By late September the tax year is three-quarters gone, but for the self-employed the most valuable work is still ahead. Almost every meaningful lever a freelancer or small business owner can pull, funding a retirement plan, buying equipment, running an S corporation salary, timing an invoice, has a hard deadline of 31 December 2026. After that the numbers are locked and April becomes an exercise in paperwork, not planning. This is a practical checklist of the moves that still count, with every 2026 figure taken from official IRS releases rather than last year's memory.
Work through it in order. The first item is a date, because missing it undoes everything else.
The one deadline you cannot move: 15 January 2027
The fourth-quarter estimated tax payment for 2026 income earned from 1 September through 31 December is due on 15 January 2027. It is the last of the four instalments, and it is the one people most often shortchange because December is busy and cash feels tight. Skipping it, or paying too little across the year, triggers the underpayment penalty, which is really daily interest on what you paid late.
You avoid the penalty by hitting a safe harbor: pay the smaller of 90% of your 2026 tax or 100% of your 2025 tax (110% if your 2025 adjusted gross income topped $150,000), spread across the four due dates. Run your projected 2026 numbers now with our quarterly estimated tax calculator so the January payment is a known figure and not a guess. One quiet advantage of doing year-end planning at all: every deduction below cuts the tax you owe, which lowers the estimated payment you need to send.
Know your 2026 self-employment tax before you plan around it
Self-employment tax is the part that surprises new business owners, and it dwarfs income tax at modest profit levels. For 2026 it is 15.3% of 92.35% of your net profit: 12.4% for Social Security plus 2.9% for Medicare. The Social Security portion applies only to the first $184,500 of net earnings, so the most Social Security tax any self-employed person pays in 2026 is $22,878. The 2.9% Medicare portion has no cap, and an Additional Medicare Tax of 0.9% applies to earnings above $200,000 for single filers or $250,000 for a married couple filing jointly. The IRS explains the mechanics on its self-employment tax page.
Two facts shape everything that follows. First, you deduct half of your self-employment tax above the line, so it never disappears entirely but it does soften. Second, self-employment tax is charged on net profit, which means a legitimate deduction that lowers profit saves you income tax and self-employment tax at once. Estimate yours with the self-employment tax calculator before you decide how aggressively to spend or defer.
Retirement contributions: the biggest lever you have left
Nothing else on this list shelters as much income as a self-employed retirement plan, and the deadlines are generous. For 2026 the IRS set the employee deferral limit at $24,500, up from $23,500, per its 2026 contribution limits release. The catch-up is $8,000 for those aged 50 and over, and a higher $11,250 catch-up applies at ages 60 to 63.
A Solo 401(k) is usually the winner for a one-person business because you contribute in two capacities. As the employee you can defer up to $24,500. As the employer you can add a profit-sharing contribution of up to 25% of your compensation. The combined total cannot exceed $72,000 for 2026 (or $80,000 with the age-50 catch-up), and because the employee deferral is a flat dollar amount, a Solo 401(k) lets you shelter far more at lower profit levels than a SEP can. The one catch: the plan itself generally must be established by 31 December 2026, even though the contributions can be funded later, up to your filing deadline including extensions.
A SEP-IRA is simpler to open, can be set up as late as your extended filing deadline, and accepts an employer contribution of up to 25% of net self-employment earnings, capped at the same $72,000. It has no employee deferral and no catch-up, so at a given profit it shelters less than a Solo 401(k). If you have not opened either plan yet, decide before 31 December: open the Solo 401(k) now if you can, and fall back to the SEP if you run out of time.
The QBI deduction and the income line that controls it
The qualified business income deduction under Section 199A lets most pass-through owners deduct up to 20% of their qualified business income, and the One Big Beautiful Bill Act made it permanent. What matters for year-end planning is the taxable-income threshold, because below it the deduction is clean and simple, and above it the rules tighten. For 2026, Revenue Procedure 2025-32 sets the threshold at $201,750 for single filers and $403,500 for married couples filing jointly, with a phase-in range of $75,000 above that for single filers and $150,000 for joint filers.
Stay below the threshold and you get 20% of QBI regardless of your trade. Rise into the phase-in range and a specified service business (consulting, law, health, accounting and similar fields) starts to lose the deduction, while other businesses face a wage-and-property limit. This is precisely why the retirement contributions above are so powerful late in the year: a Solo 401(k) contribution that pulls your taxable income back under $201,750 can rescue a QBI deduction worth thousands. Model the interaction with the QBI deduction calculator before you finalize any December contribution.
S corporation owners: run reasonable payroll before 31 December
If your business is taxed as an S corporation, the year-end priority is different. You save self-employment tax by taking part of your profit as distributions rather than salary, but the IRS requires that you first pay yourself a reasonable salary through payroll, subject to Social Security and Medicare tax. Underpay that salary and you invite reclassification, back taxes and penalties.
