Most people who file an extension are doing so because they don't have all of their information, so not having a W-2 is ok. The extension is for time to file your return, but not pay your taxes, which is why you need to estimate your total income for the year and taxes withheld. You could also base it off of last year's W-2, if it's similar, or look at your final pay stub for 2016 which will have a year-to-date total column. If you don't normally owe taxes, then you don't need to worry about it.