I have recently noticed that in my gym they have two different sizes of barbells. The difference between them is only in length, but not in diameter. the reason why I'm mentioning that is because I was doing some research and found out that standard bars, which usually weigh about 35 pounds, have a smaller diameter, but These shorter bars have the same 2" diameter as the longer bars, so do they also weigh 45 pounds?
The IPF (International Powerlifting Federation) and the IWF (International Weightlifting Federation) have set forth standards for what a barbell has to be for use in competition. These standards are largely compatible, so most manufacturers will manufacture for those standards:
Basically the bar is specified in kg, but it is close enough to the weight I listed in pounds that we use that number when loading the bar with plates in imperial measurements. NOTE: some manufacturers of cheaper Olympic style bars don't follow the standards, and sell a 35lb bar as a men's bar. Check your reviews if you are ordering from the major consumer fitness chains.
All three official bars use the same standard sleeve size, meaning the plates are interchangeable for all three bars. The Junior's bar is a little shorter than the other two.
The main differences between a powerlifting bar and a weightlifting bar are:
Standard bars were never meant to be used in competition, so they have much more variances. They are commonly 20lbs, have no spin, and many don't even have knurling. However, even the weight listed here might not be true for all standard bars.
When in doubt, weigh the bar on a scale.
Here's a link to Wikipedia http://en.wikipedia.org/wiki/Barbell - describing the various lengths and weights of barbells. Unless you're in the Olympics or some sanctioned weight/power lifting meet - the barbell's actual weight could vary slightly based on manufacturer, collars used, etc. The typical olymic bar is 45lbs and standard/universal 20lbs.
Never trust a barbell.
The gym owner will swear it's 45 pounds. It ain't. Your workout partner will swear he weighed it last year and it's 25 pounds. It ain't. There are a lot of different bars, whether Olympic, standard, curl or whatever, and they're being built to different weights. When it comes to precision, different brands have different standards. A bar being used for Olympic competition in the snatch is going to be a lot closer to its stated weight than a beater bar used in the gym. (It's manufactured to tighter tolerances.)
The only way to know how much a barbell weighs is to weigh it.