How the estimate works
Cars lose value fastest at the start: the steepest drop comes in the first year, and the curve flattens as the car ages. This calculator models that with two assumptions you can see and change — a first-year drop (default 20%) and an annual rate for the following years (default 15%), figures commonly used as a rule of thumb. They are defaults, not facts about your car: actual depreciation varies enormously by brand, model, mileage, condition, and market. Treat the output as a planning curve, not an appraisal.
Depreciation is usually a car's single largest cost — bigger than fuel, bigger than maintenance — it's just invisible because nobody sends you a monthly bill for it. Seeing it as a yearly number next to your other costs is clarifying: it's the strongest financial argument for buying a lightly used car and for keeping a good car longer.
The one variable you control
You can't choose your model's resale curve after you've bought it, and you can't drive fewer miles than life requires. What you can control is documentation. A comprehensive service history can add roughly 10–20% to a used car's value, and in a Kwik Fit survey of 2,000 car owners, nearly half said they wouldn't consider buying a used car without a full service history. At the other end, vehicle-data firm HPI has estimated that missing records can knock as much as 40% off a price.
The mechanism is simple: a buyer who can't verify maintenance assumes the worst and prices it in — used-car forums are full of buyers who deduct thousands for undocumented cars, or walk away entirely. A dated, odometer-stamped log answers the question before it's asked. On a $15,000 car, the difference between "trust me" and "here's the report" is real money.
Build the record while you own it
Service history is the one part of resale value you build in five-minute increments, years before you sell. Odobook makes the habit cheap: log each service with date, odometer, cost, and receipt photo as it happens, and when it's time to sell, Pro generates a clean PDF service history report from your records — including all the DIY and independent-shop work that never reaches CARFAX. Free for one vehicle, no account, data on your device.