Chapter 4: Capital gains and the 12-month line

Two investors buy the same stock on the same day and watch it climb the same $10,000. One sells after eleven months, the other waits until month thirteen. At the top of the income scale, the impatient seller's federal rate is 40.8% and the patient seller's is 23.8%. Same investment, same profit, and the calendar moved seventeen points of tax. No other line in the code pays this well for doing nothing.

A capital gain is the profit when you sell an investment for more than your cost basis, what you paid for it. Gains come in two flavors. A short-term gain, on anything held one year or less, is taxed at your ordinary bracket rates from Chapter 1. A long-term gain, on anything held more than one year, gets its own gentler schedule.

The long-term schedule for 2026 has three rates, set by your taxable income:

One surcharge sits on top. The net investment income tax (NIIT) adds 3.8% to gains, dividends, and interest once your modified AGI passes $200,000 single or $250,000 joint. Those thresholds are frozen in the law and never adjust for inflation, so every year they quietly catch more people. Stack the pieces and you get the signature numbers above: 37% plus 3.8% for the impatient, 20% plus 3.8% for the patient.

The clock starts the day after you buy

The line between the two schedules is exact, and people lose real money standing one day on the wrong side of it. Your holding period starts the day after your trade date, the day the order executes, and the sale must come more than one year after the purchase. Settlement dates, the two days later when cash actually moves, never matter.

Buy shares on March 3, 2026, and sell them on March 3, 2027, and you held for exactly one year. Exactly one year is not more than one year, so the gain is short-term and taxed at your full bracket rate. Sell on March 4, 2027, and it is long-term. One day, and on a $50,000 gain in the 35% bracket the difference is $50,000 at 35% versus 15%, which is $10,000 of tax.

So treat month twelve as a no-trade zone for winners. If you want to sell something you bought last spring, pull up the trade confirmation, find the exact date, and add a year and a day. Brokers report holding periods on Form 1099-B, but they report what you did, not what you should have done.

Gains stack on top of your ordinary income

The 0/15/20 table above creates the most common confusion in investor taxes: which rate applies when you have both wages and gains. The answer is a stacking rule. Your ordinary taxable income fills the measuring cup first, starting from $0. Your long-term gains pour in on top. Each dollar of gain is then taxed by where it sits in the cup: 0% below $49,450 for a single filer, 15% above that, and so on. Gains never push your wages into a higher bracket, and wages decide where your gains start stacking.

Jamie from Chapter 1 makes it concrete. Jamie's $90,000 salary leaves $73,900 of ordinary taxable income, which fills the cup from $0 to $73,900. Suppose Jamie sells fund shares for a $5,000 long-term gain. The gain stacks from $73,900 to $78,900, entirely above the $49,450 line where the 0% rate ends, so all of it is taxed at 15%: a $750 bill. Had Jamie sold a month too early, the same $5,000 would be short-term, taxed at the 22% bracket rate for $1,100. Waiting saved $350 on a small sale, and the percentage gap only widens in higher brackets.

The stacker above does this arithmetic live. Enter your wages and a gain, and watch the gain pour through the 0%, 15%, and 20% layers, with the NIIT line marked. Ten minutes with it will teach you more than most tax software ever will.

A cousin of this rule covers dividends. Qualified dividends, most regular dividends from US companies and funds, are taxed at the same friendly 0/15/20 stack. Ordinary dividends, including most REIT payouts and the interest-like distributions from bond funds, are taxed at full bracket rates. The qualification test is again a calendar: you must hold the share more than 60 days around the dividend date. Buy a stock days before its payout and flip it after, and the dividend you captured is taxed like wages.

The 0% bracket is real money

Look back at that table. A married couple can have nearly $98,900 of taxable income and pay 0% federal tax on long-term gains sitting under that line. This is the basis of 0% gain harvesting: in a low-income year, sell appreciated investments on purpose, pay nothing federal, and buy them right back. The wash-sale rule in Chapter 5 only applies to losses, so an immediate rebuy is fine, and it resets your cost basis higher, shrinking every future gain.

