GCF and LCM of 18 and 20

The GCF of 18 and 20 is 2. The LCM of 18 and 20 is 180.

Method 1: list the factors (GCF)

Factors of 18: 1, 2, 3, 6, 9, 18.
Factors of 20: 1, 2, 4, 5, 10, 20.
Common factors: 1 and 2. The greatest of these is 2.

Method 2: prime factorization

18 = 2 × 3 × 3
20 = 2 × 2 × 5

GCF: multiply the prime factors both numbers share: 2 = 2.

LCM: multiply every prime the maximum number of times it appears in either number: 2 × 2 × 3 × 3 × 5 = 180.

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. 20 = 1 × 18 + 2
  2. 18 = 9 × 2 + 0

The last non-zero remainder is 2, so GCF(18, 20) = 2.

LCM by listing multiples

Multiples of 18: 18, 36, 54, 72, 90, 108, 126, 144, 162, ...
Multiples of 20: 20, 40, 60, 80, 100, 120, 140, 160, 180, ...
The first number in both lists is 180.

Check: GCF × LCM = product

2 × 180 = 360 = 18 × 20. This always holds for two numbers, so you can find the LCM as (18 × 20) ÷ 2 = 180.

Simplifying the fraction 18/20

Divide the top and bottom by the GCF 2: 18/20 = 9/10.

GCF and LCM of similar pairs

NumbersGCFLCM
2 and 18218
3 and 18318
4 and 18236
5 and 18190
6 and 18618
7 and 181126
2 and 20220
3 and 20160
4 and 20420
5 and 20520
6 and 20260
7 and 201140

See also: factors of 18 · factors of 20 · 18/20 as a decimal.

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