Straight-Line

Why use this method?
Straight-line (SL) depreciation (the process of deducting the purchase price of a fixed asset over several accounting periods) is the most commonly used depreciation method; it's also the simplest. It can be used for both book and tax depreciation. Under straight-line depreciation, the cost of a fixed asset (property used in a productive capacity which will benefit the enterprise for longer than one year) is spread in equal amounts over its estimated useful life.

This method assumes the asset provides constant benefits. If an asset is expected to be used in the business for 10 years, then each year 1/10 of that asset's depreciable value is expensed. The dollar amount of depreciation remains constant from year to year.

How does it work? 

Annual depreciation expense = cost less salvage value estimated useful life