Calculators
Double Factorial Calculator
Calculate exact double factorials from 0 to 1000 privately in your browser, with product notation and digit counts.
Calculate the product of every other integer.
The double factorial n!! multiplies positive integers of the same parity as n, starting at n and decreasing by two. For example, 10!! = 10 × 8 × 6 × 4 × 2 = 3840, while 7!! = 7 × 5 × 3 × 1 = 105. By convention, 0!! = 1 and 1!! = 1. This is different from applying factorial twice: 3!! = 3, whereas (3!)! = 720. Enter a whole number from 0 to 1000 without signs, separators, decimal points or exponent notation. Leading zeros within the four-digit input limit and surrounding whitespace are accepted. Exact browser BigInt arithmetic preserves every digit; the result includes a digit count and abbreviated product notation for inputs above 20. Negative and non-integer extensions are outside this calculator’s scope. All calculations stay in your browser.
Frequently Asked Questions
Everything you need to know about this tool, how it works, and privacy.
What is a double factorial?
The product n × (n − 2) × (n − 4) and so on down to 1 for odd n or 2 for even n. For example, 6!! = 6 × 4 × 2 = 48.
Is n!! the same as (n!)!?
No. Double factorial skips every other integer. For example, 3!! = 3, while (3!)! = 6! = 720.
What are 0!! and 1!!?
Both equal 1. The value 0!! is defined as the empty product, which is 1.
Which inputs are supported?
Enter an integer from 0 through 1000 using one to four digits. Leading zeros and surrounding whitespace are accepted. Negative numbers, decimals, separators and scientific notation are rejected. BigInt keeps large results exact, and processing stays in your browser.