Tax year 2026 figures. Last updated .
The year of the sale, not the year you file
A capital gain belongs to the tax year in which the sale closes. Sell in December 2026 and the gain is 2026 income, reported on the return you file in early 2027.
That distinction matters for timing decisions: a sale that completes on 2 January falls into the following tax year entirely, which can be worth a great deal if it moves the gain into a year with less other income.
But the money is often due long before you file
The United States operates a pay-as-you-go system. Employment income is handled by withholding; a capital gain usually is not. If you expect to owe $1,000 or more after withholding, federal law wants the tax in quarterly instalments during the year, not in a single payment the following April.
This is the part that catches people. Waiting until filing to pay a large gain is not merely late — it can trigger an underpayment charge for each quarter the money was outstanding, even if the return is filed on time and paid in full.
The four dates
Estimated payments are generally due on 15 April, 15 June, 15 September and 15 January of the following year. When a date falls on a weekend or holiday it moves to the next business day.
The instalment that matters for a sale is the one covering the quarter in which the sale closed. A property sale completing in August belongs to the September instalment, not the following April.
The safe harbour, which is the part worth knowing
You do not have to predict your final liability precisely. IRC § 6654 provides a safe harbour: no underpayment penalty applies if your withholding and estimated payments together reach either 90% of this year’s tax or 100% of last year’s.
If your prior-year adjusted gross income exceeded $150,000, the second figure rises to 110% of last year’s tax.
The prior-year test is the useful one after an unexpectedly large gain. Pay 100% (or 110%) of what you owed last year and you are protected from penalties however large this year’s liability turns out to be — you still owe the balance at filing, but without the additional charge.
There is also a de minimis rule: no penalty applies at all if the tax owed after withholding is under $1,000.
Withholding is an alternative to estimating
Payments withheld from wages are treated as made evenly through the year regardless of when they were actually withheld. So increasing withholding late in the year — by adjusting a W-4 after a large sale — can cover an earlier quarter’s shortfall in a way that a late estimated payment cannot.
For someone with both a salary and a one-off gain, that is often simpler than filing quarterly vouchers.
Property sales have their own wrinkle
The federal timing question is separate from closing-table withholding, which several states impose on property sales and which is collected by the closing agent at the point of sale. That withholding is a prepayment against a state liability, not the tax itself, and it is not modelled by this calculator.
Non-resident sellers of US real property face FIRPTA withholding federally, which is again a prepayment rather than the final tax. Both are reconciled on the relevant return.
When the gain lands late in the year
The quarterly system assumes income arrives evenly. A single large sale in December does not, and paying a quarter of the resulting tax in each of the four instalments would mean paying tax in April on income not earned until December.
The annualised income instalment method exists for this. It lets you compute each instalment on income actually received by that point in the year, so a December gain is due with the January instalment rather than spread backwards. It requires more work at filing — a separate schedule showing the annualisation — and it is worth it when the timing is lopsided.
The simpler alternative for many people: rely on the prior-year safe harbour, which does not care when the income arrived at all.
What the penalty actually costs, and state deadlines
The underpayment charge is not a flat fine. It is computed like interest, at a rate the IRS sets quarterly, running on each instalment’s shortfall from its due date until it is paid. A small shortfall corrected early costs very little; a large one left until filing accrues for months.
That is worth knowing because it changes the decision. If you are slightly under, the cost of being wrong is small. If you have realised a seven-figure gain and paid nothing during the year, it is not.
States run their own estimated payment regimes with their own thresholds and, in some cases, different due dates. This calculator models neither federal nor state estimated payments — it computes the liability, not the schedule on which it falls due.
Why this catches retirees particularly
Someone drawing a pension and living partly off a portfolio has no employer withholding to lean on, and often a lumpy income pattern that makes the quarterly system awkward.
Pension and annuity payments can have tax withheld, and Social Security can too, but investment income generally cannot. So a year with an unusually large realisation — downsizing a property, liquidating a holding to fund care, rebalancing after a long hold — arrives with no tax withheld against it at all.
