To lose weight, you need to burn more energy than you eat. True — but it’s easy to jump to the wrong conclusion: “The less I eat, the faster it works.” An overly aggressive deficit eats into your muscle, strength, sleep and mood, and often ends in a binge a couple of months later. In this article you’ll learn how to estimate your calorie needs, how big a deficit to choose, how to keep your muscle, and how to tell when you’ve pushed too hard.
What rate of weight loss is reasonable?
A common benchmark in the sports nutrition literature is 0.5–1% of body weight per week: Helms and colleagues (2014), for example, recommend this rate in their review on natural bodybuilding contest prep to maximize muscle retention. For someone weighing 80 kg, that’s 0.4–0.8 kg a week.
- Closer to 0.5% if you’re already fairly lean, train a lot or want to keep as much muscle as possible.
- Closer to 1% if you have a lot of weight to lose: larger fat stores make a deficit easier to tolerate.
Losing more than 1% a week for a long stretch is a reason to rethink the plan. The exception is week one: as glycogen and water stores shrink, the scale can drop 1–2 kg, and that isn’t fat yet.
Step 1: Estimate your energy expenditure
First, calculate your basal metabolic rate (BMR) — the energy your body uses at rest. One of the best-validated formulas is the Mifflin-St Jeor equation (1990):
- men: 10 × weight (kg) + 6.25 × height (cm) − 5 × age + 5;
- women: 10 × weight (kg) + 6.25 × height (cm) − 5 × age − 161.
Then multiply by an activity factor to get your total daily energy expenditure (TDEE). The factors below are widely used but rough approximations, not measured values:
| Lifestyle | Factor |
|---|---|
| Desk job, little or no training | 1.2 |
| Lightly active, 1–3 workouts a week | 1.375 |
| Moderately active, 3–5 workouts | 1.55 |
| Very active, 6–7 workouts | 1.725 |
| Physical job plus training | 1.9 |
Worked example
A 35-year-old woman, 168 cm, 75 kg, with three strength sessions a week and about 7,000 steps a day.
- BMR = 10 × 75 + 6.25 × 168 − 5 × 35 − 161 = 750 + 1,050 − 175 − 161 = 1,464 kcal.
- TDEE = 1,464 × 1.55 ≈ 2,270 kcal.
Keep in mind this is an estimate. In a systematic review by Frankenfield and colleagues (2005), Mifflin-St Jeor predicted resting metabolic rate within ±10% of the measured value more often than any other equation, but for some people the error is larger — and the activity factor adds its own uncertainty. The formula is a starting point, not a verdict. Your own data will reveal your real number within two to three weeks.
Step 2: Choose your deficit
A practical approach is a deficit of 15–25% of TDEE, usually 300–600 kcal a day. For our example:
- 2,270 × 0.8 ≈ 1,800 kcal a day, a deficit of about 450 kcal.
By a common rough rule, losing 1 kg of body fat takes a total deficit of about 7,700 kcal. At 450 kcal a day that’s just over 0.4 kg a week — about 0.55% of 75 kg, right inside the target range. Treat it as a ballpark only: as Kevin Hall (2008) showed, the energy cost of each kilogram lost depends on your starting body fat, and expenditure drops over time so weight loss slows — which is why you calibrate the plan against real data.
How to calibrate:
- Weigh yourself 3–7 times a week in the morning, after using the bathroom and before eating, and calculate your weekly average.
- After two to three weeks, compare the averages.
- If your weight has stalled, cut another 100–200 kcal or add steps. If it’s dropping faster than 1% a week, add 100–200 kcal.
Don’t stay below your BMR for long. A common rule of thumb (a guide, not a fixed standard) is not to go below roughly 1,200 kcal without professional supervision. If you have a chronic condition, a history of disordered eating, or are pregnant or breastfeeding, talk your plan through with a doctor.
How to keep muscle in a deficit
In a deficit, your body is happy to burn muscle along with fat. Three levers prevent that:
- Protein at 1.6–2.2 g per kg of body weight or more. Needs are higher in a deficit than when bulking: the ISSN position stand (2017) allows 2.3–3.1 g/kg for trained people. More in the article on protein for muscle gain.
- Strength training 2–3 times a week. It tells your body the muscle is still needed. In a deficit, holding your working weights is a win.
- A moderate pace. The more aggressive the deficit, the larger the share of weight lost from muscle.
Cardio is great for health and energy expenditure, but it doesn’t replace strength work. The WHO recommends that adults get 150–300 minutes of moderate aerobic activity a week (or 75–150 minutes of vigorous activity) plus muscle-strengthening activities on at least two days — a solid foundation for fat loss, too.
Another sneaky factor is everyday movement. Many people move less in a deficit without noticing. A simple step goal helps counter that.
