[header] How to calculate double declining depreciation [title] Know when to use depreciation. [step] Before you decide on which depreciation method to use, you'll want to be sure that you need to use depreciation at all. In accounting, expenses are recorded for the same period in which revenues are produced from those expenses. 
Therefore, if you purchase an expensive asset that you will use for multiple accounting periods (usually years), you will want to spread the cost of that asset out over the years in which it produces revenue. This expense is recorded each year as depreciation.