If you've ever entered your stats into a fitness app, been told to eat 1,800 calories, followed it for two weeks, and seen nothing happen — the app wasn't wrong. You were probably just using the wrong number in the wrong way.
Calorie calculation is genuinely more nuanced than most apps let on, and the one-size-fits-all output they give you is often off by 200–400 calories for a meaningful percentage of users. This matters enormously when you're trying to lose or gain weight slowly and steadily.
Here's the full picture — from BMR to TDEE to macros — so you can work with accurate numbers.
Start with BMR: what your body burns at rest
Basal Metabolic Rate (BMR) is the number of calories your body needs just to keep you alive — breathing, circulating blood, maintaining organ function — if you did absolutely nothing for 24 hours. It's your biological floor.
The two main formulas
There are several BMR equations, but two dominate:
| Formula | Equation (metric) | Best for |
|---|---|---|
| Mifflin-St Jeor | Men: (10 × kg) + (6.25 × cm) − (5 × age) + 5 Women: (10 × kg) + (6.25 × cm) − (5 × age) − 161 | Most people — gold standard for general use |
| Katch-McArdle | 370 + (21.6 × lean body mass in kg) | Athletes with known body fat % — more accurate when muscle mass is above average |
For a 35-year-old woman, 165cm, 70kg: Mifflin-St Jeor gives a BMR of roughly 1,476 calories/day. That's before any activity. Nobody should eat at their BMR — that's not a diet, that's near-starvation.
TDEE: the number you actually need
Total Daily Energy Expenditure (TDEE) is BMR multiplied by an activity factor. This is your true maintenance — the calories needed to sustain your current weight at your current activity level.
| Activity level | Multiplier | What it actually means |
|---|---|---|
| Sedentary | 1.2 | Desk job, no intentional exercise |
| Lightly active | 1.375 | 1–3 days/week of light exercise or walking |
| Moderately active | 1.55 | 3–5 days/week of moderate exercise |
| Very active | 1.725 | Hard training 6–7 days/week |
| Extremely active | 1.9 | Physical labor job + hard daily training |
Using our 35-year-old example at "moderately active": 1,476 × 1.55 = 2,288 calories/day to maintain weight.
Why apps systematically overestimate TDEE
Most fitness apps use the activity multiplier from above but then also credit back every exercise calorie separately. So if you burn 400 calories at the gym, the app adds 400 to your daily budget on top of your already-moderate TDEE. This is double-counting — your "moderately active" multiplier already includes that gym time.
The result: apps tell people they burned 600 calories in a workout and can now eat 600 extra. The watch is overestimating exercise burn by 30–50%, and the TDEE already included that exercise in the multiplier. People eat back their "exercise calories" and wonder why they're not losing weight.
Setting your calorie target by goal
Fat loss: the 500-calorie deficit principle
One pound of fat contains roughly 3,500 calories. A 500 calorie/day deficit creates a 3,500 calorie/week deficit — theoretically one pound of fat loss per week. In practice, the body is messier than this (metabolic adaptation, water retention, muscle loss), but it's a solid starting point.
For most people, a 300–500 calorie deficit from TDEE is the sweet spot. More than 750 calories below TDEE usually results in muscle loss, extreme hunger, and eventual rebound eating. Slower is generally more effective when measured over a year.
How aggressive is too aggressive?
| Deficit size | Expected loss rate | Risks |
|---|---|---|
| 250 cal/day | ~0.5 lb/week | Very sustainable, minimal muscle loss risk |
| 500 cal/day | ~1 lb/week | Standard recommendation, manageable hunger |
| 750 cal/day | ~1.5 lb/week | Elevated muscle loss risk without high protein |
| 1,000+ cal/day | ~2+ lb/week | Significant muscle loss, fatigue, diet breaks needed |
Muscle gain: the lean bulk approach
Adding muscle requires a calorie surplus — you can't build tissue without excess energy. But a large surplus (500+ calories over TDEE) mostly adds fat, not muscle. Research supports a modest surplus of 200–300 calories over TDEE for experienced lifters, allowing 0.5–1 lb of lean mass gain per month while minimizing fat accumulation. Beginners can gain muscle faster and sometimes even in a deficit ("newbie gains").
Macros: where your calories come from matters
Once you have a calorie target, breaking it into protein, carbohydrates, and fat (macronutrients) determines what that deficit or surplus actually does to your body composition.
Protein: the non-negotiable
Protein is the macro that preserves muscle in a deficit and builds it in a surplus. The research-supported range for most active people is 1.6–2.2g of protein per kg of bodyweight per day (0.73–1g per pound). Higher protein also increases satiety — you feel fuller on the same calories. If you're in a calorie deficit, hitting your protein target is the single most important macro decision you can make.
On a 70kg person at 2g/kg: that's 140g of protein daily. At 4 calories per gram, that's 560 calories from protein alone.
Fat: the floor, not the ceiling
Fat is essential for hormone production, fat-soluble vitamin absorption, and satiety. Going too low on fat (below roughly 0.5g/kg bodyweight, or ~20% of calories) disrupts hormones, particularly testosterone and estrogen. Most people do well at 25–35% of calories from fat. Don't cut fat to near-zero chasing a calorie target — the hormonal and satiety costs aren't worth it.
Carbohydrates: fill the remaining calories
Carbohydrates are not essential in the way protein and fat are, but they're the primary fuel source for high-intensity exercise and the most flexible macro to adjust. Once protein and fat minimums are set, the remaining calories go to carbs. For sedentary people this might be 30–40% of calories; for endurance athletes, 50–60%.
A practical macro example
| Goal | Calories | Protein | Fat | Carbs |
|---|---|---|---|---|
| Fat loss (75kg person) | 1,800 | 150g (600 cal) | 60g (540 cal) | 165g (660 cal) |
| Maintenance (75kg) | 2,300 | 150g (600 cal) | 77g (690 cal) | 253g (1,010 cal) |
| Lean bulk (75kg) | 2,600 | 165g (660 cal) | 87g (780 cal) | 290g (1,160 cal) |
Why your calorie target needs to change over time
This is what most apps miss entirely: your TDEE changes as you lose weight. A 90kg person and an 80kg person have different maintenance calories — by roughly 200–400 calories/day. If you set a 1,800 calorie target based on your starting weight and never adjust, you'll be eating at a smaller and smaller deficit as the months pass, and fat loss will slow until it stalls entirely.
The fix is simple: recalculate your TDEE every 10–15 lbs of weight lost. It takes 5 minutes and keeps you on track.
Practical tips that actually change outcomes
- Use a food scale for 2 weeks. Not forever — just long enough to calibrate your eye. Most people underestimate portion sizes by 20–30%, especially for calorie-dense foods like nuts, oil, cheese, and nut butter.
- Track for awareness, not perfection. Being 100 calories off is irrelevant. Being 400–600 calories off consistently is why results stall.
- Eat protein first. If you structure meals around hitting your protein target first, the fat and carb macros are easier to manage naturally.
- Don't eat back exercise calories. As discussed: the activity multiplier already accounts for your workout schedule. Only "eat back" calories if you had an extraordinary session far above your normal.
- Two-week check-in. If you've tracked accurately for two weeks and your weight hasn't moved in the expected direction, your TDEE estimate is probably off. Adjust by 150–200 calories and try again.