Conversion Rate and Statistical Significance Last month 1000 people visited my website and 40 of them purchased my product. I can say that I have a 4% conversion rate.
What about the confidence interval? Is it correct to say that I have a 95% probability that my conversion rate is between 2,79% and 5,21%?
I have developed this simple excel tool that does the calculations for me (given the number of visits and the number of conversions):

Do you think that the numbers are correct and it does what is supposed to do?
I'm not an expert in statistics and would be really happy to receive advice from you people.
Thank you :)
 A: Your numbers appear correct, but be careful about how you describe the conclusion:

Last month 1000 people visited my website and 40 of them purchased my product. I can say that I have a 4% conversion rate.

You can't make any claim about the true, population conversion rate based on this data. What you can say is that this sample (e.g. these 1000 people) converted at 4%. We then use statistics to try to make claims about the true population conversion rate, and our claims are always in the form of an estimate (e.g. range, probability, etc..). Beware of strong claims.

Is it correct to say that I have a 95% probability that my conversion rate is between 2,79% and 5,21%?

It's correct to say that your 95% confidence interval for the true population mean for this experiment is between 2.79% and 5.21%. This is different than saying the probability of the true conversion rate is between 2.79% and 5.21%.
It's nuanced, but the definition of a confidence interval is (from here):

...means that if the same population is sampled on numerous occasions
  and interval estimates are made on each occasion, the resulting
  intervals would bracket the true population parameter in approximately
  95 % of the cases.

You can imagine running your experiment many times. This time you got a sample mean of 4%, and a 95% confidence interval of (2.79%, 5.21%). Next time you run it you might get a sample mean of 3.5% and 95% CI of (3%, 4%). Run it again, and you get a sample mean of 4.7% and 95% CI of (4%, 5.4%)*. And so on. What a confidence interval means is that the true population conversion rate falls within 95% of the confidence intervals that you created. As you can see, this is different than the claim you made above.
Given this definition of a 95% confidence interval, how useful it is, is up to you. However, note that many people use confidence intervals in their analysis and testing, so even if it doesn't feel satisfying as a measure of confidence (no pun intended) you should feel comfortable using it in reporting results. Just be sure to communicate it accurately.
*Note that I made up these numbers
