Find all divisors of a number, count them (τ), sum them (σ), classify as perfect/abundant/deficient, visualize factor pairs, and explore presets with a comprehensive divisors table.
The **Divisor Calculator** finds every positive divisor of a given integer and reveals the rich number-theoretic properties hidden within. Enter any positive integer and instantly see its complete list of divisors, the total count τ(n), the sum σ(n), and whether the number is perfect, abundant, or deficient.
A **perfect number** equals the sum of its proper divisors (e.g., 6 = 1 + 2 + 3). An **abundant number** has proper divisors that sum to more than itself, while a **deficient number** has a proper divisor sum less than itself. This classification connects to some of the oldest open questions in mathematics — it is still unknown whether any odd perfect numbers exist.
The calculator also displays all factor pairs (a, b) where a × b = n, and highlights whether the number is a perfect square (which has an odd number of divisors). The prime factorization is shown alongside, since the divisor count formula τ(n) = (e₁+1)(e₂+1)⋯(eₖ+1) depends on the prime exponents. The sum-of-divisors function σ(n) has its own formula based on geometric series of each prime power.
A comprehensive reference table shows divisor properties for integers 1–100, letting you spot patterns like highly composite numbers (those with more divisors than any smaller number). Preset buttons load famous examples — perfect numbers like 28 and 496, abundant numbers like 12 and 18, and highly composite numbers like 120 and 360. Visual bars show the relative sizes of divisors, making factor structure intuitive at a glance.
Finding every divisor of a number by trial requires dividing by each candidate up to √n, collecting pairs, and then computing sums and counts. For multi-digit numbers this is slow and easy to miss a factor. This calculator lists all divisors instantly, shows the prime factorisation, computes τ(n) and σ(n), classifies the number (perfect, abundant, deficient), and displays factor pairs. Number-theory students check homework, teachers demonstrate divisor functions, and math enthusiasts explore special numbers like highly composite and perfect numbers.
If n = p₁^e₁ × p₂^e₂ × … × pₖ^eₖ, then τ(n) = (e₁+1)(e₂+1)…(eₖ+1) and σ(n) = Π [(pᵢ^(eᵢ+1) − 1) / (pᵢ − 1)].
Result: 28: Divisors are 1
For n = 28: Divisors are 1, 2, 4, 7, 14, 28. τ(28) = 6 divisors. σ(28) = 56. Proper sum = 28 = n, so 28 is a perfect number.
If the prime factorisation of n is p₁^e₁ × p₂^e₂ × … × pₖ^eₖ, the total number of positive divisors is τ(n) = (e₁+1)(e₂+1)…(eₖ+1). Each divisor is formed by choosing an exponent between 0 and eᵢ for each prime, and the choices are independent — a counting principle. For example, 360 = 2³ × 3² × 5¹, so τ(360) = 4 × 3 × 2 = 24 divisors. This formula lets you predict divisor counts without listing them.
A positive integer is called *perfect* when the sum of its proper divisors equals itself, *abundant* when the sum exceeds it, and *deficient* when it falls short. The smallest examples are 6 (perfect: 1+2+3 = 6), 12 (abundant: 1+2+3+4+6 = 16 > 12), and 8 (deficient: 1+2+4 = 7 < 8). Euclid proved that 2^{p−1}(2^p − 1) is perfect whenever 2^p − 1 is prime (a Mersenne prime); Euler showed that every even perfect number has this form. Whether odd perfect numbers exist remains one of mathematics' oldest open questions.
A highly composite number (HCN) has more divisors than any smaller positive integer. The sequence starts 1, 2, 4, 6, 12, 24, 36, 48, 60, 120, 180, 240, 360, 720, … Engineers naturally gravitate toward HCNs: 12 inches in a foot, 24 hours in a day, 60 minutes in an hour, and 360 degrees in a circle are all highly composite, which is why — not coincidence — these numbers were chosen historically. They maximise the number of ways you can divide a whole evenly, making them ideal for partitioning, scheduling, and gear-ratio design.
A divisor (or factor) of n is a positive integer d such that n/d is also a positive integer. For example, the divisors of 12 are 1, 2, 3, 4, 6, 12.
The divisor count function τ(n), also written d(n), counts how many positive divisors n has. For 12, τ(12) = 6.
The divisor sum function σ(n) adds up all positive divisors of n. For 12, σ(12) = 1+2+3+4+6+12 = 28.
A number equal to the sum of its proper divisors (all divisors except itself). The first four are 6, 28, 496, 8128.
A number whose proper divisors sum to more than itself. The smallest is 12 (proper sum = 1+2+3+4+6 = 16 > 12).
A number with more divisors than any smaller positive integer. Examples: 1, 2, 4, 6, 12, 24, 36, 48, 60, 120, 180, 240, 360.
Nobody knows! It is one of the oldest unsolved problems in mathematics. No odd perfect number has ever been found, but none has been proven impossible either.