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Understanding Body Mass Index (BMI)
by Brad Hefta-Gaub, August 9, 2009
Understanding BMI
If you’ve been paying attention to your health and fitness or wondering if you are a healthy weight, you’ve probably heard of BMI, or the Body Mass Index. Maybe you’ve even had a health care professional measure your BMI. But what does it all mean? And maybe more importantly, what is your BMI, and what does it tell you about your health?
Weight, BMI, and Body Fat
The Body Mass Index was a system established in 1988 by the National Institute of Health. It is widely accepted as a relatively accurate estimate of an individuals body fat, and related health risks. It was designed as a more accurate replacement for standard height weight tables or simply using weight alone as a measurement of body weight related health risks. Contrary to popular opinion, although it is closely related to body fat, it is not the same as measuring body fat.
BMI is calculated by creating a ratio of your weight to your height. By considering height along with weight, the formula allows the system to differentiate between the normal weight of someone who is taller or shorter. This makes intuitive since when we consider that 180lbs is a very different weight for someone who is 5′4″ tall compared to someone who is 6′2″ tall.
BMI Calculator
The following calculator can be used to calculate your BMI. If you’re a member of the Sweat365 community, we will preload the calculator with your last known height and weight. If your height and weight changes, we will record your new values for you so you can track your progress as you work toward your fitness goals.
Understanding Your Results
Once you’ve used the calculator, how do you use the information? Well, let’s start by looking at a table of the ranges of BMI results.
| BMI | Classification | Details |
|---|---|---|
| 18.5 or less | Underweight | An Underweight BMI can indicate that your weight may be too low. It would be good for you to consult with your health care professional to determine if you should gain weight. Having a low body mass can impact your immune system leading to illness, bone loss, malnutrition and other conditions. These risks increase the lower your BMI is. |
| 18.5 to 25 | Normal Weight | According to the NIH, people with a BMI in this range have the ideal amount of body weight to their height. Studies indicate that maintaining a normal weight is associated with longer healthier lives. However, BMI is not the most accurate measure of body fat and general health, so it is still a good idea to have regular checkups and maintain a balanced nutritional diet and an active lifestyle. |
| 25 to 30 | Overweight | People in this BMI range are considered overweight according to the NIH. If you fall into this range then you would benefit from finding healthy ways to lower your weight. Regular exercise and a balanced nutritional diet are the best ways to achieve this goal. Studies have shown that a BMI in this range increases your health risk for numerous conditions. As your BMI increases so do your risks of high blood pressure, diabetes, heart disease and other illnesses. |
| 30 to 35 | Obesity (Class 1) | The NIH considers a BMI of 30 to 35 to be a physically unhealthy condition, which significantly increases your risk of heart disease, diabetes, high blood pressure, gall bladder disease, and some cancers. This is particularly true if you have a larger then recommended waist size. People who fall into this range should see their health care professional, and ideally develop a plan to modify their lifestyle. Reducing their weight by as little as 5-10% would result in considerable health benefits. |
| 35 to 39 | Obesity (Class 2) | People with a BMI of 35-40 have severe risk of weight-related health problems and even death. It is strongly recommended that you consult your health care professional and reduce your weight to a lower BMI. |
| 40 or greater | Morbid Obesity (Class 3) |
People with a BMI of over 40 have an extremely high risk of weight-related disease and premature death. If your BMI is over 40, you may already be suffering from weight-related conditions. It is strongly recommended that you consult your health care professional and get specialized care for your condition. |
15 Comments: :
Understanding Body Mass Index (BMI)
August 9th, 2009 3:25 pm
shebaduhkitty says:
So this claims that I would have to get down to 125 to be healthy weight. I haven’t seen that weight since before puberty and I think that would NOT be a healthy weight for me. I am goaling for 145, but that would still be obese according to this chart. My doctor agrees that 145 is a good goal for me to be a healthy weight. so what’s up with that?
August 9th, 2009 3:49 pm
Brad Hefta-Gaub says:
It’s great that you’ve talked to your doctor about this, and you should absolutely follow their advice. This tool is just that, it’s a tool… it can be useful for getting perspective, but it’s always better to get the advice of your doctor.
For what it’s worth, my BMI comes in between 24 and 26 depending on my fluctuating weight, which means I am on the edge of overweight according to this scale.
Also consider that as you become more fit, you will convert more body fat into muscle, and this will actually cause you to either gain weight or maintain weight, even though you are getting much healthier. The difference between body fat and lean body mass can not be determined by this tool.
August 10th, 2009 7:19 am
lassie says:
Not sure if I like this test. I am in the “overweight” category. I only go to the doctor for a “well” check up so I am healthy and fit with all the exercises I do, so I have decided to ignore this BMI result. I think my body fat is more muscle. Is there a test for that? My body likes my weight at 132 and for my age that is OK with me. Though I do wish I could be taller than 5 foot then I would be in the “normal” category!
August 10th, 2009 11:08 am
miaharney says:
I had my BMI done in the office for our health insurance and I found out that I am in the “normal” range. I even did the calculations myself beforehand, so there would not be any surprises. The one surprise was that I am a whole inch taller than I thought.
