Ratios and Proportions
The relative values between quantities  ratios and proportions.
Ratio
The ratio of one quantity to an other is a fraction  and the number of times one quantity is contained in another quantity of the same kind.
The ratio between two quantities is the quotient obtained by dividing the first quantity by the second.
Example  Ratio
The ratio between 3 and 6 can be expressed as
3 / 6 = 1 / 2 = 0.5
Ratio can also be indicated by the sign ":"  like 3 : 6 or 1 : 2 .
Reciprocal or Inverse Ratio
The reciprocal or inverse ratio is the opposite of the original ratio.
Example the Inverse Ratio
The inverse ratio between 6 and 3 can be expressed as
6 : 3 = 2 : 1 = 2
Proportion
Proportion is the equality of ratios.
Example  Proportion
1 : 2 = 3 : 6
or
1 : 2 :: 3 : 6
Related Topics

Basics
Basic engineering data. SIsystem, unit converters, physical constants, drawing scales and more. 
Mathematics
Mathematical rules and laws  numbers, areas, volumes, exponents, trigonometric functions and more.
Related Documents

Energy Balance and Energy Ratio
Energy balance and energy payback ratio. 
Fractions
Law of fractions 
Movement Ratio
Machines and movement ratio (velocity ratio). 
Percentage
Working with relative values. 
Poisson's Ratio
When a material is stretched in one direction it tends to get thinner in the other two directions. 
Poisson's Ratios Metals
Some metals and their Poisson's Ratios.