Integer factorization calculator

  1. Alpertron
  2. Web applications
  3. Integer factorization calculator

Type one numerical expression or loop per line. Example: x=3;x=n(x);c<=100;x‑1

This web application factors numbers or numeric expressions using two fast algorithms: the Elliptic Curve Method (ECM) and the Self-Initializing Quadratic Sieve (SIQS).

The program uses local storage to remember the progress of factorization, so you can complete the factorization of a large number in several sessions. Your computer will remember the state of the factorization, so you only have to reload this page.

Since all calculations are performed in your computer, you can disconnect it from the internet while the factorization is in progress. You can even start this application without an internet connection after the first run.

The source code is written in the C programming language and compiled to asm.js and WebAssembly, which are the languages used by web browsers. The latter is faster, but it is not supported in all web browsers. You can see the version while a number is being factored.

There is a list of videos related to this calculator available.

See factorization records for this application.

You can enter expressions that use the following operators, functions, and parentheses:

  • + for addition
  • - for subtraction
  • * for multiplication
  • / for integer division
  • % for modulus (remainder of the integer division)
  • ^ or ** for exponentiation (the exponent must be greater than or equal to zero).
  • <, ==, >; <=, >=, != for comparisons. The operators return zero for false and -1 for true.
  • Ans: retrieves the last answer.
  • AND, OR, XOR, NOT for binary logic. The operations are done in binary (base 2). Positive (negative) numbers are prepended with an infinite number of bits set to zero (one).
  • SHL or <<: When b ≥ 0, a SHL b shifts a left the number of bits specified by b. This is equivalent to a × 2b. Otherwise, a SHL b shifts a right the number of bits specified by −b. This is equivalent to floor(a / 2b). Example: 5 SHL 3 = 40.
  • SHR or >>: When b ≥ 0, a SHR b shifts a right the number of bits specified by b. This is equivalent to floor(a / 2b). Otherwise, a SHR b shifts a left the number of bits specified by −b. This is equivalent to a × 2b. Example: -19 SHR 2 = -5.
  • n!: factorial (n must be greater than or equal to zero). Example: 6! = 6 × 5 × 4 × 3 × 2 = 720.
  • n!! ... !: multiple factorial (n must be greater than or equal to zero). It is the product of n times nk times n2k ... (all numbers greater than zero) where k is the number of exclamation marks. Example: 7!! = 7 × 5 × 3 × 1 = 105.
  • p#: primorial (product of all primes less or equal than p). Example: 12# = 11 × 7 × 5 × 3 × 2 = 2310.
  • B(n): Previous probable prime before n. Example: B(24) = 23.
  • F(n): Fibonacci number Fn from the sequence 0, 1, 1, 2, 3, 5, 8, 13, 21, etc. where each element equals the sum of the previous two members of the sequence. Example: F(7) = 13.
  • L(n): Lucas number Ln = Fn-1 + Fn+1
  • N(n): Next probable prime after n. Example: N(24) = 29.
  • P(n): Unrestricted Partition Number (number of decompositions of n into sums of integers without regard to order). Example: P(4) = 5 because the number 4 can be partitioned in 5 different ways: 4 = 3+1 = 2+2 = 2+1+1 = 1+1+1+1.
  • Gcd(m,n, ...): Greatest common divisor of these integers. Example: GCD(12, 16) = 4.
  • Lcm(m,n, ...): Least common multiple of these integers. Example: LCM(12, 16, 24) = 48.
  • FloorDiv(m,n): integer part of the quotient of m divided by n. Examples: floordiv(10, 7) = 1 and floordiv(-10, 7) = -2.
  • Mod(m,n): value of m modulo the absolute value of n. Examples: Mod(10, 7) = 3 and Mod(-10, 7) = 4.
  • Modinv(m,n): inverse of m modulo n, only valid when m and n are coprime, meaning that they do not have common factors. Example: Modinv(3,7) = 5 because 3 × 5 ≡ 1 (mod 7)
  • Modpow(m,n,r): finds mn modulo r. Example: Modpow(3, 4, 7) = 4, because 34 ≡ 4 (mod 7).
  • Totient(n): finds the number of positive integers less than n which are relatively prime to n. Example: Totient(6) = 2 because 1 and 5 do not have common factors with 6.
  • Jacobi(m,n): obtains the Jacobi symbol of m and n. When the second argument is prime, the result is zero when m is multiple of n, it is one if there is a solution of x² ≡ m (mod n) and it is equal to −1 when the mentioned congruence has no solution.
  • Random(m,n): integer random number between m and n.
  • Abs(n): absolute value of n.
  • Sign(n): returns zero if n is zero, −1 if negative or 1 if positive.
  • IsPrime(n): returns zero if n is not probable prime, −1 if it is. Example: IsPrime(5) = −1.
  • Sqrt(n): Integer part of the square root of the argument.
  • Iroot(n,r): Integer r-root of the first argument. Example: Iroot(8, 3) = 2.
  • NumFact(n): number of distinct prime factors of n. Example: NumFact(28) = 2 because its prime factors are 2 and 7.
  • MinFact(n): minimum prime factor of n. Example: MinFact(28) = 2 because its prime factors are 2 and 7.
  • MaxFact(n): maximum prime factor of n. Example: MaxFact(28) = 7 because its prime factors are 2 and 7.
  • NumDivs(n): Number of positive divisors of n. Example: NumDivs(28) = 6 because the divisors of 28 are 1, 2, 4, 7, 14 and 28.
  • SumDivs(n): Sum of all positive divisors of n. Example: SumDivs(28) = 56 because 1 + 2 + 4 + 7 + 14 + 28 = 56.
  • NumDigits(n,r): Number of digits of n in base r. Example: NumDigits(13, 2) = 4 because 13 in binary (base 2) is expressed as 1101.
  • SumDigits(n,r): Sum of digits of n in base r. Example: SumDigits(213, 10) = 6 because the sum of the digits expressed in decimal is 2+1+3 = 6.
  • RevDigits(n,r): finds the value obtained by writing backwards the digits of n in base r. Example: RevDigits(213, 10) = 312.
  • ConcatFact(m,n): Concatenates the prime factors of n according to the mode expressed in m which follows this table:
ConcatFact function modes
ModeOrder of factorsRepeated factorsExample
0AscendingNoconcatfact(0,36) = 23
1DescendingNoconcatfact(1,36) = 32
2AscendingYesconcatfact(2,36) = 2233
3DescendingYesconcatfact(3,36) = 3322

