The formula for what percentage of values are within n deviations from the mean in a normal distribution is:

erf(n/2^.5)

(That’s really n over the square root of two, but I’m not sure how to code it here.)

erf is the error function. If you use Wolfram Alpha’s math module, you can just type in erf (1/2^.5), for instance, to get 0.682689492137, which is the percentage of values within one standard deviation on the mean. Plugging in 15, we get:

0.9999999999999999999999999999999999999999999999999926580676…

So that’s the percentage of values within 15 standard deviations of the mean. Subtract that from one (or use the erfc function for the compliment), and you get:

7.341932398625501771572179310669486972832503256080314… x 10^-51

outside the confidence interval.

Out of curiosity, are you calculating that you have a value 15 standard deviations from the norm?