Float and double are the same on AVRs - they aren't on the Due and some other fancier microcontrollers, but they are for AVRs.
A common practice is to do all the math as integers, and only at the end, display that as a decimal - for example if you want 3 decimal places, do all your math as integers a factor of 1000 higher, then divide by 1000.0 at the end (or if printing, print number/1000, the decimal, and then number %1000 - thus avoiding floats entirely). Floating point math bloats the program size and the 32-bit floats aren't quite precise enough for a lot of applications.