Use the Mifflin-St Jeor equation
For men: BMR = 10 × weight in kg + 6.25 × height in cm – 5 × age in years + 5
For women: BMR = 10 × weight in kg + 6.25 × height in cm – 5 × age in years – 161
Convert weight from pounds to kilograms by dividing by 2.2046
Convert height from inches to centimeters by multiplying by 2.54
Plug your values into the correct formula
Calculate the result in calories per day
Use the result as your estimated BMR