Knowing how many calories you need each day is the foundation of any health or fitness goal, whether you want to lose weight, gain muscle, or simply maintain your current weight. This calculator uses the Mifflin-St Jeor equation — the gold standard for estimating Basal Metabolic Rate (BMR) — combined with your activity level to calculate your Total Daily Energy Expenditure (TDEE).
Your Basal Metabolic Rate is the number of calories your body burns at complete rest to keep you alive. It accounts for approximately 60–70% of total daily calorie expenditure for sedentary individuals. BMR supports functions including:
Published in 1990, the Mifflin-St Jeor equation is the most accurate BMR formula for most adults, validated against measured resting metabolic rate across thousands of subjects:
The older Harris-Benedict equation (1919, revised 1984) is still widely used but tends to overestimate BMR by 5–15%, particularly in overweight and obese individuals. The Katch-McArdle formula is considered most accurate if you know your body fat percentage, as it uses lean body mass rather than total weight.
| Activity Level | Multiplier | Description | Example |
|---|---|---|---|
| Sedentary | × 1.2 | Little or no exercise | Desk job, minimal walking |
| Lightly active | × 1.375 | 1–3 days/week exercise | Weekend walks, occasional gym |
| Moderately active | × 1.55 | 3–5 days/week exercise | Regular gym, active commute |
| Very active | × 1.725 | 6–7 days/week hard exercise | Daily training, sports athlete |
| Extremely active | × 1.9 | Very hard exercise + physical job | Athlete + manual labour |
The UK Scientific Advisory Committee on Nutrition (SACN) and NHS recommend:
One kilogram of body fat contains approximately 7,700 kcal of stored energy. Therefore:
For most people, a moderate deficit of 300–500 kcal/day is the most sustainable approach, minimising muscle loss and metabolic adaptation while producing consistent results.
Once you know your target calorie intake, distributing those calories across macronutrients matters for health and body composition:
| Macronutrient | UK / SACN Range | Calories per gram | Practical target |
|---|---|---|---|
| Carbohydrates | 45–65% of calories | 4 kcal/g | ~50% for most people |
| Protein | 10–35% of calories | 4 kcal/g | 25–35% for active individuals |
| Fat | 20–35% of calories | 9 kcal/g | ~25–30% of total calories |
| Fibre (not a macro) | 30g/day | 2 kcal/g | From wholegrains, fruit, veg |
For weight loss, higher protein intake (1.6–2.2g per kg body weight) is recommended by sports nutrition research to preserve muscle mass while in a calorie deficit. Protein also increases satiety, helping you feel fuller for longer.
The Eatwell Guide is the UK government's visual model of a balanced diet. It recommends:
A critical concept often overlooked in calorie counting is metabolic adaptation (adaptive thermogenesis). When you sustain a calorie deficit over weeks and months, your body responds by reducing its metabolic rate — often by 10–20% more than expected from weight loss alone. This is a survival mechanism. Signs include:
Strategies to counter metabolic adaptation include: diet breaks (returning to TDEE for 1–2 weeks every 8–12 weeks), reverse dieting (gradually increasing calories), and resistance training to preserve metabolically active muscle mass.
For those wanting to track intake precisely, popular apps available in the UK include:
BMR (Basal Metabolic Rate) is the number of calories your body burns at complete rest to maintain vital functions like breathing, circulation, and cell production. TDEE (Total Daily Energy Expenditure) is your BMR multiplied by an activity factor to account for all movement throughout the day, including exercise, walking, and incidental activity. TDEE is the more practically useful number for setting calorie targets.
UK government guidelines recommend approximately 2,000 kcal/day for women and 2,500 kcal/day for men on average. However, individual needs vary widely based on age, height, weight, body composition, and activity level. A sedentary smaller woman may need as few as 1,500 kcal, while a very active larger man may need 3,500+ kcal. Use the personalised calculator above for a more accurate estimate.
A deficit of approximately 500 kcal per day below your TDEE will result in roughly 0.5 kg (1 lb) of weight loss per week, as one kilogram of fat contains about 7,700 kcal. A deficit of 1,000 kcal/day would yield around 1 kg/week but is generally too aggressive for most people and risks muscle loss, fatigue, and metabolic adaptation. A moderate 300–500 kcal/day deficit is more sustainable.
The Mifflin-St Jeor equation (1990) is generally considered more accurate for most modern adults. The original Harris-Benedict equation (1919) tends to overestimate BMR, particularly in people who are overweight or obese. A 2005 meta-analysis found Mifflin-St Jeor to be the most accurate predictor of measured resting metabolic rate in 82% of non-obese individuals tested.
There is no single universally 'best' split — the most effective diet is one you can sustain long term. UK SACN guidelines suggest carbohydrates at 45–65%, protein at 10–35%, and fat at 20–35% of total calories. Higher protein intakes of 25–35% of calories (or 1.6–2.2g per kg body weight) are recommended during weight loss to preserve muscle mass, increase satiety, and support metabolic rate.
Metabolic adaptation is the reduction in metabolic rate that occurs in response to sustained calorie restriction. The body adapts to preserve energy, reducing BMR by 10–20% beyond what is expected from weight loss alone. This causes weight loss plateaus. Counter strategies include periodic diet breaks at maintenance calories, reverse dieting, ensuring sufficient protein, and resistance training to maintain muscle mass.
If you used an activity multiplier (e.g. moderately active) when calculating your TDEE, this already accounts for your regular exercise. Eating back all exercise calories may lead to overeating. If you chose a sedentary multiplier but exercise regularly, eating back approximately 50–75% of estimated exercise calories is a reasonable approach. Exercise calorie estimates on fitness trackers are often 20–30% too high.