GCF and LCM of 18 and 25

The GCF of 18 and 25 is 1. The LCM of 18 and 25 is 450.

• GCF (greatest common factor, also called GCD or HCF) of 18 and 25 = 1
• LCM (least common multiple) of 18 and 25 = 450

18 and 25 share no common factor except 1, so they are coprime (relatively prime). When two numbers are coprime their LCM is simply their product: 18 × 25 = 450.

Method 1: list the factors (GCF)

Factors of 18: 1, 2, 3, 6, 9, 18.
Factors of 25: 1, 5, 25.
Common factors: 1. The greatest of these is 1.

Method 2: prime factorization

18 = 2 × 3 × 3
25 = 5 × 5

GCF: multiply the prime factors both numbers share: there are none, so the GCF is 1.

LCM: multiply every prime the maximum number of times it appears in either number: 2 × 3 × 3 × 5 × 5 = 450.

Method 3: Euclidean algorithm (GCF)

Divide the larger number by the smaller and repeat with the remainder until it is 0. The last non-zero remainder is the GCF.

1. 25 = 1 × 18 + 7
2. 18 = 2 × 7 + 4
3. 7 = 1 × 4 + 3
4. 4 = 1 × 3 + 1
5. 3 = 3 × 1 + 0

The last non-zero remainder is 1, so GCF(18, 25) = 1.

LCM by listing multiples

Multiples of 18: 18, 36, 54, 72, 90, 108, 126, 144, 162, ...
Multiples of 25: 25, 50, 75, 100, 125, 150, 175, 200, 225, ...
The first number in both lists is 450.

Check: GCF × LCM = product

1 × 450 = 450 = 18 × 25. This always holds for two numbers, so you can find the LCM as (18 × 25) ÷ 1 = 450.

Simplifying the fraction 18/25

18/25 is already in lowest terms because the GCF of 18 and 25 is 1.

GCF and LCM of similar pairs

See also: factors of 18 · factors of 25.

Different numbers? Use the GCF and LCM calculator (any amount of numbers) or browse all GCF and LCM pairs.