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Converting metric to imperial (12in/1ft)^3

  1. Jul 17, 2014 #1
    Hello. I am new to engineering and to imperial units, and currently learning by doing some exercises. I'm stuck on the following conversion:

    • 0.04 g / min x m^3 -> lbm / hr x ft^3
      I figured it like this:
    • 0.04 g / min x m^3 x (60min/1hr) x (1m/35,314)^3 x (1 lbm / 454g) =
      1,49x10^-4 lbm / ft^3 x hr

    However, the author of the book gives the following solution:

    • 0.04 g / min x m^3 x (60min/1hr) x (12in/1ft)^3 x (1 lbm / 454g) =
      9.14 lbm / ft^3 x hr

    It really lost me, why (12in/1ft)^3 its supposed to cancel out m^3? He does the same on a similar exercise converting cm^2 with (12in/1ft)^2.

    Thanks for your time and help.
  2. jcsd
  3. Jul 17, 2014 #2


    User Avatar
    Science Advisor

    Looks like the book has got it wrong.
    Maybe there was a typo or OCR error where "in" became "m".

    What book? what author? have you checked the web for an errata ?
  4. Jul 18, 2014 #3
    Not yet, but i will check. I was also thinking than the book might be wrong because really made no sense.

    Thanks for your help :)
  5. Jul 19, 2014 #4
    Found new information:

    Since 12in = 1ft, you can get a one by dividing both sides of the equation by 1ft, giving 12in / 1ft= 1. Because the result is 1, and the units can cancel out it makes sense after all.
    Last edited: Jul 19, 2014
  6. Jul 20, 2014 #5
    You were right the first time, the book has it wrong (yes, it does happen sometimes).
    You can evaluate the equation directly using a program called calchemy. Here is it's "evaluation" of the equation:

    0.04 g / min m^3 ? lbm / hr ft^3
    (0.04 * gramm) / (minute * [meter^3]) ? poundm / (hour * [foot^3])
    = 0.00014982723 lbm / hr ft^3

    The program runs on iPhone, but there is a "web" version at:
    that you can use for free, just cut and paste the equation above to give it a try (first line)
    The second line shows the equation it "actually" evaluated (sort of "in longhand"), the third line is the answer.

    Another web instance used to "work" at
    but that instance was broken by the service provider because of "security concerns" and has not been fixed... Still, there is a lot of good information about the program on the site.

    The program carries units with the numbers, so you don't ever have to do any conversions "by hand". It takes a little getting used to, but once you get the hang of it you can calculate all kinds of things really easy. The syntax is "what you know, with units of measure" ? "units you want the result in"
    It will even "solve" by dimensional analysis, it can do this because it knows what answer (units) you are looking for!
  7. Jul 22, 2014 #6
    My, that looks really useful for learning. I'll give it a try!

    Thanks a lot: :)
  8. Jul 22, 2014 #7
    It is sort of working at http://www.calchemy.com
    At least the answers are right, but the UI is still kind of messed up, the answer should be printed at the bottom, not in another page...
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