Cost minus salvage, divided by years
Straight-line depreciation charges the same amount each full year: the depreciable base is cost minus salvage, then that base is divided by useful life. Book value after k years is cost minus k times the annual amount, which equals salvage after the full life. Auto-lease residual is a dealer quote, not this accounting schedule. ROI measures profit against cost without spreading the cost over years. This page does not load IRS MACRS tables or a mid-year convention.
Annual depreciation = depreciable base ÷ years
Monthly depreciation = annual depreciation ÷ 12
Accumulated depreciation = annual depreciation × years elapsed
Book value = cost − accumulated depreciation
Example
Equipment costing $18,000 with $3,000 salvage and a 5-year life depreciates $3,000 a year, or $250 a month. After 2 years, accumulated depreciation is $6,000 and book value is $12,000. A $50,000 machine with $5,000 salvage and a 10-year life is $4,500 a year; after 3 years the book value is $36,500.
What this schedule leaves out
Useful life and years elapsed must be whole numbers, and elapsed years cannot exceed the life. Salvage cannot exceed cost. Section 179, bonus depreciation, MACRS, declining-balance, units-of-production, and partial first-year conventions stay outside this full-year identity.