A new car that costs $28,000 starts losing value the day it leaves the lot. If it loses 15% of its value each year, three years later it is worth only about $17,195.50, even if it still runs perfectly. A rare trading card bought for $150 can go the other way and be worth more each year. A loss of value over time is called depreciation; a gain in value over time is called appreciation. Knowing how fast something loses or gains value helps you decide whether to buy new or used, how long to keep something, and whether a "deal" really is one. There are two common ways to model the change: by the same dollar amount each year, or by the same percent each year. They give very different numbers and very different graphs.
Straight-line depreciation: the same amount each year
In straight-line depreciation, an item loses the same number of dollars every year. If it starts at a purchase price and loses dollars per year, then after years its value is . This is a linear relation: the initial value is and the rate of change is dollars per year, so its graph is a straight line sloping down. The model reaches a value of zero after years; after that the formula would give negative values, which make no sense for a price, so the model stops there. If an item is expected to be worth a salvage value (what you could still sell it for) after years, the yearly loss is .
Percent depreciation: the same percent each year
Many things, such as cars and electronics, are better described by percent depreciation (also called declining-balance depreciation): each year the item loses the same percent of its current value. Losing (as a decimal) each year leaves of the value, so each year you multiply by . A 20% loss means multiplying by every year. After years the value is . Because each year's loss is a percent of a smaller and smaller value, the dollar loss shrinks every year, and the graph is a curve that drops steeply at first and then flattens out. As long as the rate is between 0% and 100%, the model's value gets closer and closer to zero but never actually reaches it.
| Year | Straight-line value (2,000 dollars a year) | Percent value (20% a year) | Percent model: loss that year |
|---|---|---|---|
| 0 | 20,000.00 | 20,000.00 | none |
| 1 | 18,000.00 | 16,000.00 | 4,000.00 |
| 2 | 16,000.00 | 12,800.00 | 3,200.00 |
| 3 | 14,000.00 | 10,240.00 | 2,560.00 |
| 4 | 12,000.00 | 8,192.00 | 2,048.00 |
| 5 | 10,000.00 | 6,553.60 | 1,638.40 |
| 10 | 0.00 | 2,147.48 | 536.87 |
Appreciation: growing by the same percent
Appreciation works the same way as percent depreciation, but the value grows. Gaining each year means multiplying by , so after years . Each year's gain is a percent of a larger value than the year before, so the dollar gain grows and the graph curves upward more and more steeply. Things that can appreciate include some collectibles, land and homes. Appreciation is never guaranteed: home prices fell in many Canadian markets after peaking in early 2022, and the Canadian Real Estate Association's national home price index was still 3% lower in August 2026 than a year earlier (CREA, 2026). A collectible can lose value when it goes out of fashion. A model with a fixed rate is an estimate, not a promise.
Reading the graphs
- A straight line sloping down: the same dollar loss each year (straight-line depreciation). The slope is the yearly loss.
- A curve that falls quickly and then flattens: the same percent loss each year (percent depreciation). The yearly dollar loss shrinks.
- A curve that rises more and more steeply: the same percent gain each year (appreciation). The yearly dollar gain grows.
- Where two value graphs cross, the two items (or two models) are worth the same amount at that time. In the figure, the straight-line value drops below the percent value between years 8 and 9.
- The point where a graph meets the vertical axis (time 0) is the purchase price P.