The applicable averaging convention is not elective. Rather, one of three conventions (half-year, mid-month, and mid-quarter) must apply.

Half-year convention. Under this convention, property is treated as placed in service, or disposed, on the midpoint of the tax year. Thus, one-half of the depreciation for the first year of the recovery period is allowed in the tax year in which the property is placed in service, regardless of when the property is placed in service during the tax year. The half-year convention applies to property other than residential rental property, nonresidential real property, and railroad grading and tunnel bores unless the mid-quarter convention applies

Mid-month convention. Under this convention, property is treated as placed in service, or disposed of, on the midpoint of the month. The MACRS deduction is based on the number of months that the property was in service. Thus, one-half month of depreciation is allowed for the month that property is placed in service and for the month of disposition if there is a disposition of property before the end of the recovery period. The mid-month convention applies to residential rental property (including low-income housing), nonresidential real property, and railroad grading and tunnel bores.

Mid-quarter convention. Under this convention, all property (other than the property otherwise excluded) placed in service, or disposed, during any quarter of a tax year is treated as placed in service, or disposed, on the midpoint of the quarter. A quarter is a period of three months. The mid-quarter convention applies to all property (other than residential rental property, nonresidential real property, and railroad grading and tunnel bores) if more than 40 percent of the aggregate bases of such property is placed in service during the last three months of the tax year.