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