Contents
What is a calorie deficit?
A calorie deficit means you take in less energy from food than you burn in a day. Your body covers the shortfall from its reserves, and your weight goes down. There is no other mechanism of weight loss: intermittent fasting, keto and "clean eating" work exactly to the extent that they create a deficit.
The hard part is not creating a deficit but sticking to it. Most diets end not because they stop working but because the person relapses in the second or third week. And more often than not, the reason for the relapse is built into the number itself.
How to calculate your target and deficit
First you need your maintenance calories - how much you burn while your weight stays stable. It is calculated with the Mifflin-St Jeor equation: basal metabolic rate multiplied by an activity factor. The easiest way is our calorie calculator.
Example: a 29-year-old woman, 168 cm tall, 63 kg, with 3-4 workouts or a lot of walking per week.
| Metric | kcal per day |
|---|---|
| Basal metabolic rate | 1,374 |
| Maintenance (× 1.375) | 1,889 |
| 15% deficit | 1,606 |
| 20% deficit | 1,511 |
| 40% deficit | 1,134 |
A deficit is set as a percentage of maintenance, not as a fixed number: "minus 500 kcal" is a very different load for someone burning 1,800 kcal and someone burning 3,200 kcal.
Why 15-20% beats 40%
It seems that the bigger the deficit, the faster the result, but in practice a harsh deficit loses to a moderate one. The woman from the example would struggle to ever feel full on 1,134 kcal a day, with every plate small and food on her mind all day long, and a relapse in the second week in that situation isn't a lack of willpower but an expected response of the body. With a deficit that deep, especially when protein is short, the body is quicker to burn muscle, so the scale goes down while the body gets softer and metabolism slows. On top of that, people on a harsh diet move less without noticing, getting up from their desk less often and walking less, and this invisible saving eats up part of the deficit. Fatigue, irritability, poor sleep and cold hands are also common companions of eating too little.
A moderate 15-20% deficit gives a slow but steady pace, which is why the calculator on our site uses 15% by default for the "lose weight" goal. A normal pace is 0.5-1% of body weight per week, or 0.3-0.6 kg for someone weighing 63 kg. The 15% deficit from the example, about 280 kcal a day, gives roughly 1-1.2 kg a month without hunger and while keeping muscle.
Signs your deficit is too harsh
A number can look reasonable on paper, but what matters is how you cope with it. If for two weeks in a row you stay hungry even after meals, finish the evening with sweets and snacks after dinner, run out of energy in the gym or by mid-afternoon at work, sleep worse and snap at people more, or the scale drops faster than 1% a week for several weeks, it's time to raise your calories.
What helps you stay on track
The first thing to look at is protein. It fills you up more than carbs and fat and helps keep muscle, and in a deficit a good target is 1.6-2 g per kilogram of body weight, more in Dietary protein: recommended intake and sources. Vegetables are the easiest way to add volume: when half the plate is cucumbers, cabbage or stewed zucchini, lunch looks like a full meal at a modest calorie count, and the fiber adds up too.
There's no need to ban your favourite foods, because it's the ban that turns a slice of cake into a relapse. It's better to set aside 10% of your daily calories for whatever you crave and log it. Judge the week rather than a single day: a friend's birthday or a dinner party isn't a failure if your weekly average stayed in a deficit, and the app shows weekly averages right in the diary. Don't skip lunch, since it almost always comes back in the evening twice as big, and get enough sleep, because sleep deprivation raises appetite and sugar cravings and is one of the most underrated causes of relapses. You can weigh yourself every morning if you like, but compare weekly averages, since water and salt move the scale by 0.5-1.5 kg from one day to the next.
How to count calories without spending half the day on it
The most common mistake people make while losing weight is underestimating what they ate: those who count by eye miss 20-40% on average, which is exactly the size of their deficit. Oil, sauces, nuts and home cooking are where the errors are biggest. You don't need to weigh every portion, though. In MyFoodMate AI you just photograph your plate, the bot recognises the dish, estimates the weight and counts calories and macros, and the diary shows how much is left until your target. You can try it on the home page without signing up.
When to take a break
If you've been losing weight for longer than 8-12 weeks, it often helps to eat at maintenance for a week or two. In 2017 Nuala Byrne's team in Tasmania had men alternate two weeks of dieting with two weeks at maintenance, and they lost more fat than the group that dieted without a pause. Your weight may creep up a little in those days because of water and glycogen, but that isn't fat. During pregnancy or breastfeeding, with diabetes or thyroid disease, or if you've had an eating disorder, set your calorie target together with a doctor, because a harsh deficit is especially risky then.
The bottom line
The diets mentioned at the start usually fall apart in week two or three, not because they stopped working but because the number was too low from day one. A 15-20% deficit and a pace of half a percent to one percent of your weight a week look modest on paper, yet people keep them up for months when they eat enough protein and vegetables, sleep properly and look at the weekly average. If hunger hasn't let go after a couple of weeks, raise your calories a little rather than wait for it to pass.
We check blog materials against current WHO recommendations.
