The units of production method of depreciation is based on an asset's usage, activity, or parts produced instead of the passage of time. Under the units of production method, depreciation during a given year will be very high when many units are produced, and it will be very low when only a few units are produced.
To illustrate the units of production method, let's assume that a production machine has a cost of $500,000 and its useful life is expected to end after producing 240,000 units of a component part. The salvage value at that point is expected to be $20,000. Under the units of production method, the machine's depreciable cost of $480,000 ($500,000 minus $20,000) is divided by 240,000 units, resulting in depreciation of $2 per unit. If the machine produces 10,000 parts in the first year, the depreciation for the year will be $20,000 ($2 x 10,000 units). If the machine produces 50,000 parts in the next year, its depreciation will be $100,000 ($2 x 50,000 units). The depreciation will be calculated similarly each year until the asset's Accumulated Depreciation reaches $480,000.
The units of production method is also referred to as the units of activity method, since the method can be used for depreciating airplanes based on air miles, cars on miles driven, photocopiers on copies made, DVDs on number of times rented, and so on.
Depreciation is an allocation technique and the units of production method might do a better job of allocating/matching an asset's cost to the proper period than the straight-line method, which is based solely on the passage of time.