A calorie deficit is the gap between what you burn (TDEE) and what you eat. Sustain a deficit and you lose fat. Set it too aggressively and you can't sustain it; set it too small and you don't see results. Here's how to pick a number that works.
The math
Body fat stores roughly 3,500 kcal per pound. So a 500 kcal/day deficit, sustained for a week, theoretically loses 1 lb of fat:
500 kcal/day × 7 days = 3,500 kcal/week ≈ 1 lb/week
In practice, fat loss is messier because:
- Water weight fluctuates by ±2-4 lb day-to-day
- Glycogen stores shift with carb intake
- Your TDEE drops slightly as you lose weight (less mass to move)
- After 6-8 weeks, adaptive thermogenesis kicks in (your body burns less than the formula predicts)
So 500 kcal/day usually produces something like 0.7-1.0 lb/week of actual fat loss, averaged over a month.
How big a deficit should you pick?
Two competing forces:
- Bigger deficit = faster results, but harder to sustain (hunger, energy crashes, muscle loss)
- Smaller deficit = slower results, but easier to maintain for months
Use these as starting points:
| Goal | Deficit | Loss rate | Notes |
|---|---|---|---|
| Aggressive cut (event, photo shoot) | 750-1,000 kcal | 1.5-2 lb/wk | Sustainable for 4-8 weeks max |
| Standard cut | 500 kcal | 1 lb/wk | Sustainable for 3-6 months |
| Conservative cut | 250 kcal | 0.5 lb/wk | Sustainable indefinitely; best for sustainable habits |
| Slow recomp | 100-200 kcal | 0.2-0.4 lb/wk | Best for already-lean people preserving muscle |
Most people should pick 250 or 500. Picking 1,000 because you want results "fast" is the single most common reason calorie tracking fails — you can't sustain it, you binge, and you net out roughly maintenance.
The 1% body weight rule
A safer way to set your deficit: aim for 1% of body weight per week, max.
| Body weight | Max weekly loss | Max daily deficit |
|---|---|---|
| 150 lb | 1.5 lb | 750 kcal |
| 175 lb | 1.75 lb | 875 kcal |
| 200 lb | 2.0 lb | 1,000 kcal |
| 250 lb | 2.5 lb | 1,250 kcal |
| 300 lb | 3.0 lb | 1,500 kcal |
Above 1% per week and you'll lose meaningful muscle alongside fat, and your TDEE will drop hard via adaptive thermogenesis. Below 0.5% per week and you'll keep more muscle but it'll feel slow.
For most people, 0.5-1% per week is the sweet spot.
How long to stay in a deficit
Continuous dieting longer than ~12 weeks generally hurts more than it helps. Adaptive thermogenesis compounds, hunger hormones (ghrelin, leptin) shift unfavorably, and adherence drops.
A useful structure:
- 8-12 weeks at your target deficit
- 2-4 weeks at maintenance ("diet break") — eat at TDEE, let hormones reset
- Repeat
This sounds like it slows things down. It usually doesn't, because the maintenance break improves adherence and reduces the metabolic adaptation in the next cut block.
In CalBurndown
Open Settings → Goal and set your target daily deficit in kcal:
- The slider goes 0-1,000 kcal/day
- The default is 500
- We recommend 250-500 for most people
- Above 750 we'll show a warning
Your deficit is subtracted from projected TDEE to set today's food budget — the top of the burndown chart. Eat under that, you're on pace.
If you're losing faster than expected, our calibration loop will adjust over time. If you want a built-in diet break, set the deficit to 0 for those weeks.
Where to go next
- How to calculate your TDEE
- How TDEE calibration works
- Free buy-back calculator — how much walking earns back today's overeat
- Refeeds and diet breaks — long-form on cycling out of deficits