Good morning. You can use the 720 rule calculator either right here at the web site or by saving the page on your own computer. The advantage of the latter is not being at the mercy of the internet any time you want to use the calculator.

Javascript is used for the calculations so it should work regardless of your operating system. You can view the source of the page to verify that there's no malicious code, viruses, or other nonsense embedded in it - even if you don't understand javascript you should be able to see that it only deals with itself and nothing fishy's going on.

This work is derived directly from Gramslam's spreadsheet.

720 Rule Calculator