Write the quantities in the same units
Put the quantities side by side with a colon, such as a:b
Simplify both numbers by dividing by the greatest common factor
Keep dividing both parts by the same number until no common factor remains
Check that the ratio is in lowest terms
If comparing more than two quantities, write them in order and simplify all parts together
To find a missing part, use equivalent ratios or cross-multiply
To compare ratios, convert them to fractions or decimals if needed
To scale a ratio up or down, multiply or divide all parts by the same number
To turn a ratio into a fraction, write the first part over the second part
To turn a fraction into a ratio, write the numerator and denominator with a colon
To find a part from a total, add the ratio parts first, then divide the total by that sum
To find the actual amounts, multiply each ratio part by the same scale factor