What is a calorie calculator?

A calorie calculator estimates how many calories you need to eat each day to reach a specific goal - maintaining weight, losing weight, or gaining muscle. It works by calculating your TDEE (Total Daily Energy Expenditure), which combines your BMR (Basal Metabolic Rate - calories burned at rest) with calories burned through physical activity. Once you know your TDEE, eating that many calories maintains weight; eating less creates a deficit (weight loss); eating more creates a surplus (weight gain). The science is well-established: to lose 1 pound (0.45 kg) of fat requires a deficit of approximately 3,500 calories. The Mifflin-St Jeor formula used in this calculator is the most accurate predictor of BMR for general populations according to multiple meta-analyses by the American Dietetic Association.

How to use this tool

1. Choose unit system — Metric (kg, cm) or Imperial (lbs, inches) - tab at the top.
2. Enter age, sex, weight, height — All required for BMR calculation. Age affects metabolic rate (slower with age). Sex matters because men have more muscle mass on average. Be accurate for best results.
3. Select activity level — Sedentary (desk job, no exercise) to Extra active (manual labor + daily intense exercise). Most office workers with 3-4 weekly workouts are 'Moderate'. Be honest - most people overestimate.
4. Pick your goal — Maintain weight, lose at -250 or -500 kcal/day, gain at +250 or +500 kcal/day. Larger deficits = faster loss but harder to sustain and risk muscle loss.
5. Read your daily calorie target — Result shows recommended daily intake plus BMR, TDEE (maintenance), suggested protein target (1.6 g per kg body weight), and water target.

Mifflin-St Jeor formula

Step 1 - calculate BMR:

• Men: BMR = 10 × weight(kg) + 6.25 × height(cm) - 5 × age + 5
• Women: BMR = 10 × weight(kg) + 6.25 × height(cm) - 5 × age - 161

Step 2 - multiply by activity factor for TDEE:

• Sedentary (desk job, no exercise) - BMR × 1.2
• Light (light exercise 1-3 days/week) - BMR × 1.375
• Moderate (moderate exercise 3-5 days/week) - BMR × 1.55
• Very active (hard exercise 6-7 days/week) - BMR × 1.725
• Extra active (physical job + exercise) - BMR × 1.9

Step 3 - adjust for goal:

Target calories = TDEE + (-500 for fast loss, -250 for slow loss, 0 for maintain, +250 for slow gain, +500 for fast gain)

Example: 30-year-old man, 80 kg, 175 cm, moderate activity, wants to lose weight slowly:

• BMR = 10(80) + 6.25(175) - 5(30) + 5 = 800 + 1094 - 150 + 5 = 1,749 calories
• TDEE = 1,749 × 1.55 = 2,711 calories
• Target with -250 deficit = 2,461 calories/day

Examples

• 30-year-old woman, 65 kg, 165 cm, light activity, maintain weight: BMR 1,397, TDEE 1,921, eat 1,920 kcal/day
• 40-year-old man, 90 kg, 180 cm, sedentary, lose 0.5 kg/week: BMR 1,795, TDEE 2,154, eat 1,654 kcal/day (500 deficit)
• 25-year-old man, 75 kg, 178 cm, very active, gain muscle: BMR 1,754, TDEE 3,026, eat 3,276 kcal/day (250 surplus)
• 55-year-old woman, 70 kg, 160 cm, moderate, lose slowly: BMR 1,224, TDEE 1,897, eat 1,647 kcal/day

Notice how older people have lower BMR (less muscle mass, slower metabolism), so they need fewer calories.

Tips & best practices

• Track everything you eat for the first 2 weeks using MyFitnessPal, Cronometer, or LoseIt - most people underestimate calories by 30-50%
• Weigh yourself daily at the same time (morning, after bathroom) but only judge progress by 7-day moving average - daily weight fluctuates 1-2 kg
• Prioritize protein (1.6-2.2 g per kg body weight) when losing weight - prevents muscle loss and increases satiety
• Don't drop below 1,200 kcal (women) or 1,500 kcal (men) per day without medical supervision - too aggressive and unsustainable
• Re-calculate every 5 kg of weight change - your BMR decreases as you lose weight, so calorie needs drop too
• Refeed days (eating at maintenance once per week) help dieters psychologically and may boost hormones
• Strength training while in a deficit preserves muscle mass - keep lifting even when cutting

Limitations & notes

The Mifflin-St Jeor formula has a margin of error of about ±10%. Individual variation in metabolism (genetics, gut microbiome, medical conditions like thyroid disorders) can mean your actual TDEE is 200-400 calories different from the estimate. Activity multipliers are notoriously inaccurate - most people overestimate. The best approach is to use the calculator's number as a starting point, eat at that level for 2-3 weeks, then adjust based on actual weight change.

Frequently Asked Questions

How many calories should I eat to lose weight?

Generally TDEE minus 500-750 calories per day produces healthy weight loss of 0.5-1 kg per week. Going below 1,200 kcal (women) or 1,500 kcal (men) risks nutritional deficiencies and muscle loss. The 'right' number depends on your activity level - sedentary people may lose at 1,600 kcal while athletes may lose at 2,500 kcal.

Is the Mifflin-St Jeor formula accurate?

It's the most accurate BMR formula for the general population according to American Dietetic Association meta-analyses, accurate within ~10% for most people. Older formulas like Harris-Benedict overestimate by ~5% on average. For athletes or very obese individuals, the Katch-McArdle formula (uses lean body mass) is even more accurate.

How fast can I safely lose weight?

0.5-1 kg (1-2 lbs) per week is the medically recommended safe rate. Slower means easier to maintain and better preserves muscle. Faster (2+ kg/week) usually means losing muscle and water, plus rebound weight gain when you stop dieting. Slow and steady wins.

Why isn't my weight loss working at the calculated calories?

Most common reasons: (1) underestimating food intake (oil in cooking, snacking adds up fast), (2) overestimating activity (most people are between sedentary and light, not moderate), (3) water retention masks fat loss for weeks, (4) not enough patience - results take 2-4 weeks to show on the scale.

Should I eat the same calories on workout vs rest days?

Either approach works. Same calories every day (easier to plan, time-restricted feeding) or calorie cycling (more calories on workout days, less on rest). For most people, eating the same amount daily is simpler and equally effective.

How does protein intake affect calorie needs?

Higher protein intake (1.6-2.2 g/kg) doesn't change your TDEE much, but it: (1) preserves muscle during a calorie deficit, (2) has the highest thermic effect of food (TEF) - body burns 25-30% of protein calories during digestion vs 5-10% for carbs/fat, (3) increases satiety, helping you stick to the diet.

Can I trust calorie counts on food labels?

Mostly, but labels have a ±20% accuracy allowance by FDA. Restaurant menu calorie counts are often 10-25% higher than listed. For tighter accuracy, weigh raw ingredients with a kitchen scale and log them. For restaurant meals, add a 15-20% safety margin to listed counts.

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