What Is a Calorie and Why Does It Matter?
A calorie (technically a kilocalorie, or kcal) is a unit of energy. In nutrition, it measures the amount of energy that food provides when metabolised by the body. Your body requires a continuous supply of energy to maintain basic functions — breathing, circulating blood, regulating body temperature, and repairing cells — even at complete rest. This baseline energy requirement is known as your Basal Metabolic Rate (BMR).
Your Total Daily Energy Expenditure (TDEE) is your BMR multiplied by an activity factor that accounts for the energy you burn through movement, exercise, and daily activity. To maintain your current weight, you need to consume approximately the same number of calories as your TDEE. To lose weight, you need to consume fewer (a calorie deficit); to gain weight or build muscle, you need to consume more (a calorie surplus).
The Mifflin-St Jeor Equation
This calculator uses the Mifflin-St Jeor equation, published in 1990 and widely regarded as the most accurate formula for estimating BMR in the general population. A 2005 study in the Journal of the American Dietetic Association found it to be the most reliable of the commonly used BMR equations, accurate to within 10% for most people. The formulas are as follows:
| Sex | Formula |
|---|---|
| Male | BMR = (10 × kg) + (6.25 × cm) − (5 × age) + 5 |
| Female | BMR = (10 × kg) + (6.25 × cm) − (5 × age) − 161 |
Understanding Macronutrients
Macronutrients — protein, carbohydrates, and fat — are the three categories of nutrients that provide calories. Protein and carbohydrates each provide 4 kcal per gram, while fat provides 9 kcal per gram. The ratio in which you consume these macros can significantly influence your body composition, energy levels, and athletic performance, even when total calorie intake is the same.
The macro split used in this calculator (30% protein, 40% carbohydrates, 30% fat) is a balanced general-purpose split suitable for most people. Those focused on building muscle may benefit from a higher protein intake (up to 40%), while endurance athletes may require a higher carbohydrate proportion. For personalised macro targets, consult a registered dietitian.
How Many Calories Should I Eat to Lose Weight?
A deficit of 500 kcal per day below your TDEE is generally recommended for steady, sustainable weight loss of approximately 0.5 kg (1 lb) per week. More aggressive deficits can lead to faster initial weight loss but risk muscle loss, nutritional deficiencies, and metabolic adaptation. The NHS recommends a minimum intake of 1,200 kcal/day for women and 1,500 kcal/day for men — this calculator enforces a floor of 1,200 kcal to prevent dangerously low targets.
The figures produced by this calculator are estimates based on population-level equations and should be used as a starting point rather than a precise prescription. Individual metabolism varies, and factors such as sleep quality, stress levels, hormonal health, and gut microbiome composition can all influence actual calorie needs. If you are managing a medical condition or have specific dietary requirements, always consult a qualified healthcare professional or registered dietitian before making significant changes to your diet.