August 10th, 2009 11:24 am
efranlje says:
The one thing I do NOT like about the BMI calculations is that it does not take into account different body frames. While BMI provides a good guideline it cannot be viewed as the be all end all. To sheba’s point - we all have our own healthy range
August 10th, 2009 11:46 am
Brad Hefta-Gaub says:
@Lassie - Yes, one common complaint about this test is that it does not handle shorter people very well. It also does not handle “muscular” people very well. To @efranlje’s point, the BMI test tends to work well on people with “typical” frames.
If your BMI is higher than you expected, then it may be worth while to get a body fat test done. There are several types of Body Fat tests, each with their own advantages and disadvantages, but they will usually be more accurate than a BMI test for people with “atypical” frames.
August 10th, 2009 12:53 pm
Hope says:
At 5′6″ tall, I have weighed 243 pounds (2/2002) and 100 pounds (8/2005) and am now, for the first time in my life, positively delighted to say that I’m right squat in the middle of HEALTHY.
…and I hope like heck I never again see the day that I strive for that 18.5 like I once did.
August 10th, 2009 5:04 pm
Chubby Runner says:
We’ve been taking our measurements at the office on the first Monday of the month since June. Not all of us…just those of us who joined the Good Health Club. A PT/wellness center group comes to do the measurements. They check BP, weight and with a little hand held device check our BMI and fat %. (This do-dad is available on amazon.com for around $30)
My weight and BMI have been going down steadily (14 lbs to date), my fat % is much slower to go down. Will doing more strength training help this go down?
August 10th, 2009 5:14 pm
Brad Hefta-Gaub says:
@Chubby Runner - Yes doing more strength training will increase your muscle mass and make you more lean. More muscles == lower body fat. But you don’t have to become a big muscle bound body builder to see an improvement. Big muscles come from big weights — but consistently doing small and medium weights at higher and medium reps will help you tone up and get lean.
Also - “weights” are not the only way to do resistance training. For example, I was going to a spin class for a while a couple years back, and I got extremely lean. It helped that I had a good coach - who made class a lot of fun, but it proved that you can get resistance training from activities usually thought of as “cardio”. Another good example is trail running. All of the hill climbing is much more closely related to “resistance” training.
August 12th, 2009 7:08 am
sarahdaniels77 says:
Brad - I have also read (don’t ask me where!) that the BMI calculator is not particularly friendly toward women, becuase it makes no distinction for gender. According to the experts, women tend to have a larger fat percentage than men, IN GENERAL (i.e. don’t anyone jump on me for saying it, please). Is this a farce? And also, is there talk of modifying the BMI to be gender specific?
August 12th, 2009 11:07 am
Brad Hefta-Gaub says:
Sarah - This is a great question. It is true that BMI doesn’t include a gender adjustment, and so many people ask “how can it be fair”. Well, it turns out that you’re right that it probably isn’t fair, but… it’s actually less fair to men — for exactly the reason you mention. Namely it is less fair to people who are more muscular, because muscles weigh more. This test is measuring your general mass (weight) to your overall body size (height). If you have more muscles you actually have more mass not less.
The key thing to remember is that if you’ve looked at this tool and your thinking about the implications of the data, then you’ve already made a huge important step in taking on your health and wellness. Don’t get hung up on the numbers, but instead use this data as inspiration to adjust your fitness and nutrition program to reach the goals that feel right to you.
August 12th, 2009 12:34 pm
Brad Hefta-Gaub says:
Here’s a link to the article about the gap in BMI for men and women.
http://www.halls.md/bmi/gap.htm
It’s an interesting read if you’re into science and statistics.
August 12th, 2009 2:24 pm
fatrob says:
Oh boy…..
My BMI is 32, but bearing in mind I did a 24h mountain bike race at the weekend I guess I’ll live with it….
Interestingly my body mass percentage general works out to be in the “acceptable” range (I don’t believe that either !!).
August 12th, 2009 9:21 pm
suefromyourpaceormine says:
Hey Brad, I had my BMI done with calipers that measured the amount of skin that could be pinched in my arms, waist, hips and upper thighs. Then they put my age, height and weight into a program that came out with my BMI number. The interesting thing is that my BMI was 23 with that method and 23 using the BMI calculator. That’s pretty cool.
August 13th, 2009 3:46 am
Brad Hefta-Gaub says:
Sue, I’m glad to hear you like the tool.
It’s very interesting to hear that you had your BMI tested and they took several “caliper… pinch (tests)”… That sounds like they did a Body Fat test. Now, the fact that they also recorded your height and weight would suggest that they might have calculated your BMI and your body fat. Did they give you one number or two?
I know a lot of people talk about BMI as if it is the same as Body Fat… I know I’ve done that in the past too. But really, they are different numbers. BMI is a good tool to help tell you about your fitness/wellness, and it’s a good metric to indicate potential “body fat related” health issues. But it’s not an exact correlation to Body Fat. Now… there are other tools that will “estimate” body fat based on BMI and gender and age… and still other tools that estimate body fat based on tape-measure measurements of your waist, hip, thighs, arms, etc… and still other techniques for estimating body fat by use of calipers, and dunk tanks, and (I’m not joking) air compression chambers like Mork from Ork went through… But the most important takeaway from all of this is that we should all pay attention to our overall health and our nutrition and our exercise… that’s the best way to stay fit and live a long healthy life!