Weight, BMI and Percent Body Fat Explained

Weight, Body Mass Index and percent body fat. You hear these words all the time, but what do they mean for women trying to get to the best possible weight for their bodies? The Active Woman's Health and Fitness Handbook breaks it down.

woman standing on scale

Trying to lose weight post-baby and not sure how? Or maybe you just need someone to tell you once and for all what the hell BMI is and how it affects your health? Look no further.

The Active Woman's Health and Fitness Handbook breaks it all down for you...and even provides some handy charts.

More: Setting Realistic Weight-loss Goals

Weight and BMI

Three values are commonly used to measure your body composition. Weight, the most common standard, is cheap, convenient and fairly reliable, although it can fluctuate up to five pounds with body fluids.

Weight and height are used to calculate the Body Mass Index (BMI), another common term. This differs only from weight in that it adjusts for height; BMI also varies with body fluids.

BMI, measured in kilograms per meters squared, is an unreliable guideline; standards usually do not differentiate between men and women or allow for the difference in muscle or fat content.

In athletic women, BMI is especially inaccurate because muscle weight is heavier than fat weight, making you seem "fat" by BMI standards. Essentially, Body Mass Index is a number value that has replaced height and weight charts. The number is even more confusing because the recommended numbers are similar to, but slightly different from, body fat percent.

According to standard guidelines, BMI over 30 is considered obese, under 18, extremely underweight, and between 18 and 25, normal.

Body Fat Percentile

Body fat percentile provides the most accurate estimate of body composition. Body fat percentile is an indication of the percent of your body that is made of fat. Normal body fat percent for women is 20 to 30 (for men it is lower).

In women, below 17 is extreme low body fat; between 30 to 33, high body fat; and above 34, extremely high body fat or obese. The recommended healthy body fat percentiles increase slightly with age.

Body Fat Standards for Women Recommended by Age Group

 20 to 2930 to 3940 to 4950 to 5969+
Very low<16<17<18<19<20
Moderately high29-3130-3231-3332-3333-35

Body fat percentile measurement methods vary in practicality, cost and accuracy. The most accurate measurement is through DEXA, an expensive measurement done in a radiology lab or doctor's office; this is the same radiographic measurement used to determine bone density.

The second most accurate method is water displacement, although this requires your body to be underwater in a special tank, even more impractical and expensive.

One of the simplest, most common measures of body fat percentile measurement is skinfold measurements, in which a tester uses calipers to measure fat pinches at various sites on the body. This evaluation is very dependent on the tester's skills; if done by an experienced tester, it can be up to 93 percent accurate.

The other common method is bioelectric impedence; this is a machine that looks like a scale and is common in many gyms and even available for home use. This can be a very unreliable measure, as it is highly dependent on water weight, temperature, electrolytes, blood flow and other factors. Bioelectric impedence is even less accurate in athletes with a large amount of muscle.

All these measurements are interesting and can suggest health, but should not be the basis of an exercise or nutrition regime. Remember, if you are using weight as your guide, that muscle weighs more than fat. If you are increasing strength but not changing your weight, you are becoming leaner.

The following are loose guidelines for healthy weights in athletic women. The lower numbers are for petite, small-boned, less-muscled frames; the higher numbers accommodate for large bones and bigger muscle size.

Healthy Weight Guidelines for Muscular, Athletic Women (BMI 18 to 25)

Height (Inches)Recommended Weight (Pounds)
4'10"95 to 123
4'11"98 to 128
5'101 to 132
5'1"105 to 136
5'2"108 to 141
5'3"112 to 146
5'4"115 to 150
5'5"119 to 155
5'6"123 to 160
5'7"126 to 165
5'8"130 to 170
5'9"134 to 175
5'10"138 to 180
5'11"142 to 185
6'147 to 190
6'1"152 to 196
6'2"157 to 201

More: Diets: Bad, Fair, Good

Weight Loss or Gain

The best weight changes are made slowly. The slower changes are made, the more likely your body will maintain the change, as it has a tendency to return to its "steady state," or the weight it has been at for the longest.

A pound is gained by eating 3,500 calories more than your body burns. To lose this pound, 3,500 fewer calories need to be eaten or 3,500 more burned off.

The best way to gain or lose is change your dietary calorie intake by 500 kcal a day. This should equal a one-pound weight loss or gain per week. Keeping a food diary for seven days can make you more aware of your eating habits and strategize change in a healthy way. Nutritionists and support groups can be very helpful in reaching goals for weight changes.

And speaking of nutrition, check out our tips for sticking to healthy diet when you're a parent on the go!

  • The Active Woman's Health and Fitness Handbook
    The Active Woman's Health and Fitness Handbook
    The Active Woman's Health and Fitness Handbook