Depreciation Straight Line is a method of calculating the decline in value of an asset over time. It is one of the most commonly used accounting techniques, as it is easy to calculate and offers clear results. The principal behind the straight line method is that the same amount of depreciation is spread out evenly over the entire life span of the asset. To calculate straight-line depreciation, you must first determine the cost of the asset and its estimated useful life. Then divide the cost by the lifespan to get an estimated depreciation expense per year. This amount will remain the same until the asset has been fully depreciated.