Every weight-loss method — keto, intermittent fasting, low-carb, plant-based, flexible dieting — produces results through the same mechanism: a calorie deficit. The food choices change. The deficit doesn't.
Understanding the deficit lets you evaluate any diet honestly. This article covers the definition, the math, and how to set a deficit that actually works.
The math of a deficit
Body fat stores energy at roughly 7,700 calories per kilogram (3,500 per pound). To lose 1 kg of fat, you need a cumulative deficit of 7,700 calories. Spread that over a week and a 1,100 kcal/day deficit produces 1 kg/week loss — but that's an extreme rate.
More realistic numbers:
- 250 kcal/day deficit → ~0.25 kg/week, ~1 kg/month.
- 500 kcal/day deficit → ~0.5 kg/week, ~2 kg/month.
- 750 kcal/day deficit → ~0.7 kg/week, ~3 kg/month.
- 1,000 kcal/day deficit → ~1 kg/week — fast but risks muscle loss.
How to set a deficit
Two simple steps:
- Estimate TDEE. Use the Mifflin–St Jeor formula with an activity multiplier.
- Subtract the deficit. Start with 300–500 kcal/day below TDEE for most adults. Smaller for late-stage cuts; larger only for short, supervised aggressive phases.
Why the deficit isn't always linear in real life
Three real-world wrinkles can make actual weight loss differ from the 7,700-kcal math:
- Adaptive thermogenesis. Sustained deficits lower NEAT and BMR by 50–250 kcal/day, slowing predicted loss.
- Glycogen and water. The first 1–2 kg of any deficit comes off fast — it's stored carb and water, not fat.
- Tracking error. Most people underreport intake by 10–25%. Your 500-kcal deficit on paper might be 200 in reality.
Worked example
An 85 kg man with TDEE 2,700 kcal. He targets 0.5 kg/week loss → 500 kcal/day deficit → eats 2,200 kcal/day.
Over 12 weeks: theoretical loss 6 kg. Actual loss often 4.5–5.5 kg because of adaptive thermogenesis and small tracking slop. He recalibrates after week 6 — his new TDEE at lower body weight is ~2,580, so eating 2,200 is now a 380-kcal deficit, slowing loss. He drops to 2,080 to maintain pace.
Common mistakes and misconceptions
- Going too aggressive. 1,000+ kcal/day deficits work for 4 weeks and then collapse adherence.
- Ignoring weekend overshoots. A weekly deficit beats a weekday deficit. Saturdays count.
- Confusing scale weight with fat loss. Water shifts cause 0.5–2 kg swings unrelated to fat.
- Not recalibrating. As you lose weight, TDEE drops and your old deficit shrinks. Recalculate every 5 kg.
Tips for running a deficit
- Start at 300–500 kcal under TDEE. You can always go deeper later; you can't un-binge a 1,000-kcal deficit week 1.
- Protein at 1.8–2.4 g/kg of body weight defends muscle during the deficit.
- Plan one weekly higher-calorie meal so restriction feels less permanent.
- Take a 1-week diet break every 8–10 weeks of dieting — restores some lost NEAT.
Related questions
Can I lose weight without a calorie deficit?
What is a safe calorie deficit?
Does a calorie deficit slow my metabolism?
Should the deficit come from food or exercise?
Does my calorie deficit need to be perfect every day?
Keep reading on this site
- TDEE & Body Fat Calculator — see fat-loss target sized off your TDEE
- Calories to lose weight — deficit sizing guide
- What is TDEE? — the baseline you're subtracting from
- How to reduce body fat — the full fat-loss protocol