To calculate car depreciation, you typically subtract the current market value of the car from its original purchase price. This gives you the total depreciation amount. You can then divide this by the number of years you've owned the car to get an average annual depreciation.
For instance, if a car was bought for $30,000 and is now worth $20,000 after 3 years, the total depreciation is $10,000, and the average annual depreciation is approximately $3,333.