The Calorie Calculator estimates how many calories you burn in a day and converts that into targets for your goal. It first computes your Basal Metabolic Rate (BMR) with the Mifflin-St Jeor equation — the most accurate of the common predictive formulas — then scales it by your activity level to get your Total Daily Energy Expenditure (TDEE), and finally lays out seven calorie targets from extreme weight loss to extreme weight gain around that maintenance figure.
How It Works
| Activity level | Multiplier | Typical pattern | |---|---|---| | Sedentary | 1.2 | Desk work, little or no exercise | | Lightly active | 1.375 | Light exercise 1–3 days a week | | Moderately active | 1.55 | Moderate exercise 3–5 days a week | | Very active | 1.725 | Hard exercise 6–7 days a week | | Extra active | 1.9 | Very hard exercise plus a physical job |
The goal targets are fixed offsets from TDEE: mild loss −250, weight loss −500, extreme loss −1,000; mild gain +250, weight gain +300, extreme gain +1,000 calories a day.
Worked Example
A 35-year-old woman, 68 kg, 165 cm, lightly active:
- BMR = (10 × 68) + (6.25 × 165) − (5 × 35) − 161 = 680 + 1,031.25 − 175 − 161 = 1,375 calories (rounded)
- TDEE = 1,375.25 × 1.375 = 1,891 calories — her maintenance level
| Goal | Daily calories | |---|---| | Extreme weight loss (−1,000) | 891 | | Weight loss (−500) | 1,391 | | Mild weight loss (−250) | 1,641 | | Maintenance (TDEE) | 1,891 | | Mild weight gain (+250) | 2,141 | | Weight gain (+300) | 2,191 | | Extreme weight gain (+1,000) | 2,891 |
Two things stand out. The standard −500 deficit lands at 1,391 — below many people's comfort but still above her BMR. And the "extreme" row at 891 calories is below her BMR of 1,375: it is shown because the arithmetic produces it, not because it is advisable. Sustained intake below BMR means the body cannot cover basic functions from food alone and should only ever be medically supervised.
Choosing Your Target
A 500-calorie daily deficit corresponds to roughly half a kilogram of body weight per week; 250 calories is a gentler quarter-kilo pace that most people sustain longer. For gaining, note the deliberate design: the standard "weight gain" target adds only 300 calories, not 500 — a lean surplus sized to support muscle growth with minimal fat gain, with the +250 and +1,000 rows bracketing it for cautious and aggressive approaches. Whichever row you pick, hold it for two to three weeks and judge by the trend in weekly average weight, not daily fluctuations; then recalculate, because a lighter body has a lower TDEE and yesterday's deficit quietly becomes today's maintenance.
Limitations
Mifflin-St Jeor is an estimate with a typical error band around ±10% for individuals; muscle mass, thyroid function, medication, and adaptive thermogenesis all move real expenditure. The activity multipliers are coarse — the most common error is choosing one level too high, since a desk worker who trains for an hour is usually "lightly active" in expenditure terms, not "very active." The calculator accepts adult inputs (age 15 to 120) and does not adjust for pregnancy, breastfeeding, or eating-disorder recovery, all of which need professional guidance rather than a formula.
Frequently Asked Questions
What is the difference between BMR and TDEE?
BMR is the energy your body spends at complete rest on basic functions — breathing, circulation, cell repair. TDEE is BMR scaled by everything you actually do: work, walking, training, even fidgeting. You eat against TDEE, never against BMR; BMR is shown so you can see the floor your intake should not chronically undercut.
Why is the weight-gain target only +300 when weight loss is −500?
Because the two directions are not symmetric in outcome. A 500-calorie deficit mostly draws on fat stores in someone with weight to lose, but a 500-calorie surplus in a trainee outruns the rate of muscle synthesis and the excess is stored as fat. A +300 surplus captures most of the muscle-building benefit with much less fat gain.
How often should I recalculate?
Whenever your weight has moved by about 5 kg, or your activity pattern changes materially. TDEE falls as you lose weight — that is arithmetic, not a "broken metabolism" — and a target set at the start of a diet will overfeed its end if never revised.