The three-step math
Step 1: your BMR (basal metabolic rate) โ the calories your body burns at complete rest โ estimated from age, sex, height and weight using the Mifflin-St Jeor equation, the formula most dietitians use. Step 2: multiply by an activity factor, from about 1.2 (desk job, little exercise) to 1.7+ (hard training most days). The result is your TDEE โ total daily energy expenditure, the calories that keep your weight stable. Step 3: adjust for your goal โ roughly 500 fewer per day for gradual fat loss (about 0.5 kg/week), or a few hundred more to gain.
A realistic example
A 30-year-old woman, 165 cm and 70 kg, has a BMR around 1,450 calories. With moderate exercise 3โ4 days a week (ร1.55) her TDEE is roughly 2,250. For steady fat loss she would target about 1,750 a day โ not the 1,200 that crash plans push, which is below what most adults can sustain and usually ends in rebound.
The mistakes that break the math
People overestimate activity (pick the honest multiplier, not the aspirational one), underestimate portions (calorie counts drift 20โ30% without weighing), and slash too hard, which tanks energy and adherence. The equation is an estimate with a real margin of error โ use it as your starting point, watch your weight trend for two to three weeks, and adjust by 100โ200 calories rather than starting over.
Quick answers
Is 1,200 calories a day safe?
For most adults it is too low to sustain and often backfires. A moderate deficit of about 500 below your TDEE preserves energy and muscle and is far easier to stick with.
Why am I not losing weight on my calculated calories?
The formula estimates; portions drift; activity gets overcounted. Track honestly for two weeks โ if the scale trend is flat, trim 100-200 calories or add a weekly walk and reassess.
Do I need different calories on training days?
You can cycle calories, but consistency beats complexity. Most people do better hitting one sustainable daily number than juggling two.
Run your own numbers
BMR, TDEE and goal calories with the Mifflin-St Jeor formula โ free, in your browser.
Calorie Calculator โ