Calorie Deficit Calculator (Weight Loss Timeline)
Calculate the calorie deficit needed to lose weight by a target date, and project how long realistic deficits will take.
What this calculates
Fat loss is fundamentally calorie deficit over time. About 7,700 kcal is one kg of body fat — so a 500 kcal/day deficit produces roughly 0.5 kg per week. This calculator works two directions: given a target weight loss and timeline, find the required daily deficit; or given a deficit, find how long the change will take.
Formula & how it works
Daily deficit = (kg_to_lose × 7700) ÷ days. Weeks to goal = (kg_to_lose × 7700) ÷ (daily_deficit × 7). Each kg of fat ≈ 7,700 kcal. Lean tissue losses (water, glycogen, some muscle) can make early weight loss appear faster, then plateau as fat dominates. Adaptive thermogenesis (your BMR slowly drops) extends timelines beyond the simple math.
Worked example
Target: lose 10 kg. 12-week timeline → 84 days. Required deficit = (10 × 7700) ÷ 84 ≈ 917 kcal/day. That's aggressive — possible but hard to maintain. Stretching to 16 weeks → 688 kcal/day, much more sustainable. Stretching to 24 weeks → 458 kcal/day, gentle and easy to recover BMR from.
Frequently asked questions
What's a safe deficit?
300–750 kcal/day for most adults. Larger deficits accelerate muscle loss, slow BMR more, and crash energy/mood. The 'lose 10 lb in 2 weeks' programs usually trigger a rebound within 3 months.
Why do I plateau?
Adaptive thermogenesis: as you lose weight your BMR drops (less mass to maintain). The deficit you started with becomes maintenance at lower weight. Recalculate TDEE every 5 kg and adjust.
Should I exercise more or eat less?
Both, in moderation. Diet is more efficient for creating deficit (it's easier to skip 500 kcal than to burn 500 kcal exercising). Exercise — especially resistance training — protects muscle during the cut.
What about 'starvation mode'?
Severe under-eating does slow metabolism, but the dramatic 'starvation mode' lore is overblown. A 1,200 kcal diet doesn't 'lock' your metabolism. The bigger problem is unsustainability and muscle loss.
Disclaimer: Talk to a registered dietitian before aggressive calorie restriction.
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