What Is a Calorie Calculator?
A calorie calculator estimates two numbers: your Basal Metabolic Rate (BMR) — the energy your body burns at complete rest just to keep organs running — and your Total Daily Energy Expenditure (TDEE), which adds in the calories burned through daily movement and exercise. TDEE is also called your "maintenance" number, since eating that amount keeps your weight stable.
From there, the math for weight change is simple arithmetic: eat below maintenance and you lose weight, eat above it and you gain. This tool uses the Mifflin-St Jeor equation, which the Academy of Nutrition and Dietetics and most modern nutrition research recognize as more accurate than the older Harris-Benedict formula it replaced.
How to Use This Calculator
- Select your sex — the formula uses different constants for men and women
- Enter your age — metabolism slows gradually as you get older
- Enter height and weight — metric or imperial, whichever you have handy
- Choose your activity level — be honest here; overestimating activity is the most common reason people plateau on a diet
- Click "Calculate My Calories" to see your BMR, maintenance calories, and targets for losing or gaining weight at different paces
The Mifflin-St Jeor Formula
Here's the actual math, in metric units (the calculator converts imperial inputs before running these):
Notice the two formulas differ only in the last constant: +5 for men, −161 for women. That 166-point gap reflects the average difference in muscle mass and body composition between sexes at the same height and weight — muscle burns more calories at rest than fat does.
How Activity Level Changes Your Number
Same person, same BMR — but the multiplier swings the maintenance number by close to a thousand calories depending on how much you move. Using the 30-year-old, 165 cm, 65 kg example above (BMR = 1,370.25):
| Activity Level | Multiplier | TDEE | Typical Description |
|---|---|---|---|
| Sedentary | ×1.2 | 1,644 kcal | Desk job, little to no exercise |
| Lightly active | ×1.375 | 1,884 kcal | Light exercise 1-3 days/week |
| Moderately active | ×1.55 | 2,124 kcal | Moderate exercise 3-5 days/week |
| Very active | ×1.725 | 2,364 kcal | Hard exercise 6-7 days/week |
| Extra active | ×1.9 | 2,604 kcal | Physical job plus daily training |
Going from sedentary to moderately active adds 480 calories to this person's maintenance number — roughly what a 45-minute run burns. That's also why the activity dropdown matters more than most people expect: pick one level too high, and your "maintenance" number is really a small daily surplus.
Calorie Targets by Goal
One pound of body fat holds roughly 3,500 calories of stored energy — a commonly used approximation from the CDC's guidance on losing weight, though real-world weight loss slows this rate somewhat as body weight drops. Using that figure, here's what different daily deficits and surpluses mean over time for the same 2,124-calorie maintenance level:
| Daily Adjustment | Daily Calories | Weekly Change | Weeks to Lose/Gain 20 lbs |
|---|---|---|---|
| −1,000 kcal (aggressive cut) | 1,124 kcal | −2.0 lb/week | 10.0 weeks |
| −750 kcal | 1,374 kcal | −1.5 lb/week | 13.3 weeks |
| −500 kcal (standard cut) | 1,624 kcal | −1.0 lb/week | 20.0 weeks |
| −250 kcal (mild cut) | 1,874 kcal | −0.5 lb/week | 40.0 weeks |
| 0 (maintenance) | 2,124 kcal | No change | — |
| +250 kcal (mild surplus) | 2,374 kcal | +0.5 lb/week | 40.0 weeks |
| +500 kcal (standard surplus) | 2,624 kcal | +1.0 lb/week | 20.0 weeks |
A 1,000-calorie daily deficit gets you to a 20-lb loss in 10 weeks instead of the 20 weeks a 500-calorie deficit takes — but dropping this person's intake to 1,124 calories a day is aggressive for someone with a 2,124-calorie maintenance level, and sustaining it long-term usually costs muscle along with fat. Most dietitians land on the 500-calorie range for a reason: it's slow enough to preserve muscle, fast enough to see progress.
How Age Affects Calorie Needs
The formula subtracts 5 calories per year of age — small on its own, but it adds up over decades. Same person (165 cm, 65 kg, moderately active) at different ages:
| Age | BMR | TDEE (moderately active) |
|---|---|---|
| 20 | 1,420 kcal | 2,201 kcal |
| 30 | 1,370 kcal | 2,124 kcal |
| 40 | 1,320 kcal | 2,046 kcal |
| 50 | 1,270 kcal | 1,969 kcal |
| 60 | 1,220 kcal | 1,891 kcal |
| 70 | 1,170 kcal | 1,814 kcal |
Between 20 and 70, this person's maintenance number drops by 387 calories — about the size of a fast-food value meal — for identical height, weight, and activity level. That's part of why the same diet that maintained weight at 30 can slowly add pounds at 50 if intake isn't adjusted downward alongside it.
Splitting Your Calories Into Macros
Total calories decide whether you lose, maintain, or gain weight. Macronutrient split — how those calories divide between carbs, protein, and fat — mostly affects how you feel and perform along the way, plus how much of what you lose or gain is muscle versus fat. At a 2,000-calorie target, here's what three common approaches look like in grams (protein and carbs run 4 calories per gram, fat runs 9):
| Approach | Carbs | Protein | Fat |
|---|---|---|---|
| Balanced (45/25/30) | 225 g | 125 g | 67 g |
| High-protein (35/35/30) | 175 g | 175 g | 67 g |
| Low-carb (20/35/45) | 100 g | 175 g | 100 g |
All three rows add up to the same 2,000 calories — the difference is entirely in where those calories come from. Research on protein intake during a calorie deficit, including guidance summarized by the Academy of Nutrition and Dietetics, generally supports higher protein targets (the middle and right columns above) for people actively trying to lose fat while keeping muscle.
6 Tips for Using Your Calorie Number
- Treat your TDEE as a starting estimate, not a fixed law. Mifflin-St Jeor is accurate to within roughly 10% for most people — for someone with a 2,124-calorie TDEE, that's a real range of about 1,912 to 2,336 calories. Adjust based on what actually happens to your weight over 2-3 weeks.
- Weigh yourself as a trend, not a daily verdict. Daily weight can swing 2-4 lbs from water and food volume alone. A weekly average tells you far more than any single morning's number.
- Recalculate every 10-15 lbs of change. A lighter body burns fewer calories at rest — losing 20 lbs from the example above would lower BMR by roughly 92 calories (10 kcal per kg × 9.1 kg), so a deficit that worked at the start needs adjusting later.
- Don't stack multiple deficits by accident. Eating at a calculated 1,624-calorie target and then also skipping meals or over-exercising can push you well below what your body needs, which often backfires through fatigue and muscle loss rather than faster fat loss.
- Protein matters more than the total number suggests. At the same 2,000 calories, going from 125g to 175g of protein (the balanced vs. high-protein rows above) is a meaningfully different diet for muscle retention, even though the calorie count is identical.
- Extreme deficits rarely hold up long-term. A 1,000-calorie deficit hits the 20-lb mark in 10 weeks on paper, but adherence research consistently shows aggressive cuts have higher dropout rates than moderate ones — the math works until the plan doesn't get followed.