Been covered a few times on this board.

However, this article;

"How to Calculate Depth of Field"

... contains not only the 'math' behind the subject, it includes (at the bottom of the article) a number of links to some handy 'cheat sheets' and apps for a number of, and varying ... smart devices.

I.e. "PhotoCalc for iPhone (website) – Not only does it do everything for DoF, it also incorporates a sunrise/sunset function, flash exposure calculator, and Exposure Reciprocation calculator."

Great link Les, most of the maths had me bamboozled but I now have photcalc on my Iphone, thanks!