Depreciation Conventions, Explained: Which Timing Rule Applies to Your Assets

You buy a $60,000 machine on December 28 and put it straight to work. How much depreciation can you claim for those last three days of the year? If you guessed "about three days' worth," you are wrong in both directions. Under one IRS timing rule, that machine earns half a full year of depreciation. Under another, it earns about six weeks' worth. The purchase price is identical, the delivery date is identical — only the calendar math changes, and it can swing your first-year deduction by tens of thousands of dollars.

That calendar math is the depreciation convention: the rule that decides what fraction of a year counts when you place an asset in service or take it out. There are three of them — half-year, mid-quarter, and mid-month — and picking the wrong one is one of the most common depreciation errors small businesses make. This guide explains what each convention does, the exact order to apply them in, and how to book the result so your tax depreciation never drifts from your ledger.

What a Depreciation Convention Actually Decides​

Depreciation starts when an asset is placed in service — ready and available for its assigned job, not necessarily when you paid for it or took delivery. A server rack delivered December 20 but not racked and powered until January 5 is a January asset. Once that date is set, the convention answers the follow-up question: for depreciation purposes, how much of this tax year did the asset serve?

The three conventions give three different answers:

• Half-year: every asset counts as serving exactly half the year, no matter its actual date.
• Mid-quarter: every asset counts as serving from the midpoint of its calendar quarter.
• Mid-month: every asset counts as serving from the midpoint of its month.

You do not get to choose freely among them. Each asset gets exactly one, assigned by a fixed decision order: real property always takes mid-month; everything else takes half-year unless the year's purchases trip the 40% test, in which case everything takes mid-quarter. Apply them in that order and you will never assign the wrong one.

The Half-Year Convention: The Default for Equipment​

The half-year convention is the default for MACRS personal property — the 3-, 5-, 7-, 10-, 15-, and 20-year classes covering machinery, vehicles, computers, office furniture, and most equipment a small business buys. It treats every asset placed in service during the year as if it arrived at the exact midpoint of the year. Buy in January or December, you get the same half year of depreciation in year one.

That symmetry extends to the other end of the asset's life. When you sell, retire, or otherwise dispose of an asset, the half-year convention grants a half year of depreciation in the disposition year too — even if you sold it in the first week of January. And because year one only consumed half a year of recovery, a "5-year" asset actually depreciates across six tax years: a half year up front, four full years, and a final half year. The MACRS percentage tables (Table A-1 in IRS Publication 946 for the popular 200% declining-balance classes) already bake this in: 5-year property shows 20% in year one, not the 40% a full year of double-declining-balance math would suggest.

The policy logic is administrative mercy. Without a convention, every business would prorate every asset to the day and the IRS would audit placed-in-service dates line by line. The half-year rule trades a little precision for a system everyone can apply from a printed table.

The Mid-Quarter Convention: The 40% Test That Changes Everything​

The half-year rule has an obvious exploit: buy everything on December 31 and collect half a year of depreciation for one day of ownership. Congress closed it with the mid-quarter convention, triggered by the 40% test: if more than 40% of the aggregate depreciable basis of the property you placed in service during the year went into service in the last three months, mid-quarter replaces half-year for every asset placed in service that year — not just the late ones.

Under mid-quarter, each asset is treated as placed in service at the midpoint of its own quarter. That cuts both ways, and the upside surprises people:

• Q1 assets get 10.5 months of first-year depreciation — more than the half-year rule would give.
• Q2 assets get 7.5 months, Q3 assets get 4.5 months.
• Q4 assets get just 1.5 months. A 5-year asset bought in December earns 5% in year one under mid-quarter (Table A-5) versus 20% under half-year (Table A-1). On that $60,000 December machine, the convention alone is the difference between a $12,000 and a $3,000 first-year deduction.

What counts in the 40% test — and what does not​

The test is computed on depreciable basis, and several categories stay out of the fraction entirely:

• Real property is excluded from both sides. A rental house or office building you bought in November does not push you toward mid-quarter, because it lives under the mid-month convention anyway.
• Property placed in service and disposed of in the same year is excluded.
• Amounts you expense under Section 179 come out of the basis used for the test. Electing Section 179 on a big Q4 purchase can shrink the numerator enough to keep you on the half-year convention.
• Bonus depreciation basis stays in. Taking bonus on Q4 assets does not help you pass the test — a detail that catches buyers who assume any form of immediate expensing works the same way.

Note the "more than" in the rule: exactly 40% in the last three months does not trigger mid-quarter. Only a fraction above 40% does.

If year-end equipment buying is a regular habit of yours, read the companion walkthrough of the mid-quarter convention trap for planning strategies — accelerating purchases ahead of October, deferring to January, and running a tally starting in the fall. The rest of this guide covers the rules themselves.

The Mid-Month Convention: Why Real Estate Gets Its Own Rule​

Buildings get finer-grained timing than equipment. The mid-month convention applies to residential rental property (27.5-year), nonresidential real property (39-year), and railroad gradings and tunnel bores. It treats property as placed in service — or disposed of — at the midpoint of the month, regardless of the actual day. Close on a rental house on March 2 or March 30 and year one is 9.5 months either way. Sell it in June of some later year and the disposition year gets 5.5 months.

Why the special treatment? Scale. Half a year of depreciation on a $900,000 building dwarfs half a year on a $9,000 copier, so the December-31 exploit the mid-quarter rule polices for equipment would be far more lucrative for real estate. Monthly precision also matches how buildings actually transact — closings happen on specific days, rents prorate by month — while keeping the math table-driven. Real property depreciates straight-line, so the monthly slice is simply one-twelfth of the annual amount, halved for the first and last months.