Diet breaks
A long deficit wears you down physically and mentally. Many coaches use planned breaks — for example, one to two weeks at maintenance after 6–12 weeks of dieting (a practical scheme, not a scientifically fixed rule). In the MATADOR study (Byrne and colleagues, 2018), men with obesity spent 16 weeks in a deficit of about 33%: one group continuously, the other in two-week blocks alternating with two weeks at maintenance. The intermittent group lost more weight and fat, and six months after the study ended had kept off considerably more than the continuous group.
A diet break isn’t a “cheat week”; it’s eating at your estimated TDEE with the same protein and training. Your weight may tick up slightly as glycogen and water return. That’s normal.
Signs your deficit is too aggressive
If several of these sound familiar, it’s time to ease off:
- you’ve been losing more than 1% a week for over a month;
- your working weights in the gym fall week after week;
- constant, intense hunger, food on your mind all day, binges;
- worse sleep, irritability, low energy;
- dizziness, feeling cold all the time;
- for women, menstrual cycle changes — get those checked by a doctor.
Track fat, not just the scale
The scale shows the sum of water, fat, muscle and gut contents. Salt, carbs, stress and the menstrual cycle can swing your weight by 1–2 kg within days. That’s why a single weigh-in tells you very little.
Watching several markers at once is far more reliable:
| Marker | How often | What counts as progress |
|---|---|---|
| Weekly average weight | weekly | down 0.5–1% |
| Waist circumference | every 1–2 weeks | decreasing |
| Body fat percentage | every 2–4 weeks | decreasing |
| Working weights | every session | stable or rising |
Learn how to take your measurements correctly in the body measurements guide, and how not to be fooled by single data points in the article on reading progress charts.
Checklist for a sensible deficit
- Calculate TDEE with the formula and subtract 15–25%.
- Keep protein at no less than 1.6–2.2 g/kg.
- Lift weights 2–3 times a week.
- Track weekly average weight, waist and body fat.
- Adjust calories by 100–200 kcal no more than every two to three weeks.
- Take a maintenance break after 6–12 weeks.
How FitHilf helps
FitHilf doesn’t count calories, but it shows you what really matters: what’s happening to your body. Log your weight and measurements, get your body fat percentage from the U.S. Navy and Deurenberg formulas, see fat mass and muscle mass separately, and track your workouts with personal records. If your weight is going down while your muscle mass holds steady, your deficit is set up right. Start tracking in the dashboard.
Sources
- Mifflin, M. D., St Jeor, S. T., Hill, L. A., Scott, B. J., Daugherty, S. A., & Koh, Y. O. (1990). A new predictive equation for resting energy expenditure in healthy individuals. The American Journal of Clinical Nutrition, 51(2), 241–247. doi:10.1093/ajcn/51.2.241
- Frankenfield, D., Roth-Yousey, L., & Compher, C. (2005). Comparison of predictive equations for resting metabolic rate in healthy nonobese and obese adults: a systematic review. Journal of the American Dietetic Association, 105(5), 775–789. doi:10.1016/j.jada.2005.02.005
- Helms, E. R., Aragon, A. A., & Fitschen, P. J. (2014). Evidence-based recommendations for natural bodybuilding contest preparation: nutrition and supplementation. Journal of the International Society of Sports Nutrition, 11, 20. doi:10.1186/1550-2783-11-20
- Hall, K. D. (2008). What is the required energy deficit per unit weight loss? International Journal of Obesity, 32(3), 573–576. doi:10.1038/sj.ijo.0803720
- Jäger, R., Kerksick, C. M., Campbell, B. I., et al. (2017). International Society of Sports Nutrition Position Stand: protein and exercise. Journal of the International Society of Sports Nutrition, 14, 20. doi:10.1186/s12970-017-0177-8
- Byrne, N. M., Sainsbury, A., King, N. A., Hills, A. P., & Wood, R. E. (2018). Intermittent energy restriction improves weight loss efficiency in obese men: the MATADOR study. International Journal of Obesity, 42(2), 129–138. doi:10.1038/ijo.2017.206
- Bull, F. C., Al-Ansari, S. S., Biddle, S., et al. (2020). World Health Organization 2020 guidelines on physical activity and sedentary behaviour. British Journal of Sports Medicine, 54(24), 1451–1462. doi:10.1136/bjsports-2020-102955
FAQ
How many calories should I cut to lose weight?
A deficit of 15–25% of your daily expenditure, roughly 300–600 kcal a day, is usually enough. That typically produces a loss of 0.5–1% of body weight per week.
Why isn’t my weight dropping even though I’m in a deficit?
Water retention or inaccurate calorie tracking is the usual culprit. Compare your weekly average weight and waist size for at least two to three weeks before changing the plan.
Do I need cardio to lose fat?
It helps burn energy and is good for your heart, but it isn’t required. For keeping muscle, strength training and enough protein matter more.
How accurate is the Mifflin-St Jeor equation?
In comparison studies it lands within ±10% of measured resting metabolic rate more often than other equations, but for some people it’s further off. Use it as a starting point and adjust based on how your weight actually trends.
