Multiples of N Between A and B

A common counting problem is to determine the number of multiples of an integer n in a given range from a to b (inclusive) where a < b. There may be other conditions on the problem, such as counting the numbers that are multiples of both n AND m, multiples of EITHER n OR m, or multiples of n but NOT of m.

For small ranges it is easy to list them by hand, but for larger ranges it is more efficient to use a formula. Each condition scenario is analyzed below. You can also use the convenient calculator on the left to quickly count multiples and generate them as a list.

Case I: Multiples of n

To count the number of multiples of n between a and b, you need to use the floor and ceiling functions, ⌊⌋ and ⌈⌉ respectively. The number of multiples is given by the expression

⌊b/n⌋ - ⌈a/n⌉ + 1

For example, the multiples of 4 between 0 and 24 are {0, 4, 8, 12, 16, 20, 24}. To count them more quickly without having to list them, you compute

⌊24/4⌋ - ⌈0/4⌉ + 1
= 6 - 0 + 1
= 7

Case II: Multiples of both n AND m

To count integers that are multiples of both n and m, it suffices to count numbers that are multiples of the least common multiple (LCM) of n and m. The formula is

⌊b/y⌋ - ⌈a/y⌉ + 1

where y = LCM(n, m). For example, suppose you want to count numbers that are multiples of both 4 and 6 between 100 and 200. Since the LCM of 4 and 6 is 12, you compute

⌊200/12⌋ - ⌈100/12⌉ + 1
= 16 - 9 + 1
= 8

The exact set is {108, 120, 132, 144, 156, 168, 180, 192}.

Case III: Multiples of Either n OR m

To determine the number of integers that are multiples of either n or m (or both), you calculate the multiples of just n, then add the multiples of just m, and subtract the multiples of LCM(n, m). If you don't subtract this quantity, the multiples of LCM(n, m) will be counted twice. The full expression is

⌊b/n⌋ - ⌈a/n⌉ + ⌊b/m⌋ - ⌈a/m⌉ - ⌊b/y⌋ + ⌈a/y⌉ + 1

where y = LCM(n, m). For instance, suppose you need to find the number of integers between 20 and 40 that are multiples of either 4 or 6. Since LCM(4, 6) = 12, the answer is

⌊40/4⌋ - ⌈20/4⌉ + ⌊40/6⌋ - ⌈20/6⌉ - ⌊40/12⌋ + ⌈20/12⌉ + 1
= 10 - 5 + 6 - 4 - 3 + 2 + 1
= 7

The exact set is {20, 24, 28, 30, 32, 36, 40}.

Case IV: Multiples of n but NOT of m

The number of multiples of n that are NOT multiples of m is given by the expression

⌊b/n⌋ - ⌈a/n⌉ - ⌊b/y⌋ + ⌈a/y⌉

where y = LCM(n, m). For example how many multiples of 4 between are there between 0 and 60 that are not multiples of 6? To calculate the answer, evaluate the expression

⌊60/4⌋ - ⌈0/4⌉ - ⌊60/12⌋ + ⌈0/12⌉
= 15 - 0 - 5 + 0
= 10.

The full list is {4, 8, 16, 20, 28, 32, 40, 44, 52, 56}.

© Had2Know 2010