“Eat less” is the most repeated, least useful piece of weight loss advice there is, because it doesn’t say how much less. A calorie deficit that’s too small barely moves the scale; one that’s too large is miserable to sustain and often reversed within weeks. Here’s how to actually size yours.
The Maths Behind a Calorie Deficit
Roughly 7,700 kcal represents 1kg of body fat. A consistent daily deficit of 500 kcal below your TDEE therefore produces, on average, around 0.5kg of weight loss per week – a figure widely considered both effective and sustainable. This isn’t a precise physical law – individual results vary with water retention, hormonal factors and measurement noise – but it’s a solid, well-supported starting point for planning.
Deficit Size by Weekly Goal
| Daily Deficit | Approx. Weekly Loss | Suitability |
|---|---|---|
| 250 kcal | ~0.25kg | Gentle; good for smaller goals, or maintaining motivation over a longer timeline |
| 500 kcal | ~0.5kg | Standard, most widely recommended pace for sustainable fat loss |
| 750 kcal | ~0.75kg | Faster; requires closer attention to protein intake and energy levels |
| 1,000+ kcal | ~1kg+ | Aggressive; generally only appropriate short-term, and often under professional guidance |
The Calorie Deficit Calculator runs this maths against your specific TDEE automatically, rather than requiring you to work from a generic average.
Percentage-Based vs Fixed-Number Deficits
Some people prefer sizing a deficit as a percentage of TDEE (commonly 15–25%) rather than a fixed calorie number, since a fixed 500 kcal deficit represents a much bigger proportional cut for someone with a 1,800 kcal TDEE than for someone at 3,000 kcal. We cover this approach specifically in our guide to the 20% calorie deficit, which tends to scale more sensibly across different body sizes and activity levels than a flat number applied to everyone.
How Much Weight You Have to Lose Changes the Calculation
The right deficit size isn’t purely a matter of speed preference – it also depends on your starting point:
- Larger amount to lose – a slightly bigger deficit (up to around 25% of TDEE) is often more manageable, since overall intake is higher to begin with, leaving more room to cut calories while still eating a reasonable volume of food.
- Closer to a healthy weight already – a smaller, gentler deficit tends to preserve energy levels and reduce the risk of losing muscle alongside fat, which becomes a bigger relative concern the leaner you already are.
Signs Your Deficit Is the Right Size
- Steady, gradual progress on your weight trend over 2–3 week windows (not day to day – see water weight vs fat loss)
- Hunger that’s manageable, not overwhelming or constantly distracting
- Energy levels sufficient for daily life and any training you’re doing
- A pace you can realistically imagine sustaining for weeks or months, not just the first few days
If your deficit consistently fails on more than one of these, it’s worth adjusting the size rather than pushing through – see our guide on aggressive calorie deficit risks for the specific warning signs to watch for.
Why Bigger Isn’t Always Faster in Practice
It’s tempting to assume a larger deficit simply means faster results, and mathematically it does – in theory. In practice, very large deficits are harder to sustain consistently, and a diet abandoned after two weeks produces worse real-world results than a moderate deficit followed consistently for two months. The “best” deficit size is ultimately the one that balances a meaningful weekly result against a pace of eating you can actually stick to.
A Worked Comparison: Two People, Two Deficit Sizes
To make this concrete, consider two people with the same 2,200 kcal TDEE, choosing different deficit sizes:
| Person A (500 kcal deficit) | Person B (1,000 kcal deficit) | |
|---|---|---|
| Daily calorie target | 1,700 kcal | 1,200 kcal |
| Predicted weekly loss | ~0.5kg | ~1kg |
| Typical hunger level | Manageable | Often significant |
| Muscle loss risk | Lower, especially with adequate protein | Meaningfully higher |
| Realistic sustainability | Weeks to months | Often days to a couple of weeks before struggling |
On paper, Person B loses weight twice as fast. In practice, if Person B abandons the diet after 10 days due to hunger and fatigue, while Person A sustains theirs for 8 weeks, Person A will have lost considerably more total weight by the end – despite the “slower” daily rate. This is the core reason moderate, sustainable deficits so consistently outperform aggressive ones over any realistic, real-world timeframe.
Adjusting Deficit Size Over the Course of a Longer Diet
For people with a significant amount of weight to lose, it’s often sensible to plan for the deficit size to change over the course of the diet, rather than staying fixed throughout. A common, sensible pattern:
- Early stages – a slightly larger deficit (up to 25% of TDEE) is often well tolerated, since higher starting body weight and fat stores provide more buffer
- Middle stages – as weight drops and the body has less fat to draw on relative to lean mass, tightening protein intake and moderating the deficit slightly helps protect muscle
- Final stages, closer to goal weight – a smaller, more conservative deficit (10–15%) tends to be more appropriate, since the remaining fat to lose is proportionally smaller and the risk of muscle loss proportionally higher
This kind of periodic adjustment, rather than a single fixed deficit maintained from the first day to the last, tends to produce a better balance of results and muscle preservation over a longer diet.
Frequently Asked Questions
Is a 500 calorie deficit right for everyone?
It’s a reasonable default for many people, but it isn’t universal – someone with a very high or very low TDEE may be better served by a percentage-based approach, covered in our 20% deficit guide, rather than the same fixed number applied regardless of size.
Can I change my deficit size partway through a diet?
Yes – and it’s often necessary as your weight (and therefore your TDEE) changes. Recalculating every 4–6kg lost keeps your deficit accurately sized rather than gradually becoming too small or too large as your body changes.
What happens if my deficit is too small to see progress?
A deficit that’s too small relative to day-to-day tracking or estimation error can produce results too slow to detect against normal weight fluctuation, making it feel like nothing is happening even though a small deficit technically exists.
Should beginners start with a smaller or larger deficit?
Many people find it easier to start with a moderate deficit (around 500 kcal or 15–20%) rather than an aggressive one, building sustainable habits before considering a larger cut if progress needs to be faster for a specific reason.
Does exercise let me eat more while keeping the same deficit?
If your TDEE already reflects your typical activity level including exercise, no additional allowance is usually needed. See our comparison in calorie deficit vs exercise for more detail.
What if I have a lot of weight to lose – should I go bigger than 25%?
Even with significant weight to lose, pushing much beyond a 25% deficit tends to increase risks around muscle loss, energy and sustainability without proportionally faster real-world results, since very large deficits are harder to maintain consistently over the months a bigger weight loss goal typically requires.
Does my deficit need to be the same every single day of the week?
No – many people find it easier to eat a slightly smaller deficit on harder days (social occasions, higher stress) and a slightly larger one on easier days, as long as the weekly average lines up with their overall target. This flexibility can make a moderate deficit considerably easier to sustain than a rigid daily number.