What it means
For most people BMR is the biggest share of daily calorie burn, often around 60 to 70%. It depends mostly on body size, lean mass, age and sex.
True BMR is measured in a lab after an overnight fast. Calculators estimate it with equations instead. Sunny Weigh uses the Mifflin-St Jeor equation by default and the Katch-McArdle equation when you enter a body fat percentage.
BMR drops as you lose weight, partly because there is less of you to fuel. That is one reason the same calorie target that worked at the start can stop working months later.
Illness, some thyroid conditions and certain medicines can push BMR up or down, which no calculator can see. If your results differ a lot from your real-world trend, your trend is the better guide.
Why it matters
BMR is the base number every TDEE, deficit and macro target on this site is built from.