The ability to convert approximately back and forth between inferial and metric is a worth-while talent to cultivate. Here are a couple of tricks that allow you to do it mentally. Handy when you lack a calculator or want to do a mental check that you keyed the correct numbers into one.
CONVERTING FRACTIONAL MEASUREMENTS TO METRIC
Keep doubling numerator and denominator until denominator is 256
5/16 = 5*16 / 16*16 = 80/256
Now we need to multiply by 25.4 (mm/inch) to convert to millimeters. But 25.4 = 254/10, so
80/256 * 254/10 = 80/10 * 254/256
254 is close enough to 256 to make the approximation that 254/256 ~= 1 and we have
5/16" ~= 80/10 = 8 mm [exact value = 7.94 mm]
So the rule is DOUBLE NUMERATOR AND DENOMINATOR UNTIL LATTER IS 256, THEN ANSWER IS NUMERATOR WITH DECIMAL SHIFTED ONE PLACE TO LEFT.
CONVERTING METRIC MEASUREMENTS TO INFERIAL INCHES
We need to divide by 25.4
8 mm / 25.4
but 25.4 can be approximated as 25 and written as 100/4. Then...
8 / 25.4 ~= 8 * 4 /100 = 32/100 = 0.32" [exact value = 0.315]
So the rule is MULTIPLY BY 4 AND MOVE DECIMAL TWO PLACES TO LEFT.
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