MACRS convention determines the number of months for which you can claim depreciation during a partial year, either when you first placed the asset in service or when you disposed of it.
The mid-month convention only applies to residential rental property, nonresidential real property, and railroad grading or tunnel bore. It simply means that you get a half month's worth of depreciation no matter when that asset was placed into (or taken from) service during that month, whether that was at the beginning, middle, or end of the month.
The half-year convention works the same way but instead of the month it goes by the year. In other words, you'll get 6 months' depreciation if the asset was placed into service or disposed of during the year, no matter if it was in January or December.
Finally, there's the mid-quarter convention, which can be used for assets that the mid-month convention doesn't apply to, if the total depreciable basis of assets subject to MACRS placed into service during October, November, and December amounts to over 40% of the depreciable basis of all assets (other than residential rental property, nonresidential real property, railroad grading, tunnel bore, and assets that were both placed into service and disposed of during the same year) subject to MACRS that were placed into service during that year. This convention gives you a half-quarter (1.5 months' worth of depreciation) for the quarter in which the asset was either placed into service or disposed of.
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Depreciation of Business Assets - TurboTax Tax Tips & Videos
Publication 946 (2020), How To Depreciate Property | Internal ...
I placed assets into service all year long, with only about 15% in the last 3 months. So, according to this, I should be using the mid-quarter convention. But TT (Business) is calculating all of my depreciation using the half-year convention. How can I correct this?
40% of the depreciable basis needs to be placed into service in the last quarter for the M-Q convention to apply
where did you see 15%?
The mid-quarter convention. Use this convention if the total depreciable bases of MACRS property you placed in service during the last 3 months of the tax year (excluding nonresidential real property, residential rental property, any rail-road grading or tunnel bore, property placed in service and disposed of in the same year, and property that is being depreciated under a method other than MACRS) are more than 40% of the total depreciable bases of all MACRS property you placed in service during the entire year.