One related trap deserves a paragraph: cost segregation. When a study carves 5-, 7-, or 15-year components (parking lots, decorative fixtures, dedicated electrical) out of a building, those components stop being real property for depreciation purposes. They fall under the half-year or mid-quarter conventions instead — and their basis counts in the 40% test. A November building purchase with a large cost-seg carve-out can single-handedly trip mid-quarter for all your equipment that year. Run the test with the segregated components included before you celebrate the accelerated deductions.

How the Three Rules Interact on One Return​

A single tax return routinely uses two conventions at once — mid-month for the building, half-year or mid-quarter for everything inside it. Work through each asset in this order:

1. Is it real property subject to MACRS real-property rules? Residential rental, nonresidential real estate, railroad gradings and tunnel bores take the mid-month convention. Done — and it stays out of the 40% test.
2. Did more than 40% of this year's depreciable basis (after the exclusions above) go into service in the last three months? If yes, every remaining asset takes the mid-quarter convention, each at its own quarter's midpoint. Use the mid-quarter percentage tables in Publication 946, selecting the column for the asset's quarter.
3. Otherwise, the half-year convention. Every remaining asset gets a half year up front, and Table A applies.

Dispositions follow the convention the asset lived under: half-year assets get a half year in the year of disposal, mid-quarter assets get a quarter-fraction based on the disposal quarter (an asset retired in Q2 is treated as retired at that quarter's midpoint), and real property gets a half month for the disposition month.

What Section 179 and Bonus Depreciation Do to Conventions​

Immediate expensing interacts with conventions in a specific order, and getting the order right matters:

1. Section 179 comes first. The expensed portion is deducted in full with no convention applied, and it drops out of the 40% test as described above. Any remaining basis continues to step 2.
2. Bonus depreciation comes second. It applies to the remaining basis, and unlike Section 179, it does not remove basis from the 40% test. When bonus covers the full remaining cost, conventions are moot for that asset — there is nothing left to spread across years.
3. Regular MACRS comes last. Whatever basis remains after both is depreciated under whichever convention the year dictates.

The practical consequence: in years when you fully expense an asset, you can ignore conventions for it. But partial expensing — a Section 179 election capped by taxable income limits, bonus taken at less than full rates, or listed property with limited deductions — leaves a remainder that still needs the right convention and the right table. Bonus percentages have been changed by statute more than once, so confirm the rate for the year you place the asset in service rather than assuming last year's rate still holds.

Booking Conventions in Your Ledger​

Here is the bookkeeping subtlety most guides skip: conventions are a tax concept, not a book concept. Your GAAP or management books typically depreciate from the actual in-service date with monthly proration — a March 20 server gets about 9.3 months in year one on the books, while the tax return claims a half year or a quarter-fraction. Neither is wrong; they answer different questions. The books show economic cost over time, while the tax return follows MACRS tables.

That means a well-kept ledger tracks both. Record each fixed asset with its actual placed-in-service date for book depreciation, and keep the tax convention and MACRS table as attributes of the same asset for the tax schedule. In a plain-text ledger, the monthly book entry looks like this:

2026-03-31 * "Monthly depreciation — Dell server (books: 36 mo from 2026-03-20; tax: 5-yr MACRS half-year, Table A-1)"
  Expenses:Depreciation:Equipment  138.89 USD
  Assets:Equipment:Accumulated-Depreciation  -138.89 USD

The narration carries the tax attributes so the schedule can be rebuilt from the ledger at year-end: recovery period, convention, and table. When mid-quarter applies, note the quarter too (mid-quarter Q4, Table A-5) so next year's preparer — possibly you, months later — knows why December assets used a different column. If you visualize asset balances for review, the Fava dashboard renders these accounts from the same file, and the docs walk through account setup for fixed assets.

Reconcile the two tracks annually: total book depreciation for the year versus total tax depreciation claimed. The gap is your depreciation timing difference — the number that explains why taxable income and book income diverge and the starting point for any deferred-tax entry.

Common Mistakes to Avoid​

Applying the half-year convention to a building. New landlords do this constantly, claiming a half year on a November closing. Real property is always mid-month: a November purchase earns 1.5 months in year one, not six.

Assuming mid-quarter only affects Q4 assets. It re-times every asset placed in service that year, including January purchases — which is good news for those (10.5 months) but means you must switch tables for all of them. Running Q1 assets through Table A after the test trips is an under-claim that compounds for years.

Counting real estate in the 40% test. Both sides of the fraction exclude it. A December building purchase alone never triggers mid-quarter for your equipment — though cost-seg components carved out of it do count, as noted above.

Prorating tax depreciation to the actual day. Daily or exact-month proration is the book method, not MACRS. Using it on the return overstates some assets and understates others, and the error persists for the asset's whole recovery period.

Confusing purchase date with placed-in-service date. Equipment bought in December but not installed and usable until January is next year's asset — for the 40% test and the deduction alike. Delivery slips prove purchase; installation logs, utility turn-on records, and first-use evidence prove placed in service.

Your Next Equipment Purchase Starts With a Calendar​

Before you sign for the next machine, vehicle, or server rack, run the 40% test on a scratch sheet with the purchase included. If the addition keeps you at or under 40% in the last three months, the half-year convention holds and timing barely matters. If it pushes you over, you now know exactly what that costs — 1.5 months instead of six on the new asset, with new tables for everything else bought that year — and you can decide with open eyes whether to buy now, buy in January, or expense your way out with Section 179.