computer fan size HELKP

Solution
That depends on the case, how many fan locations it has, how many fan headers you have on the motherboard and what you intend to use the machine for. A rig meant only for office applications or browsing can probably do fine with a single front and rear fan. Or even a single exhaust on some systems, plus a cpu cooler.

A gaming machine with an aftermarket graphics card and higher end processor on the other hand probably demands that you have at least two intakes and an exhaust fan, and preferably at least two intakes and two exhaust fans, if not three, again, depending on the case, the processor and whether or not you're using an aftermarket graphics card or overclocking anything.

A few more details, including your hardware specs...
That depends on the case, how many fan locations it has, how many fan headers you have on the motherboard and what you intend to use the machine for. A rig meant only for office applications or browsing can probably do fine with a single front and rear fan. Or even a single exhaust on some systems, plus a cpu cooler.

A gaming machine with an aftermarket graphics card and higher end processor on the other hand probably demands that you have at least two intakes and an exhaust fan, and preferably at least two intakes and two exhaust fans, if not three, again, depending on the case, the processor and whether or not you're using an aftermarket graphics card or overclocking anything.

A few more details, including your hardware specs, would be helpful.
 
Solution
also large fans the 240 and bigger can move more air at a slower speed...slower speed..less fan sound. one error i did on my older build is plug 120mm fans into the power supply. the fans run at full speed and over time the sound does get to you. on that build i wish i had use a fan controller for the extra fans.
 
Exactly. That's why when a fan location can use 140 or 120mm fans, I almost always choose 140mm. On some cases that will accept larger fans like the 200 and 230mm fans, they sometimes make sense and are definitely quieter. I never recommend connecting fans directly to the power supply unless it's absolutely necessary, and then only as a temporary measure. As you say, it's loud, annoying, uses way more power and is in general more dangerous if you're using a fairly cheap PSU or one that's already clearly borderline with regard to it's capacity versus what the system is actually pulling.

Fans usually don't pull many amps, but some pre-built units are already so close to max capacity that any additional load can be a problem.
 
My build is a
Gigabyte ga-z97x ud5h motherboard
Corsair CX500 PSU
Intel (R) Pentium(R) CPU G3258 @ 3.2GHz
Thermaltake V4 Black Edition Case
NVIDIA Geforce GTX 750 Ti Black Edition
 
Well, at the risk of offending you, I'm going to say that is one of the worst case choices you could have by any of the major manufacturers. However, depending on your system it might be fine. If you're overclocking that G3258 you definitely want a good aftermarket CPU cooler and you want to populate all the case fan locations. Since that case only supports 120mm fans, your choices are easy when it comes to size. That's all the case fan locations support so you can't use anything else without some kind of modification.

You'd probably be fine filling the available fan locations with these unless you want high quality fans which run more like 15-25 bucks each:

PCPartPicker part list / Price breakdown by merchant

Case Fan: Cooler Master SickleFlow 69.7 CFM 120mm Fan ($5.98 @ OutletPC)
Total: $5.98
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2015-05-17 13:36 EDT-0400