There are different ways to write a ratio.
For example, 4:12
1. As a fraction.
2. As a decimal.
To turn a fraction into a decimal, divide the numerator by it's denominator.
4/12 = 0.33
3. As a percent.
To turn a decimal into a percent, move the decimal point two places to the right.
0.33 = 33%
PS: you can also simplify the ratio.
Ex: 4:12 = 1:3
( I divided both number by 4. )