How to measure handlebar diameter? I don't know my bike's stock handlebar diameter. I am going to upgrade some parts, for handlebar I tried to read any information written on it but there is only it's brand name. Can you help me please?

EDIT: Sorry for my poor English. I am trying to mean the "diameter" by "width", like the cylinder here http://www.nordenexpress.com/images/packaging.gif (For example I thought- the drop bar I want to buy has a "length" of 44cm, not width. Sorry again) 

My bike has a flat handlebar and I am willing to replace it with a drop bar. Drop bars I've found on online bike stores are mostly 31.8mm. I don't know how to measure the diameter of my flat bar, I don't have calipers. Even if I had calipers, I don't know where to measure; the middle of the bar is thicker, ends are thinner. 

I am not sure about where is 31.8mm. Just thought maybe it is the diameter of the bar, am I right?

If a handlebar (like mine) is said to be 25.4, can you tell how we get the result 25.4? Measure the diameter? Radius? Periphery?
Is it a road dropbar or a MTB riser style bar? These two images should prove useful. If it is a mountain bike style riser or straight bar it's quite simple.

If it's a road bar most people (zipp) measure center to center of the drops. 







Also if it is a Mountain bike were speaking of, most bars will be sold as 730mm or more wide and have pre-printed lines to cut them shorter. There is a lot of preference involved in bar length. I prefer a shorter length than a lot of people my size, some downhill guys prefer 800mm bars, too long for me, I'm good at finding trees! :P

A good reference if its a road bar is about shoulder width in the drops. This can vary with preference and riding style slightly but is a solid method for the best on bike ergonomics and comfort.