Even or odd

A whole number is even when it divides by two with nothing left over, and the last digit is all you need to look at: 0, 2, 4, 6 or 8 means even, and 1, 3, 5, 7 or 9 means odd. Every other digit in the number is a multiple of ten, and every multiple of ten is even, so none of them can change the answer.

Zero is even. It divides by two exactly, and it sits between two odd numbers. This is the one that gets argued about.

Negatives follow the same rule. −4 is even and −3 is odd. The minus sign does not affect whether something divides by two.

The same test works on a whole number written out in words as a cardinal (one, twenty-one, one million and five), though not on an ordinal such as twenty-first or third. A cardinal is odd exactly when its last word, or the part after the hyphen if the last word has one, is one, three, five, seven, nine, eleven, thirteen, fifteen, seventeen or nineteen.

Every other last word means even: two, four, six, eight, ten, twelve, fourteen, sixteen and eighteen; the tens from twenty to ninety; hundred, thousand and every larger scale word; and zero. So one million and five is odd, and nine hundred and ninety-eight thousand is even. American English, which drops the “and”, gives the same answer, because the “and” never comes last.