The Calculations Explained:

Individual Weights:

The Hamwi Formula:
Dr. G.J. Hamwi's formulas have become very popular since they first appeared in a publication of the American Diabetes Association in 1964. Many of the weight calculators on the web use this formula and aside from BMI based calculations, this is probably the most popular method of calculating ideal weight. It has an advantage over most other calculation methods in that Dr. Hamwi suggests that the results may be reduced by 10% for a person with a light frame, and increased by 10% for a person with a heavy frame. Thus I provide all three numbers here. The formulas are:
Men: 106 lb for the first 5 ft; 6 lb for each inch over 5 ft
Women: 100 lb for the first 5 ft; 5 lb for each inch over 5 ft

The Robinson Formula:
This is similar to the Hamwi Formula but is based on a revision of Devine. The formulas are:
Men: 115 lb for the first 5 ft; 4 lb for each inch over 5 ft
Women: 108 lb for the first 5 ft; 3.75 lb for each inch over 5 ft

The Miller Formula:
This is also derived from Devine. The formulas are:
Men: 124 lb for the first 5 ft; 3 lb for each inch over 5 ft
Women: 117 lb for the first 5 ft; 3 lb for each inch over 5 ft

BMI Based Formula:
BMI or Body Mass Index is the preferred method of calculating obesity for most clinicians. This is the method the US government pushes. It is based on a calculation that supposedly takes into consideration the lean body mass and adds in an appropriate amount of fat. For my calculations, I simply reversed the process to arrive at an ideal weight based on an average BMI of 23. Thus, instead of using your height and weight to calculate your BMI, I use the height you input to provide a weight that would be equivalent to a BMI of 23, a number towards the high end of normal. Note that BMI does not take gender, age, body composition, or activity level into consideration

Average Weights:
The first number is the mean average weight of the above calculated weights. Because both Dr. Hamwi and the BMI allow for variation based on frame size and other factors, the next result is a range based on 10% under and 10% over the resulting average. Thus if you feel the average is too low due to frame size or other factors, use the upper end of the normal range. Likewise, if you think the weight listed for average is too high based on your frame size or other factors, use the lower end of the range. I have also included the range of weights considered to be "normal" according to the BMI. This is included as a comparison to the average range right above it. In general, the BMI range will be slightly larger on both the upper and lower ends for women, meaning that it will have a "lower" low and a "higher" high. For men, the average weight will likely be significantly higher on the lower end than the BMI, and slightly higher on the upper end.

Caloric Need:
BMR = Basal Metabolic Rate - a number that represents your caloric need before factoring in activity level.

TDEE = Total Daily Energy Expenditure - This it the BMR times an energy multiplier that is designed to account for how active you are. See below for the formulas used.

Caloric Need for Weight loss = (TDEE - 500). This will give an average loss of one pound a week as 500 X 7 days = 3500 calories, or the amount needed to burn one pound of fat.

Harris-Benedict Formula
The Harris-Benedict formula uses height, weight, age, and sex to determine basal metabolic rate (BMR). The only variable it does not take into consideration is lean body mass. Therefore, this equation will be accurate for many, but will underestimate the caloric needs of muscular athletes and overestimate the caloric needs of the obese.

Because of this, I am actually using an average from the computed individual weights and NOT total weight for this calculation. Thus will have a tendency to slightly underestimate the caloric needs of the obese since fat is not actually completely metabolically inert. Even so, I believe this is a better measure than one based on total weight because in my opinion it is generally better to err on the side of too few calories than too many when discussing caloric need for the overweight.

For reference, the formula used is as follows:

Men: BMR = 66 + (6.23 X weight in pounds) + (12.7 X height in inches) - (6.8 X age)

Women: BMR = 655 + (4.35 X weight in pounds) + (4.7 X height in inches) - (4.7 X age)

The calculations for TDEE are identical to those used for the Katch-McArdle Formula on the right.

Healthy Body Fat Guidelines*:

*Adapted from: Gallagher et al., "Healthy percentage body fat ranges: an approach for developing guidelines based on body mass index." American Journal of Clinical Nutrition, 72 (3): 694.

Weight Results from % Body Fat:
This portion of the calculator will likely provide a more accurate goal weight for many people. It works by first calculating lean body mass:
Lean Body Mass =
Your Current Weight X (1 - (Your Current Body Fat / 100))

Next, two differing methods are used to calculate the results:

Method 1:
This method is called "lossy" because it purposely calculates the desired body fat as a function of the lean body mass. Thus if you were to check, you'd find that the Lean Body Mass dropped by 5 to 10%. This method is included because some research suggests that overweight people (especially the morbidly obese), tend to lose some muscle mass as they lose weight. I have added this method to take this into consideraton. It should not be considered any more, or any less accurate than method 2 below. The actual calculation method is as follows:
Ideal Weight =
(Lean Mass * (Desired Body Fat / 100) + Lean Mass)

Method 2:
This method is called lossless because it assumes that no muscle mass will be lost during weight loss. This method has no formula associated with it. Instead, it uses the power of the computer to quickly check up to two-hundred possible body weights till it finds one that fits the desired percent body fat you entered. (If you care about why it must be done this way, click here: 123-fitness-and-health).

Keep in mind that if you got your percent body fat from BIA (Bioelectrical Impedance Analysis), that either of these methods can be off by a few percent. Some reports state that this is especially true for severely obese individuals. The point is that even this method is not a 100% accurate measure of true lean body mass and thus the results of this calculator should not be considered perfect. It is highly dependent on how accurate the body fat measurement was originally and whether or not the amount of muscle mass changes during the course of weight loss.

Caloric Need:
BMR = Basal Metabolic Rate - a number that represents your caloric need before factoring in activity level.

TDEE = Total Daily Energy Expenditure - This it the BMR times an energy multiplier that is designed to account for how active you are. See below for the formulas used.

Caloric Need for Weight loss = (TDEE - 500). This will give an average loss of one pound a week as 500 X 7 days = 3500 calories, or the amount needed to burn one pound of fat.

Katch-McArdle Formula
The Katch-McArdle formula differs from the Harris-Benedict formula in that Katch-McArdle takes lean body mass into consideration. This calculation is therefore only possible if you know your percentage body fat.

Generally speaking, this is a better measure of caloric need for the obese than Harris-Benedict because it represents your actual body composition and is not based of assumptions about what one "should" weigh.

The formula is as follows:
BMR = 370 + (9.79759519 X Lean Mass in pounds)

TDEE is calculated by multiplying your BMR by your activity multiplier from the list below:

• Sedentary = BMR X 1.2 (little or no exercise, desk job)
• Lightly active = BMR X 1.375 (light exercise/sports 1-3 days/wk)
• Mod. active = BMR X 1.55 (moderate exercise/sports 3-5 days/wk)
• Very active = BMR X 1.725 (hard exercise/sports 6-7 days/wk)
• Extr. active = BMR X 1.9 (hard daily exercise/sports & physical job or 2X day training, i.e marathon, contest etc.)

Important Note:
Regardless of what the calculator tells you, going under about 1000-1200 calories for a woman and 1300-1500 for a man can be detrimental to weight loss as this can cause your body to try harder to retain weight by slowing your metabolism. This is commonly called "starvation mode" and is a natural reaction to too low a caloric intake. As such, do not go lower than these numbers without the advice and consent for your physician. If the calculator gives you numbers smaller than this, it is probably because you are small in stature and not exercising much. If so, the best advice I can give is to increase your activity level and this will allow a more normal caloric intake.