What is a calorie deficit?
A calorie deficit means eating fewer calories than your body uses in a day. A common rule of thumb is that 550 kcal a day below your daily energy is about 0.5 kg a week, though real results vary.
Updated 21 October 2026. Sources named on the page.
- What a deficit is in one line
- How to work it out from daily energy
- Why there is a floor you do not go below
- Who should not use a deficit
Keep your results in the Daily Journal
Habits, food, mood and sleep in one private place, with progress charts and a daily research card. Free to start, and your account opens straight away.
- Every claim names its study
- Alone, as a couple or with a group
- Private by default
Work out your deficit
Pick a gentle or moderate deficit and see your target in kcal and kJ, held at a safe floor.
The sums
Target = daily energy (TDEE) minus the deficit. About 7,700 kcal is often used as the energy in 1 kg of body weight, so 550 x 7 = 3,850 kcal a week is about 0.5 kg.
Worked example (man, 30 y, 80 kg, 180 cm, moderately active)
| Deficit | Target | In kilojoules | Rule of thumb |
|---|---|---|---|
| None | 2,750 kcal | 11,506 kJ | keeps weight |
| Minus 300 | 2,450 kcal | 10,251 kJ | a slower change |
| Minus 550 | 2,200 kcal | 9,205 kJ | about 0.5 kg a week |
Sources: The 7,700 kcal per kg figure is a commonly used estimate, not a measurement of any person; 1 kcal = 4.184 kJ
Figures and sources checked 21 October 2026. General information, not medical advice.
How it works
Your body uses energy every day, which is your TDEE. Eat less than that and the body makes up the gap from stored energy over time. Eat more and it stores the extra.
Why smaller is safer
Large deficits are not offered here. They can bring tiredness, lost muscle and missed nutrients, and they are hard to keep up. The calculator offers 300 or 550 kcal a day and never goes below 1,200 kcal for women or 1,500 kcal for men.
Real life is lumpier
Weight moves up and down from water, food in the gut and hormones, so a week on the scale can mislead. Look at a few weeks together, and remember the rule of thumb is a guide, not a promise.
Who should not use a deficit
Not for anyone under 19, in pregnancy or breastfeeding, or with a medical condition. If you have or have had an eating disorder, do not use a calculator like this: talk to a doctor or an accredited practising dietitian instead.
Questions
How big should a calorie deficit be?
This site offers a gentle 300 kcal or a moderate 550 kcal a day below daily energy. Bigger deficits are not appropriate without medical supervision.
How much weight will I lose with a deficit?
A common rule of thumb is about 0.5 kg a week for 550 kcal a day. It is not a promise: real change varies from week to week and person to person.
Do I have to count calories?
No. Counting is one method. Others are simple habits such as regular meals and portion sizes. Choose what you can keep up.
What is the lowest target I should use?
The calculator never goes below 1,200 kcal for women or 1,500 kcal for men.
Keep going
Track the habits behind the number
Sign up free for the Daily Journal app: habits, food, mood and sleep in one private place. Your account opens straight away.