BMI to body fat %

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2

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  • tomatoey
    tomatoey Posts: 5,446 Member
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    rabbitjb wrote: »
    Merkavar wrote: »
    so I just came across this formula which seems to line up with the % from my scale for body fat %.

    Just wondering if it's accurate for others.

    Anyone mind calculating this and posting the result, along with what you think your body fat % actually is.

    Formula is
    1.2xbmi + 0.23xage -5.4 for female and -16.2 for males.

    See my scales and this formula are 1.4% different.

    Just curious as to the accuracy of this formula.

    I might post a image of me working out as a reward :P

    How does that take account of musculature which is a pre-requisite for understanding BF isn't it?

    It doesn't, other than some generalized sex-based assumptions built into the equation (the -5.4 for women and the -16.2 for dudes).

    Where did you get this formula, @Mercavar?
  • Sued0nim
    Sued0nim Posts: 17,456 Member
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    dondotwinks wrote: »
    I got 38.8% my scales this morning gave me 39.3%. Pretty damn close

    So one bad reading matches another bad reading ?
  • tomatoey
    tomatoey Posts: 5,446 Member
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    rabbitjb wrote: »
    Without meaning to be smug that gives me a BF reading of 34.2

    That's out by around 10-11%

    It is not assuming that people are doing progressive weight routines and eating eg .8g/lb protein. It's assuming people are going to the mall and whatnot.
  • tomatoey
    tomatoey Posts: 5,446 Member
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    Should work (on average, like the bmi) for most people in the middle of the bell curve
  • Sued0nim
    Sued0nim Posts: 17,456 Member
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    ahoy_m8 wrote: »
    Gave me 28%. I think I'm <20%. Your formula seems to punish age.

    Agrees, and muscles
    vismal wrote: »
    Gave me 22.6, not even close.

    *chortles*
    Hornsby wrote: »
    Gave me 21.59. Not even close.

    *laughs*

    Puts formula in the "that's ridiculous" pile
  • Whitezombiegirl
    Whitezombiegirl Posts: 1,042 Member
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    Formula gives me 27%. My scale gave me 15% every morning this week.
  • Sued0nim
    Sued0nim Posts: 17,456 Member
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    HazelTrisEverdeen wrote: »
    This gives me 20.61%. My scale gives me around 17%. I think I should go have my body fat % checked at the doctor's next time just so I actually know.

    Dexa scan is your best bet
  • Morgaen73
    Morgaen73 Posts: 2,817 Member
    edited August 2015
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    It gives me 40% which is what my BF % was when I weighet 35kg heavier. My current BF% (measured with a calipur) is 22%.
  • Merkavar
    Merkavar Posts: 3,082 Member
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    tomatoey wrote: »
    rabbitjb wrote: »
    Merkavar wrote: »
    so I just came across this formula which seems to line up with the % from my scale for body fat %.

    Just wondering if it's accurate for others.

    Anyone mind calculating this and posting the result, along with what you think your body fat % actually is.

    Formula is
    1.2xbmi + 0.23xage -5.4 for female and -16.2 for males.

    See my scales and this formula are 1.4% different.

    Just curious as to the accuracy of this formula.

    I might post a image of me working out as a reward :P

    How does that take account of musculature which is a pre-requisite for understanding BF isn't it?

    It doesn't, other than some generalized sex-based assumptions built into the equation (the -5.4 for women and the -16.2 for dudes).

    Where did you get this formula, @Mercavar?

    A few places, I just googled something like Body fat from BMI. Im not claiming its awesome or for everyone etc. Just it seemed accurate for me and another person so I was curious about others. Its based on the BMI so its going to be inaccurate for some just like the BMI.

    http://halls.md/body-fat-percentage-formula/
    http://www.livestrong.com/article/173592-how-to-convert-bmi-to-body-fat-percentage/
    http://www.caloriecount.com/forums/weight-loss/way-calculate-body-fat-percentage-bmi
  • yarwell
    yarwell Posts: 10,477 Member
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    These things are typically correlations between measured variables in a sample population. You end up with a cloud of dots and a line passing through the middle. Many dots are a way off the line. Typically the correlation coefficient is <0.7 which implies 30% of the variations are not taken care of by the equation.

    They serve a purpose if you only know your height, weight and age but are no substitute for decent measurement.
  • sijomial
    sijomial Posts: 19,811 Member
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    Doesn't seem to work too well for me - older but fit with more muscle than most old farts my age (55).

    Gives me 26%
    My "guesstimate" is around 15% but haven't tested my BF for a quite a while.
  • WBB55
    WBB55 Posts: 4,131 Member
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    Your formula 35.25%
    Cheap scale 31.8%
    Average of tapings 29.5%
  • zyxst
    zyxst Posts: 9,136 Member
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    Formula - 39.316 BF%
    Dexa scan done in Feb 2015 - 50.9 BF%

    Though I've lost 10# since the scan, pretty confident I didn't lose 11% in body fat.
  • MissAbbee
    MissAbbee Posts: 41 Member
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    The calculator gave me 31.19%, Body fat measured last night at one of those stand on scales at the gym came to 31.1%.. I'll take that.
  • WBB55
    WBB55 Posts: 4,131 Member
    edited August 2015
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    I'm thinking this formula is assuming that, something like, a female at a BMI of 18=18%BF and a female at a BMI of 30=40%BF and it's just applying a conversion formula to reach those assumed BF% at those BMIs. **numbers are pulled out of my head** and then adding a little cojigger to assume older people have a higher BF%
  • lemurcat12
    lemurcat12 Posts: 30,886 Member
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    The formula gives me 31.35%. Based on DEXA I'm around 25%. I'm another person who is older (45), and tries to stay fit, although I wouldn't have said I have that much muscle. I do have enough (according to DEXA) that I'd have too low bf at the bottom of the BMI range for my height, though. (I think I'll be happy at 20.5-21, am 22 now, so I'd also have said I'm a person the BMI range works great for.)
  • arb037
    arb037 Posts: 203 Member
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    The formula is largely dependent on BMI being accurate. Which for anybody with muscle it is not.
    Formula = 28.74%
    Actual 15-18%
  • rjmudlax13
    rjmudlax13 Posts: 900 Member
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    Well, if the formula includes BMI, it's fundamentally flawed from the start.
  • Need2Exerc1se
    Need2Exerc1se Posts: 13,575 Member
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    Whoa! No, that wasn't even close for me. Maybe because I'm old and have quite a bit of muscle, but it was over 10% high. And I've had my BF% measured by DEXA scan.
  • Hornsby
    Hornsby Posts: 10,322 Member
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    rjmudlax13 wrote: »
    Well, if the formula includes BMI, it's fundamentally flawed from the start.

    Yup.

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