Pick a goal weight. The math tells you exactly how long it'll take.
A weight loss calculator does three things: estimates your maintenance calories, applies a deficit, then projects how long it takes to reach your target. The actual numbers behind that calculation are simple and worth understanding.
Skip the arithmetic. The free Weight Loss Calculator runs it both ways: set a deadline to get the daily deficit you need, or set a deficit to get your timeline.
The Weight Loss Math
Three numbers drive every weight loss calculation:
- TDEE: calories you burn per day at current weight
- Deficit: calories you eat below TDEE per day
- Time: weeks to reach goal
The formula
1 kg of body fat = ~7700 calories
Weeks to lose X kg = (X × 7700) / (daily deficit × 7)
Example: want to lose 10 kg. On a 500 cal/day deficit:
(10 × 7700) / (500 × 7) = 77000 / 3500 = 22 weeks
22 weeks = roughly 5.5 months.
Realistic Weight Loss Rates
Reality check: pure-fat-loss math says X kg in Y weeks. Reality is messier because of water, glycogen, hormonal changes, and adherence.
| Bodyweight | Sustainable Rate | Aggressive Rate |
|---|---|---|
| 60 kg | 0.3-0.6 kg/week | 0.6 kg/week |
| 80 kg | 0.4-0.8 kg/week | 0.8 kg/week |
| 100 kg | 0.5-1.0 kg/week | 1.0 kg/week |
Sustainable rate (0.5-0.75% bodyweight/week): doable long-term, preserves muscle, doesn't trash hormones.
Aggressive rate (1% bodyweight/week): works for short cuts (4-8 weeks), causes muscle loss if extended, hard to sustain.
Calculating Time to Goal
Quick Formula
Weeks to goal = total kg to lose / weekly loss rate
Examples:
- Need to lose 5 kg, target 0.5 kg/week → 10 weeks (2.5 months)
- Need to lose 10 kg, target 0.5 kg/week → 20 weeks (5 months)
- Need to lose 15 kg, target 0.7 kg/week → 21-22 weeks (5 months)
- Need to lose 25 kg, target 0.8 kg/week → 31 weeks (7-8 months)
- Need to lose 40+ kg, target 1.0 kg/week → 40+ weeks (10+ months, plan for diet breaks)
The bigger the total target, the longer the journey, and the more diet breaks you'll need.
Why Pure-Math Estimates Underestimate
In reality, weight loss isn't linear:
- Week 1-2: larger drops (water, glycogen depletion), often 2-4 kg
- Week 3-12: steady loss at calculated rate, most predictable
- Week 12+: rate slows because BMR drops with bodyweight; need to recalculate
- Plateaus: common around week 8-12, lasting 1-3 weeks
Plan on 10-20% longer than pure math suggests.
Adjusting as You Lose
Your TDEE drops as you lose weight (less tissue to maintain). A target that produced steady loss at 95 kg will plateau at 85 kg.
Recalculate every 5-10 kg of weight change.
Example trajectory:
- Start: 95 kg, TDEE 2900, target 2400 (500 deficit), losing 0.5 kg/week
- After losing 10 kg: 85 kg, TDEE drops to ~2750, same intake = 350 deficit, slower loss
- Recalculate: TDEE × deficit → 2250 to maintain 0.5 kg/week loss
Run your updated numbers back through the Weight Loss Calculator every 5-10 kg so your deficit and timeline stay matched to your new bodyweight.
Diet Breaks Are Mandatory for Long Cuts
Anything past 12-16 weeks of continuous deficit:
- Hormonal fatigue (testosterone drops, thyroid slows, leptin crashes)
- Adherence breaks down
- BMR drops more than the math predicts (metabolic adaptation)
The solution: every 8-12 weeks of dieting, take 1-2 weeks at maintenance calories. You won't gain meaningful fat, you'll restore hormones and motivation, and the next dieting block works better.
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