Mathlean · Online math workspace
Double factorial calculator
Multiply every other positive integer down to 1 or 2.
How to use this calculator
Replace 9 in the double-factorial expression; 9!! is 945.
Tap the math below to edit it and see the result. Once a result is ready, press Tab on a hardware keyboard to accept it. Outside a matrix, Return completes the line; the phone math keyboard has the same Return action.
Try it here
Interactive tools require JavaScript. You can read the guide and example without it.
What to know
Two factorial signs mean double factorial here, not taking a factorial twice.
Mathlean combines visual mathematical input with computation and canvas tools. Results depend on the expression and available engine support; check important answers independently.