How to use the LCM Calculator
- Type your numbers separated by commas or spaces.
- Read the LCM, the GCF of the same numbers and the prime factors.
How it works
The least common multiple is the smallest positive number that every number in your list divides into evenly. One way to find it is prime factorization: write each number as a product of primes and multiply together the highest power of every prime that appears.
Another way uses the greatest common factor: for two numbers, LCM(a, b) = a × b ÷ GCF(a, b). For more numbers the calculator repeats this one pair at a time. The LCM is what you need to find a common denominator when adding fractions.
Formula
LCM(a, b) = a × b ÷ GCF(a, b) LCM(a, b, c) = LCM(LCM(a, b), c)
Example
12 = 2² × 3, 18 = 2 × 3² and 30 = 2 × 3 × 5. Taking the highest power of each prime gives 2² × 3² × 5 = 180.
Adding fractions with the LCM
To add 5/12 and 7/18, find the LCM of the denominators: LCM(12, 18) = 36. Rewrite each fraction over 36: 5/12 = 15/36 and 7/18 = 14/36. Then add the tops: 15/36 + 14/36 = 29/36. Using the least common multiple keeps the numbers small, so there is less simplifying to do at the end.
Listing multiples
For small numbers you can list multiples until one appears in every list. Multiples of 6 are 6, 12, 18, 24, 30; multiples of 8 are 8, 16, 24. The first shared value, 24, is the LCM. For larger numbers or longer lists, prime factorization or the GCF method is much faster.
Frequently asked questions
What is the LCM of 4 and 6?
12. Multiples of 4 are 4, 8, 12… and multiples of 6 are 6, 12…, so 12 is the first one they share.
What is the difference between LCM and LCD?
The least common denominator is the LCM of the denominators of a set of fractions. To add 1/4 and 1/6, use 12 as the denominator.
Can the LCM be smaller than the numbers?
No. The LCM is always at least as large as the biggest number in the list.
Where is the LCM used in real life?
Repeating schedules. If one bus comes every 12 minutes and another every 18, they arrive together every 36 minutes.
Last reviewed October 20, 2026. Spotted an error? Tell us.