Welcome to Diablo's Armory!
Diablo's Armory is a free tool to calculate character and gear stats. To get started just click on the Stats Calc link. You do not need to create any accounts or submit any data to use the tool (see Guest Accounts for more info).
First you will need to create a character and any items on your in game character then add them the your character in the tool. I have found the best way to do this is to take a screen shot of all of your items so you can read the tool tips out of game. For the Armor value on items use the big armor number on the item tool tip. For weapons you will only need to enter the min and max damage found under the Damage Per Second value on the item tool tip.
Gems and set bonuses are not currently supported by the tool so for now just add the values as extra stats. If you don't want to add up all of the values of your sockets (I know math is hard) you can enter it on a new entry and the tool will add all of the values for you. If you do not see the attribute for an item in the drop down list let me know so I can get that fixed.
Sadly Blizzard has not released the API for Diablo 3 yet. Once they do I will look for a way to auto import all of your items from your account. Till then you will have to do it by hand.
Now that you have created an item it will appear in your saved items tab. Mousing over the item will display an arrow for the items drop down menu. In the drop down menu you can equip the item to your current character along with other actions. After all items have been added we can now go check out the stats tab. These values should be very close if not the same as your in game character sheet. If any value appears to be off check over your items again them let me know so I can look into it.
Currently I know there are a few errors with Damage but most of the time the difference is less than 10. This mainly happens when dual wield and on some weapons that have % Damage increase. There is also a small difference (~0.02) in the reduction from resistance information. Again if you find any other values that are wrong let me know.
Anyways I hope this tools helps you out when your looking for that perfect item on the auction house, I know it has helped me. If you can think of anything you would like to see as part of this tool let me know. This is just the start and I want to keep expanding the site into useful tool for every Diablo 3 player.