The classic low-income window is the gap between retirement and Social Security, which is exactly where Carlos and Elena live.

Trace the test the way the IRS does, because this is where people slip: the gains stack on top of the couple's other income. The question is never "are our gains under $98,900" but "is our ordinary taxable income plus our gains under $98,900." If Carlos and Elena had realized $70,000 instead, the first $59,100 would still be at 0% and only the $10,900 spilling over the line would be taxed, at 15%, for $1,635. The bracket works like every other bracket: crossing it costs you only the dollars above it.

Three honest caveats. Most states tax these gains even when the IRS does not. The realized gains still raise AGI, which can shrink a marketplace health insurance subsidy before Medicare and, through the two-year lookback covered in Chapter 9, raise Medicare premiums later. And the same headroom is also the raw material for Roth conversions, so Chapter 9's job is choosing which use of the window pays more.

The two endgames for a winner you never sell

Because gains are taxed only at sale, an appreciated share has two tax-free exits worth knowing about now, decades early.

The first is the step-up in basis. When you die holding an investment, your heirs' cost basis resets to its market value on that date, and every gain of your lifetime simply vanishes from the tax system. A $20,000 fund position that grew to $300,000 passes with $0 of taxable gain. This is why Chapter 5 thinks hard before realizing gains late in life.

The second is giving shares away. Donate appreciated stock to charity and nobody ever pays the gains tax; Chapter 12 runs that math. Gifting shares to a person works differently: your basis travels with the gift, so the recipient inherits your future tax bill, though a recipient in the 0% zone may pay nothing when they sell. Gifts to children collide with special rules on a child's investment income, and a CPA earns their fee here.

The December fund trap

One more calendar trap, and this one bites people who did nothing but buy and hold. By law, a mutual fund must pass its net realized gains through to shareholders, and most do it in one December distribution. The fund's trading generated those gains, so you can owe tax on profits you never saw.

The ugliest version is buying a fund in late autumn. Put $50,000 into a mutual fund in your taxable account on November 28, and on December 15 the fund might distribute 9% of its value, $4,500, as a capital gain payout. The share price drops by the same amount, so you are not a penny richer, but the $4,500 lands on your 1099 and at 15% costs you about $675. You just paid tax on gains other shareholders enjoyed for years. ETFs, by contrast, shed appreciated shares through in-kind redemptions, so broad index ETFs distribute capital gains rarely and in small amounts. This is a real reason index ETFs dominate taxable accounts, and none of it matters inside an IRA or 401(k), where distributions are invisible.

Where people go wrong

• Selling a winner in month eleven or twelve. The schedule changes at one year plus a day, and the gap can run to 17 points of tax.
• Counting the year from the settlement date. Trade date to trade date, plus a day, is the only count that matters.
• Thinking gains under the 0% threshold means total gains. The test is ordinary taxable income plus gains; gains stack on top.
• Skipping the 0% harvest in a gap year. A layoff year, a sabbatical, or the window before Social Security can make five figures of gains free, and the headroom expires every December 31.
• Buying a mutual fund in a taxable account the week before its December distribution. That is a tax bill for other people's gains.

Key takeaways

• Short-term gains are taxed like wages; long-term gains at 0/15/20%. At the top the gap is 40.8% versus 23.8%.
• Long-term means more than one year after the trade date. Exactly one year fails the test; add a year and a day.
• Gains stack on top of ordinary taxable income: wages fill the brackets first, and each gain dollar is taxed where it lands relative to $49,450 single / $98,900 joint, with NIIT adding 3.8% above $200,000 / $250,000 MAGI.
• Carlos and Elena realized $40,000 of long-term gains on $39,800 of ordinary taxable income and paid $0 federal, then rebought to reset their basis.
• Step-up at death and charitable gifts of shares erase gains entirely; check mutual fund distribution estimates before any Q4 purchase in taxable.

Sources: IRS: 2026 Tax Inflation Adjustments · IRS: Publication 550, Investment Income · IRS: Federal Income Tax Rates and Brackets · Tax Foundation: 2026 Tax Brackets · Finvest Personal Finance Guide