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Double Factorial Calculator

Work out double factorial instantly with clear inputs, formula shown and shareable results.

About Double Factorial Calculator

The double factorial n!! multiplies n by every second number down to 1 or 2, so it uses only the odd numbers when n is odd and only the even numbers when n is even. It satisfies n! = n!! × (n−1)!!.

Formula

Double factorial

n!! = n(n−2)(n−4)…, ending at 1 or 2, with 0!! = 1

Worked example

With the default values below, Double Factorial Calculator returns n!! of 945. Change any input above and every figure here is recalculated in your browser.

Inputs used

Value naccepts 0–300
9
Compare with the double factorial ofaccepts 0–300
10

Results

n!!
945
Comparison double factorial
3,840
Which numbers are multiplied
odd, product of odd numbers
Ordinary n! for reference
362,880

Step by step

  1. 1Values used
  2. 2Double factorial
  3. 3n!!= 945
  4. 4Comparison double factorial= 3,840
  5. 5Which numbers are multiplied= odd, product of odd numbers
  6. 6Ordinary n! for reference= 362,880

Assumptions and limits

  • The result is derived only from the 2 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
  • Figures are rounded for display to whole numbers, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

What is 9!!?

9 × 7 × 5 × 3 × 1 = 945.

Is n!! the same as (n!)!?

No, and the difference is enormous. 9!! = 945, whereas (9!)! is astronomically large.

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