Estimating daily calorie requirements is challenging. Even the very best calculators cannot determine an accurate metabolic rate for every individual.
Quite simply – everyone is different!
However, the formulas established by peer-review medical journals are helpful for the majority of individuals. Those who engage in very high levels of physical activity tend to find the calculator estimations to be too low.
NOTE: Our Calorie Calculator uses a selection of three formulas to estimated daily energy requirements:
- Uses Mifflin-St Jeor formula (most recent).
- Can also select an older formula (Harris-Benedict).
- And a formula based on Lean Mass (Katch-McCardle) – but you must know your body fat percentage.
You can also choose BMR (Basal Metabolic Rate) in the Exercise level. In this situation all weight loss calorie levels are blanked out.
Calorie calculators will first estimate Basal Metabolic Rate (BMR). This is effectively the amount of energy expended per day.
BMR is a measure of only the most basic functions (effectively the same as if you rested in bed the whole day). Other terms synonymous with BMR are Resting Metabolic Rate (RMR) and Resting Energy Expenditure (REE).
Once BMR has been estimated, various “activity” factors are applied. Once again these are best guesses based on contemporary research. One individual may burn 100 calories on a 10 minute run – another individual may burn 110 calories.
Mifflin – St Jeor Formula
The calculator currently uses the formula proposed by MD Mifflin and ST St Jeor1.
Why This Formula? The ADA (American Dietetic Association) published a comparison of various equations2. The Mifflin-St Jeor was found to be the most accurate.
10 x weight (kg) + 6.25 x height (cm) – 5 x age (y) + 5
10 x weight (kg) + 6.25 x height (cm) – 5 x age (y) – 161.
This can be selected on the calculator. It is a variation1 on the basic Mifflin-St Jeor equation that will base the equation on Fat Free Mass (FFM) or Lean Mass. This is more accurate for those who are leaner (and who know their body fat percentage!).
21.6 * Fat Free Mass + 370
Where Fat Free Mass = Weight – (Body Fat Percentage * Weight).
This formula was created in 1919, and due to changing lifestyles, it tends to overstate calorie needs by 5%. The results tend to be skewed towards both obese and young people3.
66.5 + (13.75 X weight in kg) + (5.003 X height in cm) – (6.775 X age in
655.1 + (9.563 X weight in kg) + (1.85 X height in cm) – (4.676 X
age in years)
Estimating Calories for Weight Loss
After calculating the BMR, exercise is factored in. Depending on the exercise level chosen, the BMR will be multiplied by anything from 1.2 to 1.9.
This provides us with maintenance calories – the amount of calories you could consume each day and neither lose or gain weight.
To get the fat loss figure – 20% of calories is subtracted.
The extreme fat loss figure has 40% subtracted. However – there is a “rock bottom” figure that equates to 8 calories per pound of body weight – the extreme fat loss will never be less than this amount. This has been put into the calculator as a failsafe to prevent users from embarking on highly-restricted diets. Such diets need medical care, advice, and monitoring.
It is also not advised to drastically reduce calories, but rather do so gradually or by a maximum of 500 calories per day.
Daily calorie needs for pregnant women.
- MD Mifflin, ST St Jeor, et al. A new predictive equation for resting energy expenditure in healthy individuals.J Am Diet Assoc 2005:51:241-247. Link
- Frankenfield DC, et al. Comparison of Predictive Equations for Resting Metabolic Rate in Healthy Nonobese and Obese Adults: A Systematic Review.J Am Diet Assoc. 2005;105:775-789. Link
- Frankenfield DC, et al. The Harris-Benedict studies of human basalmetabolism: history and limitations.J Am Diet Assoc. 1998;98:439-445.