How the calorie calculator estimates your daily needs
This calculator uses the Mifflin-St Jeor equation to estimate your Basal Metabolic Rate (BMR), then multiplies it by an activity factor to find your Total Daily Energy Expenditure (TDEE). The result is how many calories your body uses in a day given your current size and activity level. From there it shows what you'd need to eat to maintain, lose, or gain weight at various rates.
Enter your age, sex, height, weight, and activity level to get your maintenance calories instantly. Switch to "Lose or gain" mode to see a custom target for your chosen rate of weight change.
How to use it
- Select Maintain weight mode for your maintenance number, or Lose or gain to set a specific goal.
- Enter your age, sex, weight, and height.
- Choose the activity level that honestly matches your average week (most people should pick one level lower than they think).
- In goal mode, choose whether you want to lose or gain, and at what rate (0.25–1 kg per week).
- Read your daily calorie target, plus the breakdown showing maintenance, mild loss, moderate loss, and extreme loss tiers.
How daily calorie needs are calculated
The Mifflin-St Jeor equation (1990) is considered the most accurate predictive formula for resting metabolic rate in non-obese adults:
Your BMR is then multiplied by an activity factor to estimate total daily expenditure:
| Activity level | Multiplier | Description |
|---|---|---|
| Sedentary | 1.2 | Desk job, little or no exercise |
| Lightly active | 1.375 | Light exercise 1–3 days/week |
| Moderately active | 1.55 | Moderate exercise 3–5 days/week |
| Active | 1.725 | Hard exercise 6–7 days/week |
| Very active | 1.9 | Physical job plus daily training |
For weight change, the calculator uses the energy balance principle: 1 kg of body fat stores approximately 7,700 kcal. A daily deficit of 550 kcal produces roughly 0.5 kg of loss per week; a surplus of the same amount produces roughly 0.5 kg of gain.
Choosing the right activity level
This is where most people go wrong. The activity multiplier accounts for all movement in your day, not just formal exercise. A construction worker who also hits the gym is "very active." An office worker who runs 30 minutes three times a week is "lightly active" to "moderately active," not "active."
When in doubt, choose one level lower. You can always adjust upward if you're losing weight faster than expected. Overestimating activity is the single most common reason calorie targets fail.
Understanding the weight loss tiers
The calculator shows four scenarios so you can see the trade-offs at a glance:
- Maintain weight (100%) — your estimated TDEE. Eat here to stay the same weight.
- Mild weight loss (0.25 kg/week) — a 275 kcal/day deficit. Sustainable long-term, minimal muscle loss, barely noticeable hunger for most people.
- Weight loss (0.5 kg/week) — a 550 kcal/day deficit. The most commonly recommended rate. Noticeable but manageable.
- Extreme weight loss (1 kg/week) — a 1,100 kcal/day deficit. Aggressive. Higher risk of muscle loss, metabolic adaptation, nutrient deficiency, and burnout. Not recommended without medical supervision, especially if it drops you below 1,200 kcal/day.
A common mistake is picking the most aggressive tier "to get it over with." Research consistently shows that moderate deficits (0.25–0.5 kg/week) produce better long-term outcomes because they're sustainable and preserve more muscle mass.
Why the calculator has a 1,200 kcal floor
If your chosen rate of loss would require eating below 1,200 kcal/day, the calculator caps the result and shows a warning. Below this threshold, it becomes very difficult to meet basic micronutrient needs from food alone, and the risk of gallstones, hormonal disruption, and excessive muscle loss increases sharply. If you genuinely need to lose weight faster, the safer lever is increasing activity rather than cutting calories further.
How protein fits in
The calculator shows a daily protein target because protein is the most important macronutrient for body composition during weight change:
- During a deficit, high protein (1.6–2.2 g/kg) preserves muscle that would otherwise be broken down for energy.
- During a surplus, adequate protein (1.6–2.0 g/kg) ensures extra calories build muscle rather than just fat, provided you're strength training.
- At maintenance, moderate protein (0.8–1.2 g/kg) supports general health and satiety.
The target shown adjusts based on your activity level — more active people need more protein to support recovery and muscle protein synthesis.
Common mistakes to avoid
- Overestimating activity level. The most common error. Be honest about sedentary hours.
- Eating back all exercise calories. Fitness trackers overestimate burn by 20–50%. If you do extra exercise beyond your usual routine, eat back 50–75% at most.
- Using an extreme deficit for too long. Metabolic adaptation kicks in after a few weeks. Consider diet breaks (1–2 weeks at maintenance) every 8–12 weeks.
- Ignoring protein. A calorie deficit without adequate protein loses muscle alongside fat, leaving you lighter but not leaner.
- Not recalculating. Your TDEE drops as you lose weight. Recalculate every 4–6 weeks at your new weight to keep the deficit real.
Limitations
This calculator provides an estimate, not an exact measurement. Individual variation in metabolism (genetics, thyroid function, non-exercise activity thermogenesis) means your true needs may differ by 100–300 kcal from the prediction. Use the number as a starting point: track your weight for 2–3 weeks, and adjust intake up or down based on whether you're hitting your target rate.
The Mifflin-St Jeor equation is validated for adults aged 19–78. For adolescents, the elderly, or very muscular individuals, accuracy decreases. It does not account for pregnancy, breastfeeding, or medical conditions affecting metabolism.
This calculator provides estimates for informational purposes. It is not medical or nutritional advice. Consult a healthcare professional or registered dietitian before making significant changes to your calorie intake, especially if you have a medical condition or are considering eating below 1,500 kcal/day.
Frequently asked questions
How accurate is the Mifflin-St Jeor equation?
Studies show it predicts measured resting metabolic rate within 10% for most people, making it the most reliable predictive equation for adults who are not elite athletes. Individual variation (genetics, muscle mass, hormones) means your true needs may differ by 100–300 kcal.
Should I eat back calories burned during exercise?
Partially. The activity multiplier already accounts for regular exercise. If you do an unusually intense session beyond your normal routine, eating back 50–75% of those extra calories is reasonable. Don't eat back 100% — calorie-burn estimates from trackers are typically inflated by 20–50%.
Is 1200 kcal/day safe for weight loss?
1200 kcal is generally the minimum recommended for women, and 1500 for men, without medical supervision. Going below risks nutrient deficiencies, muscle loss, metabolic adaptation, and hormonal disruption. A slower, more moderate deficit is safer and more sustainable long term.
Why does my weight loss stall after a few weeks?
Your body adapts to a calorie deficit through metabolic adaptation (lower BMR), reduced non-exercise activity, and water retention masking fat loss. Recalculate your needs at your new weight every 4–6 weeks, and consider a diet break (eating at maintenance for 1–2 weeks) to reset.
How much protein do I need while losing weight?
Aim for 1.6–2.2 g per kg of body weight when in a calorie deficit to preserve muscle mass. This is higher than the general recommendation (0.8 g/kg) because your body is more likely to break down muscle for energy during weight loss, especially without adequate protein.
Does metabolism slow with age?
Yes, but less than commonly believed. Research shows BMR declines roughly 1–2% per decade after age 20, largely due to muscle loss. Maintaining muscle through resistance training substantially offsets this decline. The bigger factor is usually reduced activity, not innate metabolic slowing.
What activity level should I choose?
Be honest and pick the level that matches your average week. Sedentary is a desk job with no exercise. Light is 1–3 light sessions per week. Moderate is 3–5 sessions. Active is daily exercise. Very active is physical labor plus training. Most people overestimate their level — when in doubt, choose one lower.