calculating check digit. formula issue

"The problem is I am computing the checksum for a UPC-13 barcode, or a EAN-13 barcode. It is 13 digits long anyway. For example:
9780450022852
I know some data should be multiple by 3, then sum up all the data.
93+7+0+5+0+2+5=112 Then 120 - 112 = 8
This is the checksum I got. Then I tried online tools, the check digit showed as 2.
Hope someone could let me know simple steps.
"

It really waste of time to calculate checksum yourself. Have a try of this EAN 13 generator for .NET. And you will be able to enable or disable the checksum digit in the HR text. http://www.keepautomation(dot)com/products/net_barcode/barcodes/ean_13.html

You just miss the last step that is adding the remainder to 10. The checksum should be 10-8=2. I think you needn`t to calculate the checksum yourself. If you installed the barcode control, it will compute the checksum for you automatically.

Are you using a font for the generation of EAN13? If you use a EAN13 barcode control which supports your developmental environments, you will not need comput the check digit manually.

If you want to learn the algorithm of EAN-13, you can navigate to Wikipedia: http://en.wikipedia(dot)org/wiki/EAN-13

If you need to create ean13 barcode , I recommend Barcode Generator for .NET for you.
It is an easy-to-use .NET barcode generating DLL, which supports .NET 2.0 and later version to be used in Visual Studio 2005, VS 2008 and VS 2010.
And the project will calculate check digit automatically.
Hope it helps!