There is a technique called "unit cancellation" that may serve you well.
The technique is essentially to treat each unit as an orthogonal vector
(orthogonal to all other units), so one unit can't be mixed with any other
The only way to convert is to multiply by various forms of "1". "1" is a
fraction like (2.54 cm)/(inch) or (1 inch)/(2.54 cm), where the same thing
is in the numerator and denominator.
You can cancel (i.e. cross out) the same units appearing in the numerator
The rules free you from having to remember conversion factors. You can just
multiply by various forms of "1".
For example, if one has 1000 square inches and wishes to know how many
(1000 inch * inch) (1 foot / 12 inch) (1 foot / 12 inch) = 6.944 foot *
Notice that the "inch" x 2 in the numerator cancel with those in the
denominator. Because you have to "cancel" units (by crossing them out on
paper), it would be impossible to use "12" alone as the converstion factor,
as the units would come out to (foot * inch) rather than (foot * foot) if
you did this.
It is a system that helps to keep your head straight and prevent human
I hope there is a web page somewhere that describes this with illustrations.
It is hard to do in a text e-mail.
You can go surprisingly far with this technique if you forget conversion
factors and need to get them again. For example, suppose I've forgotten how
many kilometers per mile, but I do remember that there are 2.54 cm/inch ...
(1 mile)(5,280 foot/mile)(12 in/1 foot)(2.54cm/1 inch)(1m/100cm)(km/1000m) =
> -----Original Message-----
> From: Jay Blanchard [mailto:jblanchard@stripped]
> Sent: Thursday, April 20, 2006 10:46 AM
> To: Shaun; mysql@stripped
> Subject: RE: Working out Square Footage with Feet and Inches
> I have measurements of rooms stored in a table for each house in feet
> inches, does anyone know how can I work out the square footage?
> Convert to inches, multiply length * width, divide by 144 (one square
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