How to convert analog MQ4 sensor output to ppm

Hi everyone, 
My final year engineering involves a rover that can sense methane gas. Initially the idea was to just detect the gas as digital input but now i wanna do a bit more. I've searched all over the internet on how to convert the analog values to ppm, but couldn't understand any of it.
I'd be really great-full if someone could give me the formula or the code to do this. 
I don't intend on calibrating the sensor to find precise answers, just a rough picture of the ppm values is enough to begin with.
Thanks a lot in advance.