Calorie Deficit Score Calculator — Find Your Fat Loss Number
Enter your stats, activity level, and weight loss goal. Get your TDEE, your ideal daily calorie target, a personalized HealthIQ Deficit Score out of 100, and a week-by-week projection.
⚡ Next: See exactly how many calories your body burns
Your deficit target is only as accurate as your TDEE. Get the full calculation next.
What Is a Calorie Deficit?
A calorie deficit occurs when you consume fewer calories than your body burns in a given day. When this happens consistently, your body is forced to draw on stored energy — primarily body fat — to meet its needs. This is the fundamental mechanism behind fat loss, and it is the one principle every successful weight loss approach relies on, regardless of how it is framed or marketed.
You can create a calorie deficit through eating less, moving more, or both. The most sustainable approach for most people is a combination — a moderate reduction in food intake paired with an increase in daily activity. Neither extreme works long-term: severe restriction triggers metabolic adaptation and muscle loss, while trying to exercise away a poor diet is exhausting and largely ineffective.
📊 The maths: One kilogram of body fat contains approximately 7,700 kcal of stored energy. A daily deficit of 500 kcal produces roughly 0.5 kg of fat loss per week — 2 kg per month — without aggressive restriction. A 300 kcal deficit produces 0.3 kg per week. These are estimates, not guarantees, because individual metabolism varies.
How This Calculator Works
This calculator uses the Mifflin-St Jeor equation — the most widely validated formula for estimating Basal Metabolic Rate (BMR) in clinical nutrition research. It outperforms the older Harris-Benedict equation, particularly for people who are overweight or obese.
The formulas
For men: BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age) + 5
For women: BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age) − 161
BMR is then multiplied by an activity factor (1.2 to 1.9) to get your TDEE — Total Daily Energy Expenditure. Your calorie target is TDEE minus your chosen deficit.
Choosing the Right Deficit Size
The size of your deficit determines your rate of loss — but bigger is not always better. There is a meaningful difference between a deficit that accelerates fat loss and one that triggers muscle catabolism, fatigue, and metabolic slowdown.
| Goal | Daily Deficit | Weekly Loss | Best For | HealthIQ Score |
|---|---|---|---|---|
| Slow & Steady | ~275 kcal | 0.25 kg | Maintenance mindset, muscle preservation | 85–95/100 |
| Moderate | ~550 kcal | 0.5 kg | Most people — sustainable and effective | 75–85/100 |
| Aggressive | ~825 kcal | 0.75 kg | Short-term, requires high protein intake | 60–74/100 |
| Rapid | ~1100 kcal | 1.0 kg | Medical supervision recommended | 40–59/100 |
| Maintenance | 0 kcal | 0 kg | Body recomposition — swap fat for muscle | 80–90/100 |
| Muscle Gain | +275 kcal surplus | +0.25 kg | Lean bulking with strength training | 80–90/100 |
Why 500 kcal Per Day Is the Sweet Spot for Most People
A 500 kcal daily deficit is consistently recommended in clinical weight management guidelines because it sits in a zone where fat loss is meaningful without triggering the adaptive responses that stall progress.
Below 500 kcal deficit: progress is slow but muscle and metabolic rate are well preserved. Above 750 kcal deficit: the body begins cannibalising lean tissue for energy, cortisol rises, and the metabolic rate drops. The weight comes off faster initially but the composition of that loss worsens — and the plateau hits earlier and harder.
There is also the hunger variable. A 500 kcal deficit is manageable with the right food choices (high protein, high fibre, moderate fat). A 1,000 kcal deficit requires a level of willpower that almost nobody sustains long-term.
The Role of Protein in a Calorie Deficit
Protein is the single most important dietary variable during a calorie deficit. It does three things simultaneously: it preserves lean muscle mass while fat is being lost, it increases satiety more than carbohydrates or fat per calorie, and it has the highest thermic effect of any macronutrient — meaning your body burns more calories simply digesting it.
The research-backed target during a deficit is 1.6–2.2 g of protein per kg of body weight per day. For a 80 kg person targeting 0.5 kg loss per week, that means 128–176 g of protein daily. This is substantially more than the standard dietary recommendation and is the level at which muscle preservation becomes reliable even in an aggressive deficit.
Common Calorie Deficit Mistakes
- Underestimating TDEE — most people guess their activity level is higher than it is. Start with the conservative estimate and adjust based on real results after 2–3 weeks.
- Not tracking accurately — studies consistently show people underreport calorie intake by 20–40%. Liquids (coffee drinks, smoothies, alcohol) are the most commonly missed calories.
- Eating back exercise calories — calorie estimates from fitness trackers are notoriously inaccurate, often inflated by 30–50%. Do not automatically eat back every calorie your watch says you burned.
- Starting too aggressively — rapid deficits cause rapid early losses (mostly water and glycogen), which feel motivating but create unsustainable expectations. The rebound when restriction ends is equally rapid.
- Ignoring sleep — poor sleep raises ghrelin (appetite hormone) and lowers leptin (satiety hormone), significantly increasing hunger at the same calorie level. A sleep-deprived person finds a deficit much harder to maintain.
- Skipping protein — losing weight without adequate protein often means losing muscle. Muscle is metabolically expensive to maintain — losing it permanently reduces your TDEE, making future fat loss harder.
How Long Should You Stay in a Deficit?
There is no fixed answer, but the research on diet breaks and refeed periods suggests that prolonged continuous deficits are less effective than structured periods. Most practitioners recommend 8–12 weeks of deficit, followed by 2–4 weeks at maintenance calories (a “diet break”), before resuming. This approach preserves metabolic rate, reduces cortisol, and makes the next deficit phase more effective.
If your goal is a large total loss (10+ kg), plan for multiple phases rather than one long stretch. The results are better and the psychological experience is significantly easier.
Use our TDEE Calculator to monitor how your maintenance calories change as you lose weight — because they will drop, and your deficit target needs to be recalculated every 4–6 weeks. Also use our BMI Score Calculator to track where you are against healthy weight ranges as you progress.