Net Investment Income Tax (NIIT) Calculator United States

What the NIIT calculator does

The calculator estimates the 3.8% Net Investment Income Tax (NIIT), a federal surtax on investment income for people whose income is above a set threshold. It compares your net investment income with how far your modified adjusted gross income (MAGI) is above the threshold for your filing status, and applies the tax to the smaller amount. The NIIT comes on top of ordinary income tax and capital gains tax and is easy to overlook.

The tax comes from Section 1411 of the Internal Revenue Code and has applied since 2013. Many people first notice it on their return after a strong year for dividends or a large sale of stock or property. Estimating it in advance avoids that surprise.

How the 3.8% tax is calculated

• The calculator takes your net investment income: interest, dividends, capital gains, rents and similar earnings covered by the tax.
• It finds how far your MAGI is above the threshold. Below the threshold, the amount is zero and no NIIT is due.
• It multiplies the smaller of those two amounts by 3.8%.

So two people with the same investment income can owe very different amounts. Someone just over the threshold pays tax only on that small excess; someone far above it pays tax on all their investment income.

Understanding the MAGI thresholds

The thresholds are $200,000 for single filers and heads of household, $250,000 for married couples filing jointly and $125,000 for married couples filing separately. They are fixed by law and do not rise with inflation, so more households cross them each year even when their real income has barely changed.

For this tax, MAGI is adjusted gross income plus certain add-backs, mainly some foreign earned income. Without foreign income, MAGI and AGI are usually the same. You enter your MAGI directly.

What counts as net investment income

Net investment income generally includes taxable interest, dividends, capital gains from selling investments, rental and royalty income, non-qualified annuity income and income from businesses in which you do not materially participate, minus expenses allocable to that income, such as investment interest and certain rental costs.

Wages and self-employment income are excluded, though they raise your MAGI and can push you over the threshold. Distributions from qualified retirement plans such as 401(k)s and traditional IRAs are excluded, as is tax-exempt municipal bond interest.

Why a large sale can trigger the tax

Selling appreciated stock, a rental property or an interest in a passive business can produce a large capital gain, which raises your net investment income and your MAGI in the same year. That can push you over the threshold even if your usual income keeps you below it.

Estimating the NIIT before you sell shows the full tax cost of the sale, alongside ordinary or capital gains tax and any state tax. Spreading a sale over two tax years, or timing it around other income, can sometimes reduce the NIIT, depending on your overall situation.

Reading your result

The calculator shows your MAGI over the threshold, the amount subject to the tax and the NIIT itself. The base amount tells you whether the tax is limited by your investment income or by how far your MAGI is over the threshold.

Your actual liability depends on the full detail of your return, including deductions, other income and expenses allocable to your investment income. For a large or complex situation, confirm the figure with a qualified tax professional.

When to use this tool

Use it when planning the sale of a large asset, when your dividends and interest have grown, or when a bonus or business income might push your MAGI over the threshold. Year-end, while you can still adjust the timing of income and deductions, is a good time to check. Because the thresholds do not rise with inflation, check again each year.

This is general information only and not financial advice.

About This NIIT Calculator

Use this NIIT calculator to get an instant, accurate estimate. The figures follow official IRS guidance and every rate and threshold is kept up to date for 2026. Adjust the inputs above to match your situation, or browse all our calculators to plan your next step.