How a calorie deficit works
You lose weight when you eat less energy than you burn. The gap is the deficit. A kilo of body fat holds roughly 7,700 kcal, so a deficit of about 550 kcal a day comes to about half a kilo a week at first.
This calculator works out what you burn (Mifflin–St Jeor with your activity), takes away the deficit for the pace you choose and shows what is left to eat. It never goes under 1,200 kcal a day for women or 1,500 for men.
Why weight loss slows down
A lighter body burns less, and the body also saves a little energy when food is short. So the same calories bring less loss each month, and a straight-line forecast is always too hopeful about the last kilos. Early on, water and the food in your gut make the scale fall faster than fat does.
That is why the target should move with you. Look at your weight trend over two or three weeks, not day to day, and lower the target by 100–150 kcal when the trend stalls for a fortnight.
A pace you can keep
0.25–0.5 kg a week is the pace most people can hold for months without feeling starved. Faster is possible with a lot of weight to lose, but it costs more muscle and is harder to keep up. Plenty of protein, vegetables and some strength training make any deficit easier and protect muscle.
Foods that fill you up for few calories matter more than willpower: soups, vegetables, fruit, beans, lean protein and whole grains keep hunger quiet; drinks with sugar and alcohol do not.
Questions
- What is a good calorie deficit?
- Around 300–500 kcal a day suits most people: about 0.25–0.5 kg a week. A bigger deficit works faster on paper but is harder to keep and costs more muscle.
- How long will it take to lose 10 kg?
- At about 0.5 kg a week, roughly 20 weeks — in practice a little more, because loss slows as you get lighter. Your weight trend over a few weeks shows your own pace.
- Why am I not losing weight in a calorie deficit?
- Most often the deficit is smaller than it looks: portions are underestimated, snacks and drinks are forgotten, or the burn was overestimated. Water can also hide fat loss for a week or two. Log honestly for two weeks and watch the trend, not single days.
- Is it safe to eat 1,200 calories a day?
- For many women it is the lowest level at which a normal diet can still cover protein, vitamins and minerals; for men about 1,500. Going lower should be supervised by a doctor.
Sources
- Hall KD et al. Quantification of the effect of energy imbalance on bodyweight. Lancet, 2011.
- Hall KD, Kahan S. Maintenance of lost weight and long-term management of obesity. Med Clin North Am, 2018.
- Mifflin MD et al. A new predictive equation for resting energy expenditure in healthy individuals. Am J Clin Nutr, 1990.