The prior-year safe harbour is particularly useful here precisely because retirement income is often stable while gains are not. Paying 100% of last year’s tax, or 110% where prior-year AGI exceeded $150,000, protects against the penalty regardless of how large this year’s realisation turns out to be.
The alternative worth knowing about is voluntary withholding on pension distributions, which is treated as paid evenly across the year however late it is taken — the same property that makes late W-4 adjustments useful for employees.
A simple decision rule
For most people with a one-off gain, the whole question reduces to three checks.
First: will you owe $1,000 or more after withholding? If not, nothing is due before filing and the de minimis rule protects you.
Second: will your withholding plus any estimated payments reach 100% of last year’s total tax — or 110% if your prior-year AGI exceeded $150,000? If yes, the safe harbour applies and no penalty can arise however large this year’s bill turns out to be. You will still owe the balance at filing.
Third: if neither holds, make an estimated payment for the quarter the sale closed in, or increase withholding, which counts as paid evenly across the year regardless of when it happened.
That is the entire mechanism for a typical case. The complications — annualising, state schedules, penalty computation — matter mostly when the amounts are large or the timing is awkward.
If you have already missed a payment
The charge accrues per instalment from its own due date, so a shortfall corrected in September costs less than the same shortfall corrected in April.
Paying as soon as you notice stops the accrual on that amount. Increasing withholding for the remainder of the year can also help, because withholding is treated as paid evenly across the year rather than when it actually happened — which can retrospectively cover an earlier quarter in a way a late estimated payment cannot.
Form 2210 is where any penalty is computed at filing, and it is also where the annualised income method and the exceptions are claimed.
Worked example
Computed by the same engine that powers the calculator, at the moment this page was built — not typed in by hand. Open the derivation to see every rule and citation.
A gain large enough to require an estimated payment
A single filer with $95,000 of wages sells shares in August at a $180,000 long-term gain. Payroll withholding covers the salary but nothing else.
- Taxable income after deduction
- $258,900
- Taxable gain
- $180,000
- Tax owed without the sale
- $12,070
- Tax the sale added
- $29,850
- of which net investment income tax
- $2,850
- Total federal tax
- $41,920
- Effective rate on the gain
- 16.58%
The tax on the gain is far more than $1,000, so it needs an estimated payment for the September quarter — or a matching increase in withholding — rather than waiting for April. The safe harbour based on last year’s tax is the simplest way to be sure the amount is enough.
Caveats on this example (1)
- Net investment income was derived from capital gains and qualified dividends only. Interest, non-qualified dividends, rents, royalties and passive business income inside ordinaryIncome are also net investment income under IRC 1411(c) and are not counted here.
Show the working — 12 steps, each with its citation
- Net short-term capital gain or loss for the year$0
Assets held one year or less. Taxed at ordinary rates if a net gain.
- Net long-term capital gain or loss for the year$180,000
Assets held more than one year. Eligible for the 0/15/20% rates.
- Standard deduction-$16,100
Adjusted gross income of $275,000 less $16,100.
- Taxable income$258,900
The figure the rate tables and the capital gain ceilings are both measured against.
- Ordinary income taxed at 10%$1,240
$12,400 of taxable income between $0 and $12,400.
- Ordinary income taxed at 12%$4,560
$38,000 of taxable income between $12,400 and $50,400.
- Ordinary income taxed at 22%$6,270
$28,500 of taxable income between $50,400 and $78,900.
- Ordinary income stacked below the long-term gain$78,900
Long-term gain is taxed by reference to where it sits ON TOP of $78,900 of other taxable income, not from the bottom of the rate table.
- Long-term gain taxed at 15%$27,000
Gain between the $49,450 zero-rate ceiling and the $545,500 15% ceiling.
- Net investment income tax threshold$200,000
Modified AGI of $275,000 against the $200,000 threshold for a single filer. This threshold is statutory and is not adjusted for inflation.
- Net investment income tax at 3.8%$2,850
3.8% of $75,000, the lesser of net investment income ($180,000) and the amount by which modified AGI exceeds the threshold ($75,000). Here the binding figure is the excess of modified AGI over the threshold.
- Total tax$41,920
$41,920 on $275,000 of total income, an effective rate of 15.24%.
Run your own figures
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