Here are a few common unit conversion formulas, modified to simplify calculations in your head. For example, I replace factors such as 2.54 to an whole number estimate, 10/4, equaling 2.5.
1 inch = 2.54 cm
inch to cm: cm = (10*in) / 4
cm to inch: in = (4*cm) / 10
Fahrenheit = Celsius * (9/5) + 32
C to F: F = ((2*C)*0.9) + 32 = ((2*C) - 10%) + 32
1. multiply C by 2
2. subtract 10%
3. add 32
F to C: C = ((F-32) / 2) * 1.1 = ((F-32) / 2) + 10%
1. subtract 32 from F
2. divide result by 2
3. add 10%
4. (for better result) add 1%
