This free guide explains what a calorie deficit is and how it works, clearly and safely. It’s an educational reference — it doesn’t generate a personal calorie target or a weight-loss timeline. For your numbers, use the linked Calorie and TDEE calculators.
What is a calorie deficit?
A calorie deficit means consistently taking in fewer calories than your body uses. Over time, that gap is what leads to weight loss. It doesn’t depend on any particular diet — just a sustained, sensible gap between the energy you eat and the energy you burn.
How a calorie deficit works: energy balance
Your body uses energy for basic functions (your BMR) plus all your movement and exercise — together, your TDEE (Total Daily Energy Expenditure). Eat around your TDEE and weight stays roughly stable; eat consistently a little below it and you tend to lose weight; a little above and you tend to gain. To estimate your own TDEE, use the TDEE Calculator.
How big should a calorie deficit be?
A modest deficit — often around 300–500 calories a day — is widely considered sustainable and supports gradual loss of roughly 0.25–0.5 kg (½–1 lb) a week. Bigger isn’t better: large deficits are hard to maintain, can cost you muscle and energy, and often lead to rebound. Slow and steady tends to win.
The “3,500 calories per pound” rule
You’ll often see that a pound of fat is about 3,500 calories (roughly 7,700 per kilogram), so a 500-calorie daily deficit “equals” a pound a week. It’s a useful rough guide, but not a precise law — your body adapts to eating less, water weight shifts, and real-world results rarely match the maths exactly. Treat any weight-loss projection as a loose estimate, not a promise.
When to be cautious with a deficit
- Avoid very low intakes — they’re hard to sustain and can harm health. Very low-calorie diets need medical supervision.
- Some situations need professional guidance first — pregnancy, breastfeeding, growth, illness, or a history of disordered eating.
- Don’t push through feeling unwell — dizziness, exhaustion or persistent hunger are signals, not obstacles to power past.
Making a calorie deficit sustainable
- Keep it gentle rather than aggressive.
- Prioritise food quality — protein, fibre, fruit and vegetables help you feel full.
- Stay active and sleep well, which support both adherence and results.
- Be patient, and adjust based on real results over several weeks.
Get your numbers safely
To turn the concept into figures, use the Calorie Calculator — it estimates your maintenance calories and applies a gentle, capped deficit for the “Lose” goal. For a plan suited to your health and goals, a registered dietitian or doctor is the best guide.
Can I use this as a calorie deficit or weight loss calculator?
Yes. Choose the "Lose" goal and the tool applies a gentle deficit of about 500 calories below your maintenance, roughly half a kilo (1 lb) a week. It keeps the deficit modest and won't show very low targets.
How big should a calorie deficit be?
A moderate deficit of about 300–500 calories a day is widely considered sustainable, supporting gradual loss. Larger deficits are harder to maintain and can backfire; very low intakes need medical supervision.