You can use the prefix 0x for hexadecimal numbers, for example 0x38 is equal to 56.

The notation km (mod n) means that the remainder of the division of k by n equals the remainder of the division of m by n. The number n is called modulus.

This method computes points in elliptic curves, which are represented by formulas such as y² ≡ x³ + ax + b (mod n) where n is the number to factor.

In the following graphic, you can see the points (x, y) for which y² ≡ x³ + 4x + 7 (mod 29) holds. Since the computation uses modular arithmetic (in this case using the remainder of the division by 29), the points that belong to the elliptic curve cannot be represented as a continuous line. That would be the case if the operations were performed using real numbers.

Apart from the points shown above, we use another point named O, or the point at infinity.

Using complex formulas, we can define a sum of points. In this way a point (x3, y3) that belongs to the mentioned curve can be the sum of other points (x1, y1) and (x2, y2) that also belong to the curve.

We can add a point (x, y) to itself several times, obtaining a new point (x4, y4) that is a multiple of (x, y).

When the modulus is a prime number, and 4a³ + 27b² 0 (mod p), the points that belong to the elliptic curve (including the point O) form a mathematical structure called group. The group order is the number of points. In the graphic, we can see 31 points, so the group order is 32. Since O + O = O, if we multiply any point by a multiple of the group order, we obtain the point O.

Even though the value of the group order is difficult to find, it can be shown that it is near the prime number used as the modulus. By changing the curve, we obtain a different group, and its order also changes.

To factor a number n, we have to find a multiple of the group order corresponding to any of the prime factors of n.

For each elliptic curve to process we try to find the point at infinity starting from a random point (x, y) belonging to a random elliptic curve y² ≡ x³ + ax + b (mod n). Since it is very difficult to solve quadratic or cubic equations modulo a composite number, it is better to select random values for x, y, and a. Then we can easily compute by² − x³ − ax (mod n)

In the first step of the algorithm, we multiply points by powers of different prime numbers less than a bound named B1. By computing the greatest common divisor between the coordinate x of the computed point and the number to factor, we can obtain the prime factor of n, provided that all prime factors of the group order are less than B1.

