GCF and LCM of 21 and 28
The greatest common factor and least common multiple of 21 and 28.
GCF of 21 and 28
7
LCM of 21 and 28
84
How it works
Break each number into its prime factors:
- 21 = 3 × 7
- 28 = 2² × 7
The GCF (7) multiplies the primes both share, at the lowest power each appears. The LCM (84) multiplies every prime that appears in either number, at the highest power — which is also 21 × 28 ÷ 7 = 84.
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Frequently asked questions
What is the GCF of 21 and 28?+
The greatest common factor (GCF) of 21 and 28 is 7 — the largest number that divides both 21 and 28 with no remainder.
What is the LCM of 21 and 28?+
The least common multiple (LCM) of 21 and 28 is 84 — the smallest number that both 21 and 28 divide into. It equals 21 × 28 ÷ GCF = 588 ÷ 7 = 84.