Calorie deficit: how many calories to lose weight?
Estimate a starting target, then check it against 14 to 21 days of weigh-ins. The calculation provides a hypothesis; the actual trend determines the adjustment.
Short answer
The calculator estimates your energy expenditure using the Mifflin-St Jeor equation and your activity level, then suggests a starting target. It is only a working assumption: your weight trend over 2 to 3 weeks tells you whether it works. A loss of 0.25 to 1% of body weight per week serves as a safeguard.
Calculator
Validated inputsHypothesis to test
Your starting calorie target
- Estimated rest
- --
- Estimated maintenance
- --
- Applied deficit
- --
- Order of magnitude
- --
The larger the deficit, the more hunger, recovery and adherence should weigh in the decision.
This target must not be used as a stand-alone plan. Low-energy diets require a professional indication and supervision.
The projection in kilograms uses a static conversion of 7,700 kcal. It is only intended to illustrate the order of magnitude and does not predict your actual weight loss.
Real-world calibration
Test the hypothesis over 14 days
Weigh yourself when you wake up, under comparable conditions. A single measurement never triggers an adjustment.
- Initial average
- --
- Recent average
- --
- Trend
- --
Enter at least 14 comparable weigh-ins before interpreting the trend.
The thresholds of 0.25% and 1% per week are internal caution rules for this tool, not universal clinical recommendations.
What the tool measures
A calorie calculation is only a starting point
A calorie deficit exists when energy intake remains below expenditure over time. The principle is robust, but a person's expenditure cannot be determined to the nearest kilocalorie. Equations estimate resting metabolism, then a coefficient attempts to represent daily activity.
The tool therefore offers two routes: start with the Mifflin-St Jeor equation, or enter an already observed maintenance expenditure. It then applies a deficit of 10, 15 or 20%, presented as a cautious editorial setting, not a prescription.
The most useful part comes after the result. Weight averages over 7 days, hunger, performance and adherence let you test the hypothesis for 14 to 21 days without reacting to every fluctuation in water weight.
Interpreting the result
Interpret without overpromising
The "starting calories" value is neither an obligation nor a direct measurement of your metabolism. Keep the same target long enough to observe a comparable trend.
The tracking module compares the average of the first 7 measurements with that of the last 7. Its benchmarks of 0.25% and 1% of body weight per week serve as practical safeguards, not universal medical thresholds. Someone who is already very lean, older, ill or highly trained may need a different strategy.
If hunger becomes difficult to manage, performance drops or weight loss is very rapid, do not increase the deficit. If the trend remains flat after 21 consistent days, a small trial adjustment of around 100 kcal per day is easier to interpret than a drastic cut.
Calculation and assumptions
Methodology
In "estimate my maintenance" mode, resting metabolism is calculated using the Mifflin-St Jeor equation:
- male profile: 10 × weight (kg) + 6.25 × height (cm) − 5 × age + 5
- female profile: 10 × weight (kg) + 6.25 × height (cm) − 5 × age − 161
The result is multiplied by an activity factor between 1.2 and 1.725. This step accounts for a large share of the uncertainty. The target is then calculated as follows: estimated maintenance × (1 − selected deficit).
The weekly projection uses the static convention of 7,700 kcal per kilogram only to provide a rough estimate. It does not predict actual loss, because expenditure, appetite and the composition of the loss change over time. The NIDDK dynamic model documents this limitation in detail.
Tracking calculates a 7-day moving average and only suggests an adjustment when 21 measurements, consistent adherence and good tolerability are all present.
Sources consulted
Sources
- Mifflin et al. (1990): predictive equation for resting energy expenditure
Original study of the Mifflin-St Jeor equation, derived from 498 adults using indirect calorimetry measurements.
- Frankenfield et al. (2005): comparison of resting metabolic rate equations
Systematic review showing the practical usefulness of Mifflin-St Jeor while highlighting notable individual errors.
- NIDDK: research behind the Body Weight Planner
Introduction to Hall’s dynamic model and how energy needs adapt during weight loss.
- HAS (2024): care pathway for overweight and obesity in adults
French framework for a comprehensive, personalized assessment attentive to eating disorders.
- NICE NG246: physical activity and dietary approaches
Recommendations governing low- and very-low-energy diets under specialist supervision.
Scope of validity
Limitations to know
Do not use this tool on its own if you are under 18, pregnant or breastfeeding, have unintentional weight loss, a current or past eating disorder, a disease or treatment affecting weight, or if you are an athlete with very low body fat or a high training volume.
- the original equation offers only two constants based on female and male groups
- activity coefficients greatly simplify differences between jobs, training and everyday movement
- the convention of 7,700 kcal per kilogram is static and does not describe biological adaptation
- a target below 1,200 kcal per day requires professional supervision and is not offered as a standalone plan
- the data entered stays in your browser and disappears when the page reloads
Frequently asked questions
Before using the result
What calorie deficit should you choose to lose weight?
Generally, start with 10 to 15% if tolerability is your priority. A 20% deficit is more demanding and should be reduced if hunger worsens, recovery suffers, or performance declines.
Can you safely lose 1 kg per week?
It depends on starting weight and context. One kilogram represents a very different proportion at 60 or 140 kg. The tool therefore uses a percentage of body weight and treats more than 1% per week as a signal to slow down and reassess.
Why does the calculation not match my actual loss?
Actual activity, portion-size errors, water, glycogen, bowel transit and adaptations in energy expenditure create a gap. The average of 14 to 21 weigh-ins is used to calibrate the estimate.
How can I protect muscle during a deficit?
Avoid aggressive restriction, maintain appropriate resistance training, meet your protein needs and monitor strength, recovery and the rate of weight loss.