A clearer starting point.
Your daily calories.

We use the commonly rounded Mifflin–St Jeor equation: 10 × weight (kg) + 6.25 × height (cm) − 5 × age, then add 5 for the male coefficient or subtract 161 for the female coefficient. This estimates resting metabolic rate (RMR). An approximate activity multiplier estimates maintenance calories. Your chosen 10–20% reduction gives the starting fat-loss target. These activity multipliers and deficit choices are coaching estimates, not measured metabolism.

The tool withholds fat-loss targets for a BMI below 18.5 or a calculated target below 1,200 kcal (female coefficient) or 1,500 kcal (male coefficient). These are conservative limits for this tool, not universal safe minimums. Your needs may be higher. Very muscular people, older adults, and people with unusual activity or health needs may need individualized assessment.

Sources: Mifflin et al., AJCN (1990) · NIH NIDDK Body Weight Planner guidance.