What is a calorie deficit?
A calorie deficit means you take in less energy from food than you use. Total daily energy expenditure (TDEE) is the sum of your basal metabolic rate, movement and the digestion of food. The calculator works it out with the Mifflin-St Jeor equation, just like the calorie calculator in Junai Lab, and then subtracts the deficit that matches the chosen pace.
A common rule of thumb is that 1 kg of body mass equals about 7700 kcal. Research, for example Hall's paper from 2008, shows this is a simplification: the body adapts to a deficit, so the pace slows down over time. The result is therefore a guideline, not a forecast.
Pace and the lower limit
The calculator offers three paces: 0.25, 0.5 and 0.75 kg a week. At 0.5 kg a week the daily deficit is about 550 kcal. A slower pace is easier to stick with, while a faster one needs a bigger deficit and, with lower energy use, quickly gets close to the lower limit.
The lower limit is 1200 kcal a day for women and 1500 kcal for men. If the chosen pace would take your intake below the limit, the calculator sets the intake at the limit and warns you. Eating below this limit without medical supervision is not advisable, because it is hard to get enough protein, vitamins and minerals. The calculator does not know your health, your medicines or your day-to-day differences in activity. Treat the result as a starting point that you can adjust after two to three weeks, based on how your weight actually moves.
A safeguard against a goal weight that is too low
The World Health Organization (WHO) defines underweight as a BMI below 18.5. If your goal weight at your height would mean a BMI below that limit, the calculator does not work out a plan and shows the lowest weight in the normal range instead.
The deficit is not the only factor that counts. On the scales, weight changes daily because of water, salt in food and digestion, so a single day is not a measure. The average of a week or two is more telling. Someone who moves more uses more energy, so the estimate of your expenditure also depends on how accurately you choose your activity level.
For a healthy diet, the WHO recommends a varied diet with vegetables, fruit, wholegrain foods and enough protein. The calculator does not include a meal plan. If you have an illness, are pregnant or breastfeeding, or have an eating disorder, talk to a doctor or dietitian before changing your diet.
Frequently asked questions
How many calories a day to lose 0.5 kg a week?
About 550 kcal less than you use in a day. The calculator above works out your expenditure and subtracts the deficit from it.
Why does the calculator use 7700 kcal per kilogram?
It is a common approximation of the energy value of a kilogram of body mass. In reality the body adapts, so the calculation is a guideline.
What is the lower calorie limit?
The calculator does not go below 1200 kcal a day for women and 1500 kcal for men. Going below that without medical supervision is not advisable.
Why does the calculator not accept a very low goal weight?
If the goal weight would mean a BMI below 18.5, the calculator does not show a plan, because the WHO classes that as underweight.
Is the calculator suitable for pregnant women?
No. It is made for healthy adults. During pregnancy and breastfeeding, talk to a doctor about your diet.
Sources
- Mifflin MD et al. – A new predictive equation for resting energy expenditure in healthy individuals (1990)
- Hall KD – What is the required energy deficit per unit weight loss? (2008)
- WHO – Healthy diet (fact sheet)
Updated: October 2026
For information only, not a substitute for medical advice.