macronutrients — protein, carbohydrates, and fat — determine what that weight change looks like in terms of body composition, and affect your energy levels, hunger, and performance along the way.
A commonly used starting split for general health and body composition is:
These ratios are not fixed rules. Someone prioritising muscle building might aim for 35% protein and less carbs. Someone following a low-carb approach might swap carbs and fat. The most important factor remains total calories — macro ratios are a tool for optimising within that constraint.
Why protein matters most
Of the three macros, protein has the greatest impact on body composition. It is the most satiating macronutrient (helping you feel fuller for longer), it has the highest thermic effect (your body burns more calories digesting protein than fat or carbs), and adequate protein intake is essential for preserving muscle mass during a calorie deficit. Most people benefit from aiming for at least 1.6–2.2 g of protein per kilogram of body weight.
Tips for hitting your calorie target
Knowing your calorie target is one thing. Consistently hitting it in real life is another. These practical habits make the biggest difference: