How Many Calories Do You Need? TDEE and BMR Explained
What BMR and TDEE actually mean, how the Mifflin-St Jeor equation works, and how to pick a realistic calorie target for losing or gaining weight.
“How many calories should I eat?” sounds like it should have one answer, but it depends on four boring numbers — age, height, weight, sex — plus one honest guess about how much you actually move. Get those right and the math is straightforward. Our Calorie Calculator does it in your browser as you type.
BMR: what your body burns doing nothing
Basal metabolic rate is the energy cost of simply being alive — heart beating, lungs working, brain running (the brain alone takes roughly 20% of it). For most people BMR is 60–75% of everything they burn in a day, which surprises people who assume exercise is the main event. It isn’t. A 70 kg, 175 cm man of 30 has a BMR around 1,650 kcal before he takes a single step.
The calculator uses the Mifflin-St Jeor equation, published in 1990:
- Men: 10 × weight (kg) + 6.25 × height (cm) − 5 × age + 5
- Women: 10 × weight (kg) + 6.25 × height (cm) − 5 × age − 161
A 2005 review by the American Dietetic Association compared the common prediction formulas against measured metabolism and found Mifflin-St Jeor the most reliable, which is why it has mostly replaced the century-old Harris-Benedict equation.
TDEE: the number you actually plan around
Total daily energy expenditure is BMR multiplied by an activity factor:
- Sedentary (desk job, little exercise): × 1.2
- Light (exercise 1–3 days a week): × 1.375
- Moderate (3–5 days a week): × 1.55
- Active (6–7 days a week): × 1.725
- Very active (hard physical work or two-a-days): × 1.9
Here’s where most people go wrong: they pick a level too high. Three gym visits a week on top of a desk job is “light” or “moderate”, not “active”. If you’re unsure, choose the lower option — an inflated multiplier quietly adds 200–300 kcal to your target, and then the scale refuses to move and you blame the formula.
Setting a target that survives past week two
Eat at your TDEE and your weight stays put. To change it:
- −250 kcal/day — slow loss, about 0.25 kg per week. Barely feels like a diet.
- −500 kcal/day — the classic pace, about 0.5 kg (1 lb) per week.
- +250 to +500 kcal/day — for gaining, where a smaller surplus means less of the gain is fat.
Cutting harder than 500 works on paper and fails in kitchens. Big deficits cost muscle along with fat, and hunger tends to win the argument eventually. The 0.5 kg-per-week pace is what the CDC and NHS recommend, and not because it’s exciting — because people actually stick to it.
The formula is a starting point, not a verdict
Any equation predicts the average person with your stats; you might burn 10% more or less. So treat the number as a first draft. Eat at your calculated TDEE for two or three weeks, weigh yourself a few mornings a week, and watch the trend. Weight stable? The number’s right. Drifting up? Subtract 100–150 and re-test. Your real TDEE also drops as you lose weight — a 5 kg loss shaves roughly 50 kcal off your daily burn, so recalculate now and then.
One more thing: none of this requires an account, an app subscription, or handing your body stats to anyone. The Calorie Calculator runs entirely in your browser — type your numbers, get your targets, close the tab.