Definition of rule of three in US English:

rule of three


  • A method of finding a number in the same ratio to a given number as exists between two other given numbers.

    • ‘The rule of three is the familiar way of solving problems of this type: if a man earns 50 dinaras in 8 days how much will he earn in 12 days?’
    • ‘If the fruit diminish as the requisition increases, or augment as that decreases, they, who are skilled in accounts, consider the rule of three to be inverted.’
    • ‘The mathematics involves a study of proportion and percentages and introduces the rule of three for solving proportion problems.’


rule of three

/ˌro͞ol əv ˈTHrē/