Method allocates depreciable cost of an asset over its useful life to total estimated productive output.Depreciable Cost/Estimated Total Production = Depreciation Rate per Unit of Production Then multiplied by the Actual Production for the period to get Depreciation ExpenseFrom period to period, depreciation expense, accumulated depreciation, and book value VARY directly with the units produced. In the units-of-production method, depreciation expense is a VARIABLE expense because it varies directly with production or use.If total estimated productive output differs from actual total output, final adjusting entry to depreciation expense should be for the amount needed to bring the asset's net book value equal to the asset's estimated residual value.