What is the GCF of 28 and 100?

The greatest common factor of 28 and 100 is 4.

What is the LCM of 28 and 100?

The least common multiple of 28 and 100 is 700.

Are 28 and 100 coprime?

No. They share the common factors 2 and 4, and their GCF is 4.

GCF and LCM of 28 and 100

The GCF of 28 and 100 is 4. The LCM of 28 and 100 is 700.

GCF (greatest common factor, also called GCD or HCF) of 28 and 100 = 4
LCM (least common multiple) of 28 and 100 = 700

Method 1: list the factors (GCF)

Factors of 28: 1, 2, 4, 7, 14, 28.
Factors of 100: 1, 2, 4, 5, 10, 20, 25, 50, 100.
Common factors: 1, 2 and 4. The greatest of these is 4.

Method 2: prime factorization

28 = 2 × 2 × 7
100 = 2 × 2 × 5 × 5

GCF: multiply the prime factors both numbers share: 2 × 2 = 4.

LCM: multiply every prime the maximum number of times it appears in either number: 2 × 2 × 5 × 5 × 7 = 700.

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.

100 = 3 × 28 + 16
28 = 1 × 16 + 12
16 = 1 × 12 + 4
12 = 3 × 4 + 0

The last non-zero remainder is 4, so GCF(28, 100) = 4.

LCM by listing multiples

Multiples of 28: 28, 56, 84, 112, 140, 168, 196, 224, 252, ...
Multiples of 100: 100, 200, 300, 400, 500, 600, 700, 800, ...
The first number in both lists is 700.

Check: GCF × LCM = product

4 × 700 = 2,800 = 28 × 100. This always holds for two numbers, so you can find the LCM as (28 × 100) ÷ 4 = 700.

Simplifying the fraction 28/100

Divide the top and bottom by the GCF 4: 28/100 = 7/25.

GCF and LCM of similar pairs

Numbers	GCF	LCM
2 and 28	2	28
3 and 28	1	84
4 and 28	4	28
5 and 28	1	140
6 and 28	2	84
7 and 28	7	28
2 and 100	2	100
3 and 100	1	300
4 and 100	4	100
5 and 100	5	100
6 and 100	2	300
7 and 100	1	700

See also: factors of 28 · factors of 100.

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