Using the result of the first step, the second step obtains multiples of that point up to the upper bound B2. We then multiply the x-coordinates of all the points found in this step. Finally, we compute the greatest common divisor between the product and the number to factor. In this case, we can obtain the prime number we are searching for if all prime factors (except one) of the group order are less than B1, and the greatest prime factor of the group order is less than B2.

If the greatest common divisor equals 1, we must try a different curve by changing the initial point (x, y) and the parameter a, and computing the new parameter b from the formula.

The program uses many optimizations that are outside the scope of this help to explain here.

The execution time depends on the magnitude of the second-largest prime factor and on your computer's speed.

Optimal values of B1 and expected curves
DigitsValues of B1Expected curves
351 0000001800
403 0000005100
4511 00000010600
5043 00000019300
55110 00000049000
60260 000000124000
65850 000000210000
702900 000000340000

The program runs 25 curves with limit B1 = 2000, 300 curves with limit B1 = 50000, 1675 curves with limit B1 = 1000000 and finally it uses curves with limit B1 = 11000000 until all factors are found.

The ECM factoring algorithm can be easily parallelized in several machines. In order to do it, run the factorization in the first computer from curve 1, run it in the second computer from curve 10000, in the third computer from curve 20000, and so on. In order to change the curve number when a factorization is in progress, press the button named More, type this number on the input box located on the new window and press the New Curve button.

When one of the machines discovers a new factor, you should enter this factor in the other computers by typing it in the bottom-right input box and pressing Enter (or clicking the Factor button).

The notation km (mod n) means that the remainder of the division of k by n equals the remainder of the division of m by n. The number n is called modulus.

Let N be the number to be factored. This number must not be a perfect power. If somehow we find two integers X and Y such that X² ≡ Y² (mod N) and XY (mod N), then gcd(X+Y, N) will reveal a proper factor of N.

In order to find these values X and Y, the method finds relations which have the form t² ≡ u (mod N) where u is the product of small prime numbers. The set of these primes is the factor base. These relations will be found using sieving, which is outside the scope of this introduction.

The relations found are combined using multiplications. The left-hand side will always be a square because it is a product of squares, so the goal is to have a square at the right-hand side. A number is a square when all its prime factors appear an even number of times.

For example: let the number to factor be N = 1817 and we have found the following relations with factor base = {2, 7, 13}:

45² ≡ 24 × 70 × 131

123² ≡ 210 × 70 × 131

Both relations have non-square RHS because the exponent of 13 is not even. But multiplying them we get:

84² ≡ 214 × 13²

84² ≡ (27×13)²

Since 27×13 ≡ 1664 we get the factor gcd(84+1664, 1817) = 23.

Which relations have to multiplied to find a square in the RHS is a linear algebra problem and it is solved using matrices.

The main problem with this method is that it is harder to find relations when we increase the number to be factored, so we need a variation of this method.

The large prime SIQS uses large primes along with the factor base. The size of the largest prime depends on the number to be factored, but normally it is about 50 to 100 times larger than the greatest element of the factor base.

A partial relation is an identity: the LHS is a square and the RHS is a product of primes of the factor base times a large prime. If we get two partial relations that share the same large prime, we can merge them into a full relation. This enables us to find relations about twice as fast as the non large prime variation.

For example, we select N = 1817 again, and assume we found the following partial relations, where the number 67 is a large prime:

71² ≡ 3 × 7 × 67

116² ≡ 11 × 67

To merge these partial relations into a full relation, we multiply them and then divide by the square of the large prime:

(71 × 116 / 67)² ≡ 3 × 7 × 11

367² ≡ 3 × 7 × 11

The modular division requires an extended GCD computation.

When the number to be factorized is in the range 31 to 95 digits, after computing some curves in order to find small factors, the program switches to SIQS (if the checkbox located below the applet enables it), which is an algorithm that is much faster than ECM when the number has two large prime factors. Since this method needs a large amount of your computer's memory to store relations, if you restart the applet the factorization begins from scratch. In order to start factoring immediately using SIQS, you can enter 0 in the New Curve box.