The problem in Q4 is timing: payroll for 2026 has to actually run, with taxes withheld and deposited, by 31 December 2026. If you have been under-paying yourself all year, or paying nothing, you may need to run a large catch-up payroll or bonus run in December so your W-2 reflects a defensible reasonable salary for the full year. Wait until January and the wages count for 2027, not 2026. Use the S-corp reasonable salary calculator to set a figure you can defend, then confirm your payroll provider can process it before the final pay date of the year.
Time your income and expenses across the 31 December line
Most self-employed people file on the cash basis, which hands you a simple, legal timing lever: income counts when you receive it, and expenses count when you pay them. If 2026 was a strong year and you expect 2027 to be similar or lighter, you can defer income by sending December invoices so they arrive and pay in January, and accelerate expenses by buying supplies, renewing subscriptions, prepaying certain costs or settling vendor bills before 31 December.
Reverse the logic in a low year. If 2026 profit is unusually thin and you expect a much better 2027, it can pay to pull income into December and push deductible spending into January, so the deductions land against higher-taxed income next year. The point is not to spend money you do not need to spend. A dollar deducted saves only your marginal tax rate, so buying something useless to cut tax still leaves you poorer. Time the purchases you were going to make anyway.
Section 179: write off equipment placed in service by year end
When you do need equipment, Section 179 lets you deduct the full cost in the year you place it in service rather than depreciating it over many years. For 2026 the expensing limit is $2,560,000, with the deduction beginning to phase out once you place more than $4,090,000 of qualifying property in service, figures confirmed in the IRS 2026 inflation adjustments. Qualifying property includes machinery, computers, off-the-shelf software and business vehicles over 6,000 pounds, with a $32,000 cap on the amount deductible for a sport utility vehicle.
The words "placed in service" are what make this a year-end item. The asset must be delivered, installed and ready for its intended use by 31 December 2026, not merely ordered or paid for. A machine sitting on a loading dock on New Year's Eve does not qualify. Remember too that the deduction lowers net profit, so it cuts self-employment tax as well as income tax, and it cannot create a business loss beyond your active income. Bonus depreciation can pick up any cost that Section 179 will not.
The HSA: the deduction hiding in your health plan
If you carry a qualifying high-deductible health plan, a health savings account is one of the few deductions that requires no spending at all. For 2026 you can contribute up to $4,400 for self-only coverage or $8,750 for family coverage, with an extra $1,000 catch-up once you reach age 55. The contribution is deductible above the line, the money grows tax-free, and withdrawals for qualified medical costs are never taxed.
The deadline here is friendlier than most: you have until your filing deadline in April 2027 to fund a 2026 HSA. That makes it a useful backstop if a late-year income spike leaves you needing one more deduction after 31 December. Confirm you were HSA-eligible for the months you are counting, because contribution room is tied to the months you held qualifying coverage.
Your 2026 numbers at a glance
Keep these figures in front of you as you work through the checklist. Every one is drawn from the IRS releases and Revenue Procedure 2025-32 for tax year 2026.
| Item | 2026 figure |
|---|---|
| Q4 2026 estimated tax due date | 15 January 2027 |
| Self-employment tax rate | 15.3% (12.4% Social Security + 2.9% Medicare) on 92.35% of net profit |
| Social Security wage base | $184,500 (max Social Security portion of SE tax: $22,878) |
| Additional Medicare Tax | 0.9% above $200,000 single / $250,000 married filing jointly |
| Solo 401(k) / 401(k) employee deferral | $24,500 (catch-up $8,000 at 50+, $11,250 at 60-63) |
| Total defined-contribution limit (Solo 401(k), SEP) | $72,000 |
| HSA contribution limit | $4,400 self-only / $8,750 family ($1,000 catch-up at 55+) |
| Section 179 expensing limit | $2,560,000 (phase-out begins at $4,090,000) |
| QBI deduction | 20%; threshold $201,750 single / $403,500 married filing jointly |
| Standard deduction | $16,100 single / $32,200 married filing jointly / $24,150 head of household |
Putting the checklist to work
The order matters more than any single move. Start by projecting your 2026 profit and the tax it produces, so you know the size of the problem. Fund retirement first, because it shelters the most and often drops you under the QBI threshold as a bonus. If you are an S corporation, get the reasonable-salary payroll run before the last pay date of December. Then use income and expense timing, Section 179 on equipment you actually need, and the HSA to fine-tune the result. Finally, recompute the January estimated payment against the lower tax the plan produces.
None of this is aggressive, and none of it is new. It is simply the ordinary tax code used on time. The self-employed who pay more than they should are rarely the ones who took a risky position: they are the ones who let 31 December pass without doing the arithmetic. Do it in October, and December is calm.
This article is general information, not tax advice for your specific situation. Confirm the current-year figures on irs.gov and check with a qualified tax professional before acting on any of these moves.