Fit3D uses the Mifflin St. Jeor formula to calculate BMR.
There are multiple formulas used to calculate BMR. Nowdays, the Mifflin-St Jeor equation is believed to give the most accurate result and it's what we use in this calculator.
This BMR formula is as follows:
BMR = (10 * weight / 1kg + 6.25 * height / 1cm - 5 * age / 1 year + s) kcal / day, where s is +5 for males and -161 for females.