Threshold for switching to SIQS

You can change settings for this application by pressing the Config button when a factorization is not in progress. A new window will pop up where you can select different settings:

  • Digits per group: In order to improve readability, big numbers are separated by spaces forming groups of a fixed number of digits. With this input box, you can determine the number of digits in a group.
  • Verbose mode: It shows more information about the factors found.
  • Pretty print: If this checkbox is set, the exponents are shown in superscripts and the multiplication sign is " × ". The application also shows the number of digits for numbers with more than 30 digits. If the checkbox is not set, the exponents are preceded by the exponentiation sign " ^ " and the multiplication is indicated by asterisks. Also, the number of digits is never displayed. This mode eases copying the results to other mathematical programs.
  • Hexadecimal output: If this checkbox is set, the numbers are shown in hexadecimal format instead of decimal, which is the common notation. To enter numbers in hexadecimal format, you will need to precede them by the string 0x. For instance, 0x38 = 56. The program shows hexadecimal numbers with monospaced font.
  • Keyboard: This enables the user to select between numeric or complete (alphanumeric) virtual keyboard. The virtual keyboard appears on the screen when the user selects an input box on touch screens.
  • Use Cunningham tables on server: When selected, if the number to be factored has the form ab ± 1, the application will attempt to retrieve the known factors from the Web server. In order to reduce the database, only factors with at least 14 digits are included, so the application will find the small factors. These factors come from Jonathan Crombie list and it includes 2674850 factors of Cunningham numbers.

The configuration is saved in your device, so when you start again the calculator, all settings remain the same.

Write an expression per line, then press the appropriate button. The output will be placed in the lower pane.

Blank lines or comment lines (which start with a numeral '#' character) will be replicated on the lower pane.

Expression loop: with the following syntax you can factor or determine primality of several numbers typing only one line. You have to type four or five expressions separated by semicolons:

  • First expression: It must start with the string 'x=' and it sets the first value of x.
  • Second expression: It must start with the string 'x=' and it sets the next value of x.
  • Third expression: It holds the end expression condition. If it is equal to zero (meaning false) the loop finishes, otherwise the loop continues.
  • Fourth expression: It holds the expression to be factored.
  • Optional fifth expression: If this expression is different from zero (meaning true), the fourth expression is displayed or factored, and if is zero (meaning false), the fourth expression is ignored.

Except for the first expression, all other expressions must include the variable x and/or the counter c.

If the end expression is false after processing 1000 numbers, the Continue button will appear. Pressing this button will make the program to process the next 1000 numbers and so on.

Example 1: Find the factors of the first 100 numbers of the form odd prime minus one. The line to type is: x=3;x=n(x);c<=100;x-1.

Example 2: Find the Smith numbers less than 10000. A Smith number, according to Wikipedia, is a composite number for which, in a given base (in base 10 by default), the sum of its digits is equal to the sum of the digits in its prime factorization. The line to type is: x=1;x=x+1;x<10000;x;sumdigits(x, 10)==sumdigits(concatfact(2,x),10) and not isprime(x).

The fourth expression can be changed to a format string and several expressions. The format string indicates what is displayed. Inside this string you can specify conversion clauses that start by a percent sign and they are case insensitive:

  • %D: show expression as a decimal number.
  • %X: show expression as a hexadecimal number.
  • %L: show yes if the expression is not zero and no if it is zero.
  • %FD: show factorization using decimal numbers.
  • %FX: show factorization using hexadecimal numbers.

The expressions are written after the string, separated by colons. There must be a colon also between the string and the first expression.

To print a percent sign, you have to write two percent signs, and a quote is represented by a percent sign followed by a quote.

Example 3: For each number between -100 and 100, show whether the number is prime and then show the factorization both in decimal and hexadecimal.

The line to write is: x=-100; x=x+1; x<=100; "%d is prime: %l, %Fd, %Fx":x:isprime(x):x:x

Example 4: Get 33331 digits of π using Gauss-Legendre algorithm.


In this script written by Samuel Eraut: b = lowest 33333 digits of x, t = next higher 33333 digits, a = all higher digits of x. Variable names a, b and t conform to the current example shown on Gauss-Legendre algorithm on Wikipedia.

Example 5: Get 33331 digits of π using Bailey–Borwein–Plouffe (BBP) formula (script written by Samuel Eraut).


You can download the source of the current program and the old factorization applet from GitHub. Notice that the source code is in C language and you need the Emscripten environment in order to generate JavaScript.

Written by Dario Alpern. Last updated 9 June 2024.