Calorie Deficit Calculator
Our calorie deficit calculator helps you understand exactly how many calories per day you need to reduce to reach your goal weight within a specific timeframe. Enter your current weight, target weight, and the number of weeks you want to achieve this in. The calculator uses the established rule that 1 kg of body fat equals approximately 7,700 kilocalories (3,500 kcal per pound), and divides your total required deficit over the chosen period to give you a daily target.
A calorie deficit is the foundation of weight loss — consuming fewer calories than your body expends forces it to use stored fat for energy. However, the size of the deficit matters greatly for safety and sustainability. A moderate deficit of 300–500 kcal/day leads to gradual, sustainable fat loss of 0.3–0.5 kg per week, which most health organisations recommend as the safe and effective rate. Deficits above 750–1,000 kcal/day may cause muscle loss, nutritional deficiencies, fatigue, and metabolic adaptation, so always consult a healthcare professional if your calculated deficit exceeds safe limits.
Frequently Asked Questions
What is a calorie deficit?
A calorie deficit occurs when you consume fewer calories than your body burns in a day. Your body then uses stored energy (primarily fat) to make up the difference, leading to weight loss over time. A deficit of 3,500 kcal roughly corresponds to losing 0.45 kg (1 lb) of fat.
How many calories equal 1 kg of body fat?
One kilogram of body fat contains approximately 7,700 kilocalories of stored energy. This is why losing 1 kg of fat requires a cumulative deficit of ~7,700 kcal. In imperial units, 1 pound of fat ≈ 3,500 kcal.
What is a safe calorie deficit for weight loss?
Most health authorities recommend a deficit of 500–750 kcal/day as safe for most healthy adults, leading to 0.5–0.75 kg per week of fat loss. Deficits over 1,000 kcal/day are generally considered aggressive and should only be undertaken under medical supervision.
How quickly can I safely lose weight?
The NHS, WHO, and most dietitians recommend a safe rate of 0.5–1 kg (1–2 lbs) per week. This rate preserves muscle mass, avoids nutritional deficiencies, and is much more likely to result in permanent weight loss versus rapid crash diets that lead to weight regain.
What is TDEE and how does it relate to a calorie deficit?
TDEE (Total Daily Energy Expenditure) is the total calories your body burns in a day, including exercise and daily activities. Your calorie deficit is relative to your TDEE. If your TDEE is 2,500 kcal and you eat 2,000 kcal, your deficit is 500 kcal/day.
Should I count macros or just calories?
For weight loss, total calories are the primary factor (calories in vs. calories out). However, macronutrient balance significantly affects how you feel and whether you lose fat vs. muscle. High protein intake (1.6–2.2 g/kg body weight) during a calorie deficit helps preserve muscle mass and increases satiety.
Does exercise create a calorie deficit?
Yes — exercise increases your TDEE, making it easier to create a deficit without cutting food dramatically. A 30-minute run burns 250–400 kcal depending on weight and intensity. Combining moderate diet reduction with regular exercise is the most sustainable weight loss strategy.
Why do I stop losing weight even in a deficit?
This is called a weight loss plateau. Your body adapts to a lower calorie intake by reducing metabolic rate (adaptive thermogenesis). After 4–8 weeks, your TDEE may be lower than when you started. Solutions: take a diet break at maintenance calories for 1–2 weeks, re-calculate TDEE, or increase exercise.
Can I gain weight while in a calorie deficit?
True fat gain is impossible in a consistent calorie deficit. However, weight on the scale can temporarily increase due to water retention (especially with high salt, carb refeeds, or during menstrual cycle), muscle gain (rare but possible for beginners), or increased gut content. Trust long-term trends over daily fluctuations.
What is the difference between a calorie deficit for cutting vs. crash dieting?
Cutting (bodybuilding term) means a moderate, sustained deficit of 300–500 kcal/day, designed to maximise fat loss while preserving muscle. Crash dieting involves severe restriction (800–1,200 kcal/day total), which causes rapid initial weight loss but includes muscle loss, nutrient deficiencies, and almost always leads to weight regain.
Does the type of food affect weight loss in a deficit?
For fat loss, total calorie balance is paramount. However, food quality affects satiety, muscle preservation, and health. High-volume, high-fibre, high-protein foods (vegetables, lean meats, legumes) make it much easier to maintain a deficit without constant hunger, compared to calorie-dense processed foods.