Is 28 a prime number?
28 is not a prime number. Here is a way to check it.
The answer
No. 28 is not a prime number: 2 × 14 = 28.
How to see that 28 is not prime
28 is even, and every even number can be divided by 2, so 2 is a factor as well as 1 and 28.
28 has 6 factors (1, 2, 4, 7, 14 and 28), which make 3 factor pairs.
Split 28 into prime factors
Split off the smallest prime each time, and keep going until every part is prime:
- 28 = 2 × 14
- 28 = 2 × 2 × 7
With repeated primes written as powers, that is 2² × 7. A small raised number tells you how many times to use that factor: for example, 2³ means 2 × 2 × 2.
28 in the times tables
28 turns up in 2 different times tables, the 4 and 7: 4 × 7 and 7 × 4.
Quick checks that catch 28
- The last digit, 8, is even, so 2 goes in.
- Half of it is 14, which is still even, so 4 goes in.
28 at a glance
Optional extra number facts:
- In words
- twenty-eight
- In Roman numerals
- XXVIII
- Odd or even
- Even: it ends in 8
- Triangular number
- 1 + 2 + … + 7 = 28, a triangle of counters with 7 on the bottom row
- Its factors add up to
- 56, or 28 without 28 itself. 28 is perfect: Perfect means this sum equals the number.
The numbers either side of 28
- 27 is not prime. Its digits add up to 2 + 7 = 9, which is a multiple of 3. That means 27 is a multiple of 3 too, so 3 is a factor. Check 27 in full
- 29 is prime: 2, 3 and 5 do not go into it, and 7 × 7 is already past 29. Check 29 in full
Turn the digits round
28 backwards is 82. 82 is not prime. 82 is even, and every even number can be divided by 2, so 2 is a factor as well as 1 and 82. Check 82 in full
Related pages
More numbers
Questions parents ask
Which prime numbers are either side of 28?
23 below it and 29 above it, a gap of 6.
What is 28 squared?
28 × 28 = 784.
What is the smallest number with as many factors as 28?
12. It has 6 factors, the same as 28.
Continue your number practice
Explore the diagnostic for another set of practice questions.