Straight-line method

With the straight-line method, you choose to depreciate your property an equal amount for each year over its useful life span. Here are the steps to calculate monthly straight-line depreciation:

First subtract the asset's salvage value from its cost, in order to determine the amount that can be depreciated.

Total depreciation = Cost - Salvage value

Next, divide this amount by the number of years in the asset's useful lifespan, which you can find in tables provided by the IRS.

Annual depreciation = Total depreciation / Useful lifespan

Finally, dividing this by 12 will tell you the monthly depreciation for the asset.

Monthly depreciation = Annual deprecation / 12