How do I know which PSU would fit in my computer case if I'm buying it online?

I have an average sized computer, not an old huge one, but not a slim one like those mini hp's.





How do I know which PSU will fit in my case?How do I know which PSU would fit in my computer case if I'm buying it online?
Almost all new ones are standard size.





just make sure it fits the connection of your motherboard, if you buy new one it should fit.





buy 600 W , don't forget. barely enough will waste your money.How do I know which PSU would fit in my computer case if I'm buying it online?
if you mean power supply unit, most of them are pretty much the same size physically, it's more important to know whether or not the one that your buying has all the right connectors for the other parts of your computer.
It's probably an ATX, most computer companies have standardized to that form factor. Tell us what system you've got and we'll tell you what's in it.