How this instrument works
This instrument isolates one multiplication: how much of a car's cost gets written down each year if the loss is spread in equal dollar slices from the purchase price down to an assumed salvage value. It exists for the moment you've already worked the arithmetic by hand — on a homework set, in a driver's mileage log, on a spreadsheet tracking a two-car rental fleet — and want a second, independent answer before trusting the first one. Straight-line depreciation treats every year of ownership as equally hard on the car, which is a bookkeeping convention, not a claim about how used-car prices actually move.
The shape of the formula is deliberately flat: subtract salvage value from purchase price to get the amount actually expected to be lost, then divide that remainder by however many years the car stays in service. That flat shape is the opposite of the declining-balance model this site's full car depreciation calculator uses to estimate resale value, where a shrinking balance means a shrinking dollar loss each year, front-loaded instead of spread evenly. Straight-line is the method a driver or small fleet owner reaches for when the goal is a simple, predictable number for internal books rather than a forecast of what a buyer will actually pay.
The arithmetic slip this quiz is built to catch is skipping the subtraction: dividing purchase price straight by useful life, with no adjustment for what the car is expected to be worth when it's retired, inflates the yearly figure by however much that salvage value was worth. A second, quieter mix-up is treating the straight-line result as a resale estimate — it is a flat, calendar-driven bookkeeping charge, and it will not track what a dealer or private buyer actually offers, which tends to fall faster in the first year or two than any equal slice can predict.
- Enter Purchase price, $ — the amount paid for the car, the top of the depreciable amount.
- Set Salvage value, $ — plug in the resale figure you're assuming for the day it leaves service; zero is a valid answer.
- Give Useful life, years — the span you're spreading the loss across, matching whatever problem or ledger entry you're checking.
- Read Straight-line depreciation per year, $, and compare it against whatever figure you'd already scratched out on paper.
Worked example — checking a five-year straight-line figure
A car cost $30,000 new, and the owner figures it'll still fetch about $5,000 whenever it's sold off after five years of service. Punch Purchase price, $ = 30000, Salvage value, $ = 5000, and Useful life, years = 5 into the fields, and the first step the instrument does under the hood is the subtraction: $30,000 less $5,000 leaves $25,000 as the amount genuinely expected to disappear over those five years, well before that remainder ever meets the years divisor.
Splitting $25,000 evenly across 5 years lands on Straight-line depreciation per year, $5,000.00 — the same flat number every one of those five years, since straight-line never front-loads or tapers the charge. That $5,000 is what a textbook answer key, a driver's own napkin math, or a two-line ledger entry ought to land on too; a different number usually means the salvage subtraction got skipped.
Questions
Why subtract salvage value before dividing by useful life?
Because straight-line depreciation only counts value the car is actually expected to lose. If a $30,000 car is still worth $5,000 after five years, only $25,000 of the original price was ever consumed by ownership — divide that $25,000, not the full $30,000, or the yearly figure comes out too high by exactly the salvage amount.
Does this number match what a dealer would pay for the car?
No. Straight-line depreciation is a flat bookkeeping convention — equal dollar slices every year — while resale and trade-in prices typically fall fastest in the first year or two and level off afterward. Treat this figure as an internal accounting number, not a forecast of a dealer's offer or a private-sale price.
Does the full car depreciation calculator on this site use the same formula?
No — it runs declining-balance depreciation instead, shrinking the car's value by a fixed percentage of whatever it's already worth, which front-loads the dollar loss into the early years and aims to estimate resale value. This quiz drills the opposite method, a flat equal-dollar straight-line charge, built for checking one homework or ledger figure fast rather than pricing a used car.
What happens if I set useful life to zero?
The instrument won't return a figure — dividing by zero years has no defined answer, and it flags the entry and asks for a useful life above zero instead. Useful life has to be a real span of years the car is expected to serve, even a fractional one, before the formula can run.
Can I put this figure straight onto a business tax return?
Not directly. Tax depreciation for a business vehicle in the US generally follows MACRS or the standard mileage rate, not straight-line, and which one applies depends on how the car is used and what was elected. This instrument checks the straight-line arithmetic itself; IRS Publication 946 or a preparer confirms which method actually governs a given return.
References
- IRS Publication 946 — How To Depreciate Property
- IRS Publication 463 — Travel, Gift, and Car Expenses
Read this first: This instrument shows arithmetic, not advice. Real offers add fees, taxes and terms that vary by lender and place — verify the figures against your actual paperwork before deciding anything.