A surcharge on income tax is an extra tax layered on top of what you already owe under the regular brackets. At the federal level it takes two forms: the 3.8% Net Investment Income Tax on investment earnings, and the 0.9% Additional Medicare Tax on wages and self-employment income. Both kick in only after your income passes a threshold set by your filing status, and both apply only to the income above that line. A handful of states add their own high-income surcharges on top.
What a Surcharge Actually Is
A surcharge is an additional tax that sits on top of your existing tax bill. Some surcharges are written as a percentage of the tax you already owe, which is literally a tax on tax. Others, including both current federal surcharges, are calculated directly on income above a threshold. Either version gets called a surcharge or surtax.
Governments use them because they can be added, adjusted, or repealed through a single piece of legislation without rewriting the underlying brackets. That structural separation is why the two federal surcharges below have their own forms, their own thresholds, and their own quirks that regular withholding can miss.
The 3.8% Net Investment Income Tax
The Net Investment Income Tax, or NIIT, is a 3.8% tax on the smaller of two amounts: your net investment income for the year, or the amount by which your modified adjusted gross income exceeds the threshold for your filing status.1Office of the Law Revision Counsel. 26 USC 1411 – Imposition of Tax
The thresholds are:
- Married filing jointly or qualifying surviving spouse: $250,000
- Single or head of household: $200,000
- Married filing separately: $125,000
Those numbers have not changed since the NIIT took effect in 2013 and are not indexed for inflation.2Internal Revenue Service. Questions and Answers on the Net Investment Income Tax Income levels that sat comfortably below the line a decade ago now cross it routinely, especially in years with a home sale or large capital gain.
Net investment income includes interest, dividends, capital gains, rental and royalty income, non-qualified annuities, and income from passive business activities or financial trading businesses.2Internal Revenue Service. Questions and Answers on the Net Investment Income Tax It does not include wages, Social Security benefits, unemployment compensation, self-employment income, tax-exempt interest, or distributions from qualified retirement plans like a 401(k) or IRA.3Internal Revenue Service. Instructions for Form 8960 – Net Investment Income Tax
Estates and trusts pay the same 3.8% rate but face a far lower threshold. For 2026, the NIIT applies to an estate or trust with adjusted gross income above $16,000 and undistributed net investment income.3Internal Revenue Service. Instructions for Form 8960 – Net Investment Income Tax Trustees who leave investment earnings inside the trust can generate a meaningful surcharge bill on a modest balance.
The 0.9% Additional Medicare Tax
The Additional Medicare Tax adds 0.9% to your Medicare tax on wages, self-employment income, and railroad retirement compensation once those earnings cross a filing-status threshold.4Internal Revenue Service. Topic No. 560 – Additional Medicare Tax The thresholds mirror the NIIT:
- Married filing jointly: $250,000
- Single or head of household: $200,000
- Married filing separately: $125,000
For employees the tax applies to W-2 wages. For self-employed taxpayers it applies to net self-employment income after combining it with any wages.5Office of the Law Revision Counsel. 26 USC 1401 – Rate of Tax These thresholds are also not indexed for inflation.
Employer withholding creates a common trap. Your employer must start withholding the extra 0.9% once your wages from that single job exceed $200,000 in a calendar year, regardless of your filing status.6Internal Revenue Service. Questions and Answers for the Additional Medicare Tax That flat $200,000 trigger doesn’t know about your spouse. A married couple each earning $180,000 will have no Additional Medicare Tax withheld at either job, yet they owe 0.9% on $110,000 of combined wages above their $250,000 joint threshold. The shortfall shows up as a balance due when you file.
The opposite can happen too. If you’re married filing jointly, your individual wages triggered withholding, and your combined household income is still below $250,000, you can claim a refund for the overwithholding on your return.6Internal Revenue Service. Questions and Answers for the Additional Medicare Tax
How the Math Works
Both federal surcharges apply only to income above the threshold, not to your entire income once you cross the line. There’s no cliff where one extra dollar of income taxes everything below it.
For the NIIT, compare your net investment income against the amount by which your modified adjusted gross income exceeds the threshold, then pay 3.8% on whichever number is smaller. A single filer with $260,000 of MAGI including $40,000 of investment income exceeds the $200,000 threshold by $60,000. Because $40,000 is smaller than $60,000, the surcharge applies to $40,000, for a bill of $1,520.1Office of the Law Revision Counsel. 26 USC 1411 – Imposition of Tax
The Additional Medicare Tax is simpler. Take your total wages and self-employment income, subtract your filing-status threshold, and multiply the excess by 0.9%. A married couple filing jointly with $300,000 in combined wages owes 0.9% on the $50,000 above their $250,000 threshold, or $450.7Office of the Law Revision Counsel. 26 USC 3101 – Rate of Tax
When both apply in the same year, they stack. A high earner with significant wages and investment income can owe the 0.9% on employment income and the 3.8% on investment income at the same time, on top of regular income tax. Neither reduces the other.
State Surcharges
Several states impose their own high-income surcharges, typically a flat percentage on income above a set threshold that functions as an additional bracket over the state’s ordinary rates. Rates and thresholds vary widely, and some are indexed for inflation while others use fixed dollar amounts. If you live in or earn income from one of these states, that surcharge combines with the two federal ones to raise your effective marginal rate, and the combined surcharge load can reach 5% or more on top of ordinary federal and state income tax. Check your state’s current law each year, since new surcharges have been enacted or expanded in multiple states as recently as 2025 and 2026.
Reporting the Surcharges
Each federal surcharge has its own form. The Additional Medicare Tax is calculated on Form 8959, which walks through wages, self-employment income, and railroad retirement compensation, then reconciles anything your employer already withheld.8Internal Revenue Service. About Form 8959 – Additional Medicare Tax The NIIT is calculated on Form 8960, which tallies investment income, subtracts allowable deductions, and applies the 3.8% rate to the required base.3Internal Revenue Service. Instructions for Form 8960 – Net Investment Income Tax Both forms feed the “Other Taxes” section of Form 1040.
Form 8959 is required even when withholding was correct, because it’s the only way to claim a refund for overwithholding or