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If you're interested in a web-based calculator, I've found this one to be useful: http://www.usereffect.com/split-test-calculator

Yup, but do note that the calculator you linked is based on Chi-square and the one in excel is based on z-test. So, results may differ slightly!

In case you need to use a web-based calculator (z-test), here is one on my site: http://visualwebsiteoptimizer.com/ab-split-significance-calc...

Why did you choose to use the z-test?

Z-test gives you confidence intervals on conversion rate with G-test or Chi-square test doesn't

Good point. Confidence intervals that shouldn't be trusted too much, but confidence intervals none the less.

I really prefer Chapra's due to the cleanliness of the design and clarity of results.