What a calorie deficit is
A calorie deficit is when you consume fewer calories than your body burns. That gap forces your system to draw energy from its reserves — mainly body fat. It is the only proven mechanism for weight loss: whatever diet you follow, if it works, it works because it created a deficit.
The size of the deficit determines how fast you lose and how sustainable it is. Small deficits (10–15%) produce slow but comfortable loss, with less hunger and less risk of muscle loss. Large deficits (above 25%) speed things up but raise the odds of binge eating, fatigue and a metabolic plateau.
How to calculate your deficit
The calculation has three steps:
1. Find your BMR (Basal Metabolic Rate) — what your body burns at rest. This calculator uses the Mifflin-St Jeor formula, the most widely accepted one. 2. Multiply by your activity factor to get your TDEE (total daily burn): sedentary ×1.2, light ×1.375, moderate ×1.55, intense ×1.725. 3. Apply the deficit you want: subtract 10%, 20% or 25% from your TDEE. The result is your daily calorie target.
For example: if your TDEE is 2,400 kcal and you choose a moderate 20% deficit, your target lands at 1,920 kcal/day. The gap (480 kcal/day) adds up to 3,360 kcal over a week — roughly 0.44 kg of fat.
Which deficit is safe
Research and clinical practice converge on a few limits:
• A deficit of 10–20% of your TDEE is considered safe and sustainable for most people. It is the range that preserves muscle mass, keeps your metabolism active and causes fewer side effects (excessive hunger, irritability, fatigue). • A deficit above 25% can work in the short term, but it significantly raises the risk of binge eating, muscle loss and metabolic slowdown (the dreaded "plateau"). • Never go below the safety floor: 1,500 kcal/day for men and 1,200 kcal/day for women. Below that it is very hard to get all the micronutrients you need without medical supervision.
This calculator applies that floor automatically. If the deficit you pick would take you below the minimum, it adjusts and tells you.
How much you can actually lose per week
The theoretical math is simple: every 7,700 kcal of deficit equals roughly 1 kg of fat. With a 500 kcal/day deficit, that works out to about 0.45 kg per week; at 250 kcal/day, about 0.23 kg.
In practice, though, the scale will swing more than that — and that is no reason to give up. Water retention, your menstrual cycle, the sodium in yesterday's dinner and even stress all affect daily weight. What matters is the trend across weeks.
A healthy, realistic pace for most people is between 0.3 and 0.7 kg per week. Losing faster is possible, but it usually costs you muscle and sanity. Slow and consistent wins.
Frequently asked questions
- Do I need to be in a deficit every single day?
- No. What matters is the weekly average. If you ate more at a celebration, pull back slightly over the following days — no drama. Consistency across weeks matters far more than daily perfection.
- If I exercise more, can I eat more and still lose weight?
- In principle, yes. Exercise raises your TDEE, which allows a deficit with more food. But be careful: the calories burned in exercise are usually overestimated. Treat exercise as a tool for health and wellbeing, not as a license to eat without limit.
- Is a light deficit for longer better than an aggressive one for a short time?
- For most people, a light to moderate deficit sustained for longer is more effective. Aggressive deficits produce fast early results but have a high dropout rate and a rebound effect. A deficit you can hold for months is worth more than one that lasts two weeks.
- How do I count calories without weighing everything?
- Use an app like Nutricalor: photograph your plate and the AI estimates calories and macros in seconds. You do not need to weigh every ingredient — a photo estimate is practical enough to hold a deficit.
- Does a calorie deficit work for everyone?
- A calorie deficit is the universal mechanism of weight loss. But the ideal size of the deficit, the composition of your diet and the rate of loss vary from person to person. Medical conditions (thyroid, insulin resistance, medications) can affect the process. Whenever possible, consult a registered dietitian.