Unit conversion without a calculator

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.

For temperature measurements, this is head-doable math.

C to F
Double it
Take away 10%
Add 32.

So 20c is 20x2=40. 10% is 4, 40-4=36+32=68F.

F to C
Take away 32
Add 10%
Half it.

68F - 32 = 36. 10% of that is about 4, so 36+4 = 40. Half that is 20C.