Form 8960 line by line: the 3.8% net investment income tax

The short answer. Form 8960 is where you compute the Net Investment Income Tax (NIIT), a 3.8% surtax under Internal Revenue Code (IRC) section 1411. It applies the 3.8% rate to the smaller of two figures: your net investment income, or the amount by which your modified adjusted gross income (MAGI) tops a fixed threshold — $250,000 for married couples filing jointly, $200,000 if you are single, $125,000 if married filing separately. If your MAGI sits at or below that line, you owe nothing, regardless of how much investment income you earned.

What the form is actually doing

It helps to read Form 8960 as a single sentence rather than a stack of boxes. The form gathers everything the tax code treats as investment income, strips out the expenses tied to earning it, measures how far your income runs past a threshold, and then taxes whichever of those last two numbers is smaller. That “smaller of” mechanic is the whole point — and the reason two people with identical portfolios can owe wildly different amounts, or nothing at all. The glossary entry on the net investment income tax covers what the surtax is in the abstract; this guide stays on the form itself, walking its three parts in the order the numbers flow.

The 3.8% rate never changes, and neither do the thresholds. They were set by statute and, unlike most figures in the tax code, they are not indexed for inflation. A married couple crossed the $250,000 line in 2013 at the same dollar figure they cross it today. As wages and portfolios drift upward year after year, the practical effect is a quiet expansion: more households land inside the NIIT each filing season without any deliberate change in the law. If you have edged near the threshold before, assume you are closer this year.

Part I: building gross investment income

The first part of the form is an inventory. You list the categories of income the statute counts as investment income: taxable interest, dividends, annuities, royalties, rents, and net capital gains — including the gains from selling investments. It also sweeps in income from passive businesses and from trading activity. Most of these figures are ones you have already reported elsewhere on your return; Form 8960 simply re-collects them in one place. Where dividends and interest are concerned, the amounts trace straight back to the boxes on your broker statements, which is why it helps to understand how the figures on a 1099-DIV are split across boxes before you transcribe them.

Just as important is what Part I leaves out. Wages and salaries do not count. Neither does self-employment income, Social Security benefits, or tax-exempt municipal bond interest. Distributions from qualified retirement plans — an IRA or a 401(k) — are excluded too, which surprises retirees who assume any portfolio withdrawal is “investment income.” And income from a business in which you materially participate is non-passive, so it stays out as well. The line that separates passive from non-passive business income does real work here: the same dollars can be inside or outside the NIIT depending on how involved you are.

Part II: deductions down to net

Part I gives you a gross figure. Part II turns it into a net one by subtracting the expenses allocable to that investment income. The form allows the costs genuinely tied to earning the income to reduce it: investment interest expense, the portion of your state, local, and foreign income taxes that is allocable to net investment income, and miscellaneous investment expenses. The word “allocable” matters — you are not deducting your entire state tax bill, only the slice attributable to the investment income itself.

When you finish Part II, you arrive at the total net investment income line. This is the (a) side of the “smaller of” comparison — the real number the tax can reach, after expenses. For a filer with heavy investment costs, net investment income can run meaningfully below the gross figure, and since the 3.8% is eventually applied to a “smaller of,” every allocable dollar of deduction can be worth chasing.

Part III: MAGI, the threshold, and the 3.8%

The final part is where the two halves meet. Part III computes your MAGI, then subtracts the threshold for your filing status to produce the MAGI excess — the amount by which your income runs past the line. For most domestic filers, MAGI is effectively the same as your adjusted gross income (AGI); the statutory definition is AGI with certain foreign-earned-income add-backs, which only matter if you have that foreign income. If you want to see how AGI and MAGI are constructed from the ground up, the explainer on building AGI from a W-2 walks the chain.

With both figures in hand, the form takes the smaller of your net investment income or that MAGI excess, applies 3.8%, and lands on the tax — the final line of the form. The consequence is worth sitting with. A single filer with $300,000 of MAGI runs $100,000 past the $200,000 threshold, but if only $20,000 of that is net investment income, the 3.8% applies to $20,000, not $100,000. Reverse it — modest income just over the line, large investment gains — and the MAGI excess becomes the binding number instead. The tax is always governed by whichever is the bottleneck.

How it stacks on what you already owe

The 3.8% does not replace your other taxes on investment income; it rides on top of them. A long-term capital gain already faces its own rate schedule, and the NIIT is layered above that for filers over the threshold — which is why a high earner’s true marginal cost on a gain can be several points higher than the headline capital-gains rate suggests. The mechanics of those underlying rates, and how the surtax sits on them, are the subject of the capital-gains tax guide; Form 8960 is simply the worksheet that adds the final layer.

If there is one habit to carry into your return, it is to check your MAGI against the threshold before you assume the form applies. The thresholds do not move, your income probably does, and the entire calculation collapses to zero the moment your MAGI lands at or below the line for your filing status. Run that one comparison first, and the rest of Form 8960 either becomes a short, mechanical exercise — or disappears entirely.