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How can I convert US gravity points? If, for example, a pound per gallon of sugar gives 46 gravity points (1.046 OG), how can I calcualate the corresponding value when 1kg of sugar is added to 10l of whater?

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up vote 5 down vote accepted

You do it by multiplying the ratios of the imperial/English units to metric units:

  • 1 kg is 2.204 lb
  • 10l is 2.641 gal

To convert from pounds per gallon to kilos/decaliter you multiply by 2.204/2.641 = 0.834

The answer is then

  • 46*0.834 = 38.4 points per kilo/10 l.

Edit: I always find these things confusing with "cascaded" ratios - points per pound per gallon, what is it in points per kilo per decaliter - do I multiply or divide? By kilos-to-pounds or pounds-to-kilos etc..?

The way I try to see through the muddle is that I take each unit at a time. Taking the weight first - imagine changing the unit from pounds to kilos, by just substituting the units and keeping the same quantity we would be using over twice as much substance (actually 2.204 times as much), so we expect the final points value to be 2.204 times bigger. Hence the initial multiplication. And then the change in liquid volume - from gallons to decaliters. Here, we are diluting in much more water, so we expect the value to be smaller - smaller by the ratio of the dilution - hence the division by gal/decaliter.

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