P=I.V yea i know but i didnt think we need to calculate over watt! I was calculating over V=I.R. now i get it surely goes opposite becauae they r in same side of (=) yea. But i will use 5V and my limit is 10A in project. I dont want thicker or thinner i want to find the suitable but i m still stuck because i dont have a number about watts of my leds and i dont know why i need watts of my total strips. I dont care the money of electricitt hehe. Umm okay. I will use 5V and Until 10A (usually between 5A-7A) and total lenght is 200inch. So? 

I dont know how much watts i will drive so i cant calculate

Calculating the Wattage is not too complicated - like I mentioned before, there is a lot of minutia that we can kind-a skip for now.

To calculate the wattage, see also the WS2812 article (under the paragraph "Power");

Each LED block actually contains a tiny controller and 3 LEDs. Each of these LEDs pulls about 20mA, so each white little "block" pulls about 60mA (3x20mA).

So when you have LED strands with 60 LEDs/meter, then this would be 

60 * 60 mA = 3600 mA = 3.6 A

This is however when all LEDs are lit up a max brightness - which doesn't happen very often in most scenarios.

Since we run it all on 5V, this means 18 Watts per meter of 60 LEDs.

But again this is under maximum load ... I've used 300 LEDs for my Boblight project (5 Meter), which comes down to 18 A at 5V (90 Watts). This again under maximum load, which doesn't really happen that often. So I took a 10 A power supply (50 Watts) to see if it would work, and kept monitoring it for a while to see if it would become warm or even hot. It didn't - it remained cold, so the power supply had no problems providing power for the 300 LEDs.

Granted that I didn't use real science to determine that, but key is to monitor power supply and wires when running your project. If cables or power supply get warm, then you're pushing it, and a thicker wire or better power supply might be a good idea. 

Oh yes i Did this calculation via datasheet and in my subject it is ;
30leds X 60mA = 1.8A per meter
5 strips X 1.8A = 9A so
9A X 5V = 45W and so in ur formula
45W = 5V X 9A and if we think 5v as 6v
So in the list i will look for 9x2=18ga?

That sounds about right (another source: Wiki).
For the record; I have never done any calculations when it comes to wires and how thick they should be. I've always eyeballed it hahah ...

Oh and I think I might have made a mistake earlier;
Increasing gauge numbers denote decreasing wire diameters (when you asked about 22ga vs 12ga - I corrected my answer).

Hehe i checked that awg wiki today. What a coincidence and yes Hans the number increase in ga measurement is opposite of the thickness. So 12ga is thicker than 22ga yes i ve seen this today on here ; 
http://www.firemountaingems.com/resources/encyclobeadia/charts/6404
But what u think if we check ur colorful list. 9A X 2=18 ga?

Well u have ur eyeballs but i dont have thqt much time and materials around me to play Try&Experience. So i try to calculate.. i will need 45W and 5v until 10A so umm i multiply and look for 20A in list and for 250" and it makes 8ga.. thats damn thick.. oh my.. i ve a headache

Thank goodness I have my eyeballs hehehe ....

Well, 10A is a lot, but what I really would do: check the wires of the power-supply, they should be rated correctly, no calculation needed - unless you order online, then you'd have to measure the thickness of the core of the wire ...

But, the wires will not be very thin, ... but not overly thick either.

No thin no thick yea thats why dig into awg tables

Okay i made a deep research about a pre built sample project for 4A 5V unit and it shows no thinner than 18Ga on the chart but in that project in internet they used 22ga which so thin comparing to 18ga . I dont get why those guys did that mistake. I need an engineer because the chart tells me use about 6-8ga in My project specs and thats damn thick. I ll use 5v! In the end i can have about 4V which is not accceptable in my project.i need an engineer. ( By the way i work on AWG standard not UWG)

Well, I always try to follow the rule: Better a wire a little too thick than too thin ... so I'd probably pick the 18ga just to be sure. 

But Hans. 18 ga is thicker than 22ga. And they use 22ga instead of 18ga. They did opposite of ur advise

Well, I would have to guess that they were successful with 22 ... and that might very well work just fine in their situation (or yours for that matter).
As I understand, the "ga" is based on a constant load - which is not always the case. So in those cases a thinner wire might work just fine.
Now you get an idea why I do not bother looking at the "ga" calculations ... 

Ga comes from Gauge and it has a standard for every Ga number shows a thickness number for the diameter of the wire and no i didnt get why u dont spend time for looking Ga calculations.. hmm because of ur eyeballs again? My friend i will drive much amperage ( 9A is the limit ) and i need science for that cus i dont want my room get fired up ;)

Oh I know where ga comes from, and there is definitely a need in certain scenario's to do the proper calculations to avoid shorts/fire/etc. And your concern is a valid one when it comes to that. The wires straight from the powersupply should be chosen so it can handle 5V/9A properly - I totally agree. And if not 100% sure, see what the calculation suggests and go one level up in thickness ... 

p.s. fire caused by electricity is one of my biggest fears, and that's why I always monitor the temperature of wires when making a new device/design.