Why Congress Created the Half-Year Convention

Before MACRS was enacted in 1986, businesses had incentive to make large asset purchases on December 31 and claim a full year of depreciation for owning equipment for just one day. Congress designed the half-year convention specifically to eliminate this abuse while still simplifying record-keeping — instead of calculating exact days in service, every personal property asset simply earns 6 months of depreciation in year one regardless of whether it was purchased in February or November. This standardization also simplified IRS audit procedures: rather than verifying exact acquisition dates for every item on a depreciation schedule, examiners only need to confirm the calendar year of acquisition for half-year convention assets.

The half-year convention is now the default for all MACRS personal property — equipment, vehicles, computers, furniture — covering the vast majority of depreciable assets purchased by businesses each year. Real property (buildings) uses the mid-month convention, and the mid-quarter convention overrides half-year when Q4 acquisitions become too concentrated. Understanding when each convention applies is one of the most practically important aspects of U.S. tax depreciation.

The Mechanics: How Half-Year Adds an Extra Calendar Year

The half-year convention's most counterintuitive effect is that "5-year property" actually appears in your depreciation schedule for six tax years. The first year receives half the normal annual deduction, and the sixth year receives the remaining half-year not claimed in year one. A $50,000 computer under 5-year MACRS with the half-year convention generates these deductions: Year 1: $10,000 (20%); Year 2: $16,000 (32%); Year 3: $9,600 (19.2%); Year 4: $5,760 (11.52%); Year 5: $5,760 (11.52%); Year 6: $2,880 (5.76%). The percentages sum to 100%, recovering the full $50,000 cost, but they stretch across six filing years. This schedule applies whether you purchased the machine on January 2 or December 30 — the exact date only matters for determining which tax year begins the schedule. Businesses that sell a 5-year-property asset in the sixth year still claim the 5.76% final deduction in that disposition year.

Planning Purchases to Keep the Half-Year Advantage

The half-year convention becomes a liability rather than an asset when it is replaced by the mid-quarter convention, which assigns Q4 acquisitions only 1.5 months of depreciation rather than 6 months. The IRS triggers mid-quarter when more than 40% of the year's total depreciable asset basis lands in Q4. A manufacturing company that purchases most of its annual equipment budget in November and December can easily trip this threshold. The planning solution is to spread major acquisitions across the year, particularly targeting Q1 (which under mid-quarter earns 10.5 months of depreciation — more than half-year convention's 6 months). Even without triggering mid-quarter, businesses benefit from purchasing significant assets in Q1 because the 12-month clock on the next year's depreciation begins sooner, advancing all future year deductions forward in time. A $500,000 equipment purchase made January 3 rather than December 15 of the same year has the same year-one MACRS deduction but generates the year-two (32%) deduction one full year earlier.