The most basic way to estimate resting metabolic rate, RMR, is to use the Mifflin-St Jeor equation. This equation, derived in 1990, came about from measuring indirect calorimetry in human subjects. As far as energy expenditure equations go, it’s currently the most accurate, although it has limitations is normally used as a starting-off point.

Mifflin-St Jeor ousted the previously popular Harris-Benedict equation, which was created in 1919 and overestimates RMR by 5 percent.

The Mifflin-St Jeor Equation is as follows, for ages 19–78:

Men: 10 x weight (kg) + 6.25 x height (cm) – 5 x age (y) + 5
Women: 10 x weight (kg) + 6.25 x height (cm) – 5 x age (y) – 161

To give yourself an idea of the total calories you’re burning in a day, you take that number and multiply it by your personal activity factor:

Sedentary = 1.2
Lightly active = 1.375
Moderately active = 1.550
Very active = 1.725
Extra active = 1.9

If you’re like most Americans, you need to convert your weight and height to metric units before entering into the equation:

• For your weight in kilograms, divide your weight in pounds by 2.2: For example, if you weigh 165 pounds, divide by 2.2 to get 75 kg.

• For your height in centimeters, multiply your height in inches by 2.54: If you’re 5’5”, or 65 inches, multiply by 2.54 to get 165.1 cm.

For example, if you’re a 45-year-old woman, you’d calculate your RMR like this:

(10 x 75kg) + (6.25 x 165.1) – (5 x 45) – 161 = 750 + 1,032 – 225 – 161 = 1,396 calories

Then, you’d multiply by your activity factor, which takes into account your day-to-day movement and planned exercise:

• If you’re sedentary (you work at a desk job and do very little exercise or housework): 1,396 x 1.2 = 1,675 calories

• If you’re lightly active (you go for long walks 1–3 days per week or do housework like cleaning and gardening): 1,396 x 1.375 = 1,920 calories

• If you’re moderately active (you’re moving most of the day and/or exercise with a moderate amount of effort 3–5 days of the week): 1,396 x 1.550 = 2,164 calories

• If you’re very active (you’re vigorously exercising or playing sports most days): 1,396 x 1.725 = 2,408 calories

• If you’re extra active (vigorous exercise or sports 6–7 days of the week plus a job which requires physical exertion): 1,396 x 1.9 = 2,652 calories