A calorie deficit is the only mechanism that makes you lose fat. Not keto, not fasted cardio, not cutting carbs after 6pm — those are just different routes to the same place: eating fewer calories than you burn. Once that clicks, fat loss stops being a guessing game and becomes a number you can set. This guide explains the number, shows you how to find yours, and walks through two worked examples so you can copy the maths for your own body.
What a calorie deficit actually is
Your body spends energy every day — to keep your heart beating, digest food, move around, and train. Add it all up and you get your maintenance calories, also called TDEE (total daily energy expenditure). Eat that amount and your weight holds steady. Eat consistently below it and your body makes up the shortfall by burning stored energy, mostly fat. That gap is the deficit.
The often-quoted figure is that one pound of fat holds roughly 3,500 calories (about 7,700 calories per kilogram). So a 500-calorie daily deficit predicts about a pound of fat loss a week. Useful as a starting estimate — but treat it as a forecast, not a guarantee. As you diet, your body adapts: it burns slightly less at rest, you fidget less, and the scale slows even when your deficit is unchanged. This is normal (it's called adaptive thermogenesis), and it's why the textbook arithmetic always overpredicts real-world loss over a few months. Plan for the trend, not the formula.
Step 1: estimate your maintenance calories
You can pay for a lab test, but a validated equation gets you close enough to start. The Mifflin-St Jeor formula is the standard. First calculate your resting burn (BMR):
- Men: (10 × weight in kg) + (6.25 × height in cm) − (5 × age) + 5
- Women: (10 × weight in kg) + (6.25 × height in cm) − (5 × age) − 161
Then multiply BMR by an activity factor to account for daily movement and training: about 1.2 if sedentary, 1.375 lightly active, 1.45–1.55 if you train a few times a week, higher if your job is physical. That product is your maintenance estimate. Don't agonise over the exact multiplier — it's a starting point you'll correct against the scale in two weeks.
Step 2: pick a deficit that fits your body
Here's where most advice goes wrong: it hands everyone the same flat "eat 500 less" or "1,200 calories" target. A 500-calorie cut is a sensible ~18% off a 90 kg lifter's maintenance, but for a 55 kg person it's closer to 30% — deep enough to bite into muscle and energy. Same number, very different stress depending on body size. Scale the deficit to your size instead. Two rules that travel well:
- Aim to lose 0.5–1% of your bodyweight per week. Faster than that and you start sacrificing muscle and energy.
- Or set your deficit at 15–25% below maintenance. Leaner or smaller? Stay near the lower end. More body fat to lose? You can sit at the higher end comfortably.
A moderate deficit beats an aggressive one almost every time, because the best diet is the one you can actually run for the months fat loss takes. Crash deficits torch muscle, wreck training quality, and rebound hard.
Two worked examples
Numbers make this concrete. Both people below train three to four times a week, so they use a ~1.45 activity factor.
Example 1 — Marcus, 82 kg, wants to lean out
Man, 32 years old, 180 cm, 82 kg, lifts three times a week.
- BMR = (10 × 82) + (6.25 × 180) − (5 × 32) + 5 = 1,790 calories
- Maintenance = 1,790 × 1.45 ≈ 2,600 calories
- 20% deficit = 520 calories. Target intake ≈ 2,080 calories/day
- Weekly deficit ≈ 3,640 calories → about 0.47 kg (1 lb) per week, right in his 0.5–1% range
Example 2 — Priya, 64 kg, wants to drop body fat
Woman, 29 years old, 166 cm, 64 kg, trains four times a week.
- BMR = (10 × 64) + (6.25 × 166) − (5 × 29) − 161 = 1,370 calories
- Maintenance = 1,370 × 1.45 ≈ 2,000 calories
- 18% deficit = 360 calories. Target intake ≈ 1,640 calories/day
- Weekly deficit ≈ 2,520 calories → about 0.33 kg (0.7 lb) per week
Notice Priya's deficit is smaller in both calories and percentage. She's lighter, so the 500-calorie cut Marcus runs at 20% would be about 25% of her maintenance — the very top of the sensible band, and more aggressive than she needs. The point of doing the maths is exactly this: it scales the plan to the person instead of handing both of them the same number.
| Marcus (82 kg) | Priya (64 kg) | |
|---|---|---|
| Maintenance (TDEE) | ~2,600 kcal | ~2,000 kcal |
| Deficit size | 20% (520 kcal) | 18% (360 kcal) |
| Daily intake target | ~2,080 kcal | ~1,640 kcal |
| Protein target | ~165 g | ~128 g |
| Expected loss | ~0.47 kg/week | ~0.33 kg/week |
Protein and training: keep the loss as fat, not muscle
A deficit makes you lose weight; what you do alongside it decides whether that weight is fat or hard-won muscle. Two levers protect lean mass:
- Eat enough protein. Aim for 1.6–2.2 g per kg of bodyweight, pushing toward the top of that range (2.0–2.2 g/kg) the leaner you get. That's roughly 165 g for Marcus and 128 g for Priya. Protein also keeps you full, making the deficit easier to hold.
- Keep lifting. Resistance training tells your body to hold onto muscle while it burns fat. Cutting calories without training, or living on cardio alone, sheds muscle along with the fat. If you're weighing how to spend your training time, our breakdown of cardio vs weights for fat loss covers what each one earns you, and you can build sessions straight from the exercise library.
Cardio is a tool for spending energy, not a requirement. To burn calories in less time, HIIT workouts pack a meaningful cost into 15–20 minutes — handy on busy days, though they don't replace lifting.
Setting it up in practice
The plan only works if you actually hit the number, so make the number easy to hit.
- Build meals around protein and volume. Lead each plate with protein and pile on vegetables — high-volume, low-calorie food fills your stomach for little. Our recipe collection is filtered by calories and protein so a day lands on target, and the FitBot Coach app tracks your intake against these numbers automatically.
- Weigh yourself consistently. Same time, same conditions, ideally most mornings. Day-to-day scale weight is mostly water and food in transit, so judge by the weekly average, never a single reading.
- Adjust every two to three weeks, not every day. If your weekly average isn't dropping after two solid weeks, trim another 100–150 calories or add some activity. If you're losing faster than 1% a week and feeling drained, eat a little more.
Mistakes that quietly erase your deficit
- Eyeballing portions. Oils, nut butters, and dressings are calorie-dense — a "splash" of oil can be 120 calories. Weighing food for two weeks recalibrates your eye fast.
- Forgetting liquid calories. Juice, lattes, and alcohol slip in hundreds of uncounted calories that can wipe out the gap entirely.
- Trusting the calories your watch says you burned. Wearables routinely overestimate. Base your plan on intake and let the scale trend be the feedback.
- Going too low, too fast. A bigger deficit isn't a faster shortcut — it just brings more muscle loss, more hunger, and a quicker quit. Patience is the strategy.
The short version
Estimate your maintenance, eat 15–25% below it, anchor the day with protein at 1.6–2.2 g/kg, keep lifting, and steer by your weekly average weight. Aim to lose 0.5–1% of bodyweight a week and adjust every couple of weeks rather than chasing the scale daily. Do the arithmetic once, like Marcus and Priya did, and you'll have a target built for your body instead of a generic number that fits no one.
Key takeaways
- A calorie deficit — eating less than you burn — is the only mechanism that causes fat loss; every diet just gets you there differently.
- Estimate maintenance with Mifflin-St Jeor, then eat 15-25% below it, or aim to lose 0.5-1% of bodyweight per week.
- Scale the deficit to your size: a flat 500-calorie cut is a moderate ~20% off an 82 kg lifter's maintenance but about 25% for someone at 64 kg, near the top of the sensible range.
- Eat 1.6-2.2 g of protein per kg and keep lifting so the weight you lose is fat, not muscle.
- The 3,500-calorie-per-pound rule overpredicts real loss because metabolism adapts; steer by your weekly average weight and adjust every 2-3 weeks.
Frequently asked questions
How big should my calorie deficit be?
Set it at 15-25% below your maintenance calories, or size it to lose about 0.5-1% of your bodyweight per week. Smaller or leaner people should stay near the lower end, while those with more body fat can use a larger deficit comfortably. A moderate deficit you can sustain beats an aggressive one you quit.
Will I lose muscle in a calorie deficit?
You can, but you can largely prevent it. Eating 1.6-2.2 g of protein per kilogram of bodyweight and continuing to lift weights signals your body to hold onto muscle while it burns fat. Crash dieting with no resistance training is what strips muscle alongside the fat.
Why has my weight loss stalled even though I'm in a deficit?
As you diet your metabolism adapts and burns slightly less, so the original deficit shrinks over time. First confirm you're tracking accurately, including oils, drinks, and weekend meals. If your weekly average truly hasn't moved in two to three weeks, cut another 100-150 calories or add some activity.