Calorie Deficit Calculator
Plan a safe, realistic weight loss timeline
What Is a Calorie Deficit?
A calorie deficit occurs when you consume fewer calories than your body burns in a day. This energy shortfall forces your body to draw on stored fuel, primarily body fat, to make up the difference. Over time, this consistent gap between intake and expenditure leads to weight loss. The size of the deficit determines how quickly you lose weight, while the duration determines how much total weight you lose.
This calculator takes a goal-oriented approach. Instead of asking you to guess at a deficit size, you provide your current weight, your target weight, and how many weeks you want to take. The calculator then works backward to determine the daily deficit required and tells you whether that rate of loss is safe, aggressive, or unrealistic.
The Mathematics of Weight Loss
One kilogram of body fat stores approximately 7,700 calories of energy (roughly 3,500 calories per pound). To lose one kilogram per week, you need a cumulative weekly deficit of 7,700 calories, which works out to about 1,100 calories per day. This is the fundamental equation that governs all weight loss, regardless of the diet method you follow.
| Weekly Loss Rate | Daily Deficit | Safety Rating |
|---|---|---|
| 0.25 kg (0.55 lbs) | 275 kcal | Very safe |
| 0.5 kg (1.1 lbs) | 550 kcal | Safe (recommended) |
| 0.75 kg (1.65 lbs) | 825 kcal | Moderate |
| 1.0 kg (2.2 lbs) | 1,100 kcal | Aggressive |
| 1.5 kg+ (3.3 lbs+) | 1,650+ kcal | Not recommended |
Safe Deficit Rates
Most health professionals recommend losing no more than 0.5 to 1.0 kg (1-2 lbs) per week for the general population. This rate of loss can typically be achieved with a daily deficit of 500-1,000 calories, which is large enough to produce visible progress but moderate enough to preserve lean muscle mass, maintain energy levels, and avoid the worst effects of metabolic adaptation.
There are exceptions. People with a high starting body fat percentage (BMI over 30) can often safely sustain larger deficits in the early weeks because they have more stored energy available. Conversely, people who are already relatively lean (men under 15% body fat, women under 25%) should use smaller deficits of 250-500 calories to minimize muscle loss during the final stages of a cut.
Metabolic Adaptation: Why Deficits Slow Down
When you eat in a calorie deficit, your body gradually adapts to the reduced energy intake through a process called metabolic adaptation (sometimes called "adaptive thermogenesis"). Your body becomes more efficient, burning fewer calories through several mechanisms:
- Reduced NEAT: Your body unconsciously reduces non-exercise movement, including fidgeting, posture maintenance, and spontaneous activity. This can account for a reduction of 200-400 calories per day.
- Lower thermic effect of food: Eating less food means less energy spent digesting food.
- Hormonal changes: Thyroid hormones (T3, T4) decrease, leptin drops (increasing hunger), and cortisol rises. These hormonal shifts further reduce metabolic rate and increase appetite.
- Reduced body mass: A lighter body simply requires fewer calories to move and maintain. For every kilogram lost, your TDEE drops by approximately 15-20 calories per day.
This is why linear weight loss projections are estimates. The first few weeks often produce faster results (partly due to water and glycogen loss), while later weeks may slow down as adaptation takes hold. Plan for this by building in periodic diet breaks or refeed days.
Weight Loss Projections vs. Reality
The projection this calculator provides assumes a linear rate of loss, but real-world weight loss is rarely linear. Day-to-day weight fluctuations of 1-2 kg are completely normal and are caused by water retention, sodium intake, carbohydrate intake, bowel contents, and hormonal cycles. The trend over weeks and months is what matters, not any single weigh-in.
To get the clearest picture of your progress, weigh yourself daily under the same conditions (morning, after using the bathroom, before eating) and look at the 7-day moving average. This smooths out the noise and reveals your true rate of loss.
Combining Deficit with Exercise
A calorie deficit can be created through diet alone, exercise alone, or a combination of both. The most effective and sustainable approach is a combination: reduce your food intake by 300-500 calories and increase your activity to burn an additional 200-300 calories. This split is easier to maintain than extreme dietary restriction and has the added benefit of preserving muscle mass, improving cardiovascular health, and supporting mental wellbeing.
Resistance training is particularly important during a deficit. Without strength training, up to 25% of weight lost can come from muscle rather than fat. With a structured resistance program and adequate protein intake (1.6-2.2 g per kg), muscle loss can be reduced to less than 5% of total weight lost.
Setting a Realistic Timeframe
One of the most common mistakes is setting an overly aggressive timeline. Losing 20 kg in 8 weeks requires a daily deficit of over 2,750 calories, which is virtually impossible and dangerous for most people. A realistic and healthy timeline for 20 kg of fat loss is 20-40 weeks (5-10 months). Use this calculator to experiment with different timeframes until you find a deficit that is both achievable and sustainable.