To estimate the carbon footprint of your business use the the small business greenhouse gas emission calculator from the U.S. Environmental Protection Agency
For each purchase, a credit representing the reduction of greenhouse gas emission by one metric ton will be "retired" on behalf of your organization. By retiring the credit, this ensures the credit can not be re-sold and results in a permanent reduction.
Based on your companies individual needs, you can purchase as many offsets required. For example, if your company has a carbon footprint of 100 tons per year enter 100 into the quantity field to offset your footprint for one year.