The Excel FACTDOUBLE function returns the double factorial of a number — the product of every other integer down to 1 or 2, skipping every second value.
Syntax
| Argument | Description | |
|---|---|---|
number | Required | A non-negative number. Non-integers are truncated first; negatives return a #NUM! error. |
How to use it
The double factorial multiplies every second integer rather than every integer. For odd numbers it steps down through the odds; for even numbers it steps down through the evens.
Double factorials turn up in combinatorics and certain statistical and physics formulas (for example counting pairings). Note this is different from the ordinary factorial, where FACT multiplies every integer.
Odd vs even: FACTDOUBLE(7) = 7×5×3×1 (odds) while FACTDOUBLE(8) = 8×6×4×2 (evens). The parity of the argument decides which chain you walk down.
Try it: interactive demo
Pick a FACTDOUBLE example to see the formula and its result.
Practice workbook
Frequently asked questions
What is a double factorial?
=FACTDOUBLE(7) = 7×5×3×1 = 105.How is FACTDOUBLE different from FACT?
Does the answer depend on whether the number is odd or even?
=FACTDOUBLE(8) = 8×6×4×2 = 384.What about negative or decimal inputs?
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