How many calories should I eat to lose weight?

TL;DR: Calorie targets for weight loss come from a simple formula, but biology makes the number a moving target. As you lose weight, TDEE drops more than mass loss alone predicts. Treat your starting target as a hypothesis, test it for two weeks, and adjust based on actual progress. Static plans drift out of calibration.

Table of contents

1. How daily calorie targets are calculated, and why they change as you lose weight
2. How to calculate your starting calorie target, and adjust it as you lose weight
3. FAQs

How daily calorie targets are calculated, and why they change as you lose weight

A daily calorie target for weight loss is built in three steps: estimate your resting energy expenditure (REE), multiply it by an activity factor to get your total daily energy expenditure (TDEE), and subtract some calories for a deficit. The Mifflin-St Jeor equation is the most widely used REE estimator in clinical practice and underpins most calculator apps. The biology beneath it, however, is messier than the equation suggests.

A 2018 review in the European Journal of Clinical Nutrition maps REE to organ-tissue mass: brain, liver, heart, and kidneys account for the bulk of resting heat production despite being a small fraction of body mass, while adipose tissue contributes only about 4.5 kcal/kg/day. Two people of the same weight but different body composition can have meaningfully different REEs, which is why statistical equations like Mifflin-St Jeor capture average cases well but lose precision when fat-free mass is unusually high or low.

This matters when you start losing weight, because the target moves. A 2014 review in the Journal of the International Society of Sports Nutrition on metabolic adaptation summarized the literature: TDEE drops with weight loss not only because of reduced body mass, but because of an additional component, adaptive thermogenesis, that lowers expenditure beyond what mass loss alone would predict. The drop persists during maintenance and is part of why long-term weight maintenance is hard.²

The size of that adaptation has been measured directly. A 2020 study in the American Journal of Clinical Nutrition of 171 women who lost an average of 12 kg found a metabolic adaptation of approximately −54 kcal/day below the predicted RMR after weight loss, when measurements were taken under conditions of weight stability. Notably, the adaptation was not sustained at one or two years of follow-up, and was not associated with weight regain in this cohort. 

The authors concluded that "metabolic adaptation at the level of RMR is minimal when measurements are taken under conditions of weight stability and does not predict weight regain up to 2 years follow-up." 

The literature is genuinely split on how much adaptive thermogenesis matters long-term, but it's well-agreed that the calorie target you started with isn't the calorie target you'll need at 10 lb lighter.

The behavioral consequence is that adaptive tracking outperforms fixed targets. A reasonable opening estimate from a Mifflin-St Jeor calculator gets you a starting deficit (Fitia handles this automatically when you set up your profile), but the target you start with isn't the target you'll need at 5 kg lighter, and a static plan will quietly drift out of calibration as both body mass and adaptive thermogenesis pull expenditure downward. 

The real advantage belongs to approaches that close the loop: read your logged weight trend, infer how your real expenditure is tracking against the prediction, and adjust the calorie target accordingly. That's the model some calorie tracking apps handle very well — the goal recalibrates based on your actual rate of progress, tightening the deficit when the trend stalls and easing it when you're losing too fast, so plateaus get corrected before they turn into weeks of wasted effort.

Tired of calorie targets that drift out of sync as you lose weight? Fitia keeps the deficit calibrated to your actual progress. Download Fitia and use code FITIANOW to save on Premium.

How to calculate your starting calorie target, and adjust it as you lose weight

Whatever number an app or online calculator hands you on day one is a guess. A reasonable one, but a guess. Treat it as a number to test for two weeks, and be ready to adjust it based on your progress.

Here's the starting math:

• REE (Mifflin-St Jeor): for women, REE = 10 × kg + 6.25 × cm − 5 × age − 161. For men, the same formula but +5 instead of −161.
• TDEE: REE multiplied by an activity factor. Using the FAO/WHO/UNU reference ranges, sedentary or light lifestyles fall between 1.40 and 1.69, active or moderately active between 1.70 and 1.99, and vigorously active between 2.00 and 2.40. Most people sit at the lower end of their bracket because structured exercise contributes less to total expenditure than it feels like.
• Weight-loss target: TDEE minus around 500 kcal/day, or a 20–25% reduction from maintenance. Both are well-supported and roughly equivalent in practice. Larger deficits speed up weight loss in the short term but tend to drive muscle loss, hormonal disruption, and adherence problems that wipe out the speed advantage.

A 165 lb (75 kg), 5'8" (173 cm), 35-year-old woman with a desk job and short daily walks would calculate roughly: REE ≈ 1,495 kcal, TDEE at a 1.45 factor ≈ 2,170 kcal, weight-loss target ≈ 1,670 kcal (500-kcal deficit) or about 1,735 kcal (20% deficit). Either is a reasonable starting hypothesis.

Two weeks in, the scale tells you whether the starting hypothesis was right. A loss of roughly 0.5–1% of body weight (about 0.8–1.6 lb in this example) means the target is working as designed. More than 1% per week usually signals the deficit is too aggressive, and adding 100–150 kcal back will protect adherence and muscle mass without stalling progress. 

No change at all typically comes down to one of two things: tracking accuracy is off (most people under-report intake by 15–20%), or the activity factor was too generous. In either case, drop the target by 100–150 kcal and re-test.

The single biggest mistake people make is locking the same calorie target in for three months. As body mass drops and adaptive thermogenesis pulls TDEE downward, the target that started as a deficit can drift close to a new maintenance number.

This is, mechanically, what good calorie-tracking apps handle in the background. The starting target is calculated from a Mifflin-St Jeor-based equation, but the daily calorie goal and meal plans recalibrate based on your current weight and recent rate of progress, so the target doesn't ossify. You don't have to recalculate manually; the system reflects your current physiology, not your starting physiology.

Ready to put this into practice? Download Fitia and use code FITIANOW to save on Premium.

FAQs

What's a safe rate of weight loss per week? 

Roughly 0.5 to 1% of body weight per week. For most adults that lands between 0.5 and 2 lb. Faster than that usually signals a deficit too aggressive to sustain, and tends to drive muscle loss and adherence problems that erase the speed advantage.

How often should I recalculate my calorie target? 

Every two to four weeks, or any time the scale stalls for two weeks in a row despite consistent tracking. Apps that recalibrate automatically based on your weight trend handle this for you, which is why they tend to outperform static plans over a 3–6 month cut.

Why did my weight loss stall when nothing else changed? 

Two reasons usually. Your body mass is lower, so your TDEE is lower. And adaptive thermogenesis lowers expenditure further, beyond what the mass loss alone would predict. The target you set on day one slowly approaches your new maintenance number. Dropping intake by 100–150 kcal or recalculating against your current weight typically gets the trend moving again.

Can I just trust the calorie target my app gives me? 

Treat it as a starting hypothesis, not a prescription. The Mifflin-St Jeor equation that most apps use is accurate for the average case but can be off by several hundred calories for individuals with unusually high or low fat-free mass. The two-week scale test is what tells you whether your specific number was right.

About the Author

Fiorella Ricardi is a licensed nutritionist from Universidad Científica del Sur, where she graduated in the top fifth of her class. She brings hands-on experience across clinical, public health, and food service nutrition. For the past two years, she has worked at Fitia as Operations Lead, focused on improving the accuracy of internal food entry data and ensuring users see correct, reliable nutritional insights inside the app.