Introduction to the 3.8% Net Investment Income Tax
The Net Investment Income Tax, usually shortened to NIIT, is a separate 3.8% federal tax that may apply when a taxpayer has both net investment income and modified adjusted gross income above a filing-status threshold. This calculator estimates that tax for an individual using the core lesser-of rule. It is useful for testing a projected capital gain, dividend increase, rental-income change, or other investment-income scenario before a return is prepared.
NIIT is easy to misunderstand because crossing the income threshold does not automatically place a 3.8% tax on all income. The calculation first measures how far modified adjusted gross income, or MAGI, exceeds the applicable threshold. It then compares that excess with net investment income. Only the smaller amount becomes the NIIT base. If either amount is zero, the estimate is zero.
This page uses annual US-dollar amounts. It does not prepare a tax return or determine whether every item belongs in MAGI or net investment income. Instead, it gives you a transparent planning estimate once those two amounts and the filing status are known.
How to use the NIIT calculator with your tax figures
Choose the filing status expected on the federal return, then enter annual MAGI and annual net investment income. Use full dollar amounts rather than amounts stated in thousands. For example, enter 260000 rather than 260 for $260,000. Cents are accepted, although planning estimates are often entered as whole dollars.
- Select Single, Married filing jointly, Married filing separately, or Head of household.
- Enter modified adjusted gross income for the same tax year and filing scenario.
- Enter total net investment income after the deductions you have determined are properly allocable to that income.
- Select Calculate NIIT to display the estimated taxable base and 3.8% tax.
- Change one assumption at a time if you want to understand what drives the result.
The calculator uses thresholds of $200,000 for single and head-of-household filers, $250,000 for married couples filing jointly, and $125,000 for married individuals filing separately. These statutory individual thresholds are not automatically indexed for inflation. A qualifying widow or widower generally uses the married-filing-jointly threshold, but that status is not separately offered here, so confirm the correct treatment before relying on an estimate.
Choosing MAGI and net investment income inputs
Modified adjusted gross income for NIIT begins with adjusted gross income and generally includes an adjustment for certain foreign earned income exclusions. For many taxpayers who do not use those exclusions, NIIT MAGI may match AGI, but that is not universal. Use a tax-return projection or professional calculation when the adjustment applies rather than assuming that every definition of MAGI is the same.
Net investment income can include taxable interest, ordinary and qualified dividends, capital gains, nonqualified annuity income, royalties, rents, and income from businesses treated as passive activities. It may also include income from trading financial instruments or commodities. Properly allocable expenses can reduce the net amount. Tax-exempt municipal-bond interest, wages, unemployment compensation, Social Security benefits, and distributions from many qualified retirement arrangements generally are not net investment income merely because they increase cash available to invest.
Some transactions require more care. A gain from selling a primary residence may be excluded from gross income under the home-sale exclusion and therefore may not enter net investment income to that extent. Business-sale gains can depend on whether the business was passive to the owner. Rental activity, materially participating businesses, installment sales, and controlled foreign investments can also require rules that this simple form cannot classify for you.
Keep the two dollar inputs internally consistent. If MAGI includes a proposed $50,000 taxable gain, net investment income should reflect the applicable portion of that same gain. Do not mix a current-year MAGI figure with investment income from a different year, and do not enter gross sale proceeds when only gain belongs in income.
Formulas for the NIIT taxable base and estimated tax
Let M represent modified adjusted gross income, T the threshold for the selected filing status, and N net investment income. The calculator first finds the nonnegative amount of MAGI above the threshold. It then limits that amount to net investment income. The resulting NIIT base, B, is:
The estimated tax is 3.8% of that base:
This lesser-of structure creates two possible limits. When excess MAGI is smaller than net investment income, excess MAGI limits the tax base. When net investment income is smaller, investment income is the limit. Once the entire net investment income amount is already in the base, increasing MAGI alone will not increase NIIT unless net investment income also rises.
Worked example: a single filer with $260,000 of MAGI
Suppose a single filer expects $260,000 of MAGI and $40,000 of net investment income. The single-filer threshold used by the calculator is $200,000. MAGI therefore exceeds the threshold by $60,000. The calculator compares that $60,000 excess with $40,000 of net investment income and uses the smaller number.
The NIIT base is $40,000, and the estimate is $40,000 × 3.8%, or $1,520. The tax is not $2,280, because that would apply 3.8% to the entire $60,000 MAGI excess even though only $40,000 of net investment income is available to enter the base.
If the same filer had $260,000 of MAGI but no net investment income, the estimate would be zero. If the filer had $40,000 of net investment income but MAGI of only $195,000, the estimate would also be zero. Both parts of the rule must be present for a positive result.
Comparison table: NIIT sensitivity above the single-filer threshold
The following scenarios keep filing status set to Single and net investment income fixed at $80,000. Only MAGI changes. Because $80,000 is greater than every excess-MAGI amount shown, excess MAGI is the limiting value in all three rows.
| MAGI |
MAGI above $200,000 |
Net investment income |
NIIT base |
Estimated NIIT |
| $220,000 |
$20,000 |
$80,000 |
$20,000 |
$760 |
| $240,000 |
$40,000 |
$80,000 |
$40,000 |
$1,520 |
| $260,000 |
$60,000 |
$80,000 |
$60,000 |
$2,280 |
The progression is linear because each additional dollar of excess MAGI adds one dollar to the base while net investment income remains high enough to cover it. If net investment income were only $30,000, the base would stop at $30,000 and the estimated NIIT would stop at $1,140 even if MAGI continued to rise.
How to interpret your NIIT result
The result panel reports both the taxable NIIT base and the estimated tax. The base explains more than the tax number alone: compare it with net investment income and with MAGI above the threshold to see which side of the lesser-of test is controlling. A zero result ordinarily means MAGI did not exceed the selected threshold, net investment income was zero, or both conditions applied.
A positive estimate is an additional federal tax estimate, not the total income tax on the investments. Dividends and gains may still be subject to regular income-tax or capital-gain rates, and state taxes can apply separately. NIIT also differs from the Additional Medicare Tax on earned income, even though both are sometimes described as Medicare-related surtaxes.
For planning, run separate scenarios rather than combining unlike amounts into a single total. You might compare a sale this year with a sale next year, or test how a larger deductible investment expense affects net investment income. Keep a written note of each scenario because this page displays the current calculation but does not save or download tax records.
Limitations and assumptions of this NIIT estimate
This calculator applies the individual NIIT thresholds and the basic 3.8% formula. It assumes the MAGI and net investment income figures you provide are already correctly classified. It does not calculate adjusted gross income, capital gains, passive-activity limitations, investment-expense deductions, foreign-income adjustments, or home-sale exclusions.
- Individuals only: estates and trusts use a different threshold framework and are not supported.
- Federal estimate: state income taxes and other federal taxes are outside the result.
- No return-year determination: confirm current law and filing-status eligibility for the year being planned.
- No negative investment income carryover: the form accepts nonnegative net investment income and does not model losses or their use elsewhere.
- Rounded display: the result is shown to cents, while a filed return may use whole-dollar conventions and additional worksheets.
Use the output as a planning aid, not as tax, legal, or investment advice. IRS Form 8960 and its instructions provide the detailed federal computation. A tax professional can help when an investment sale, rental property, passive business, foreign income item, or filing-status question changes the classification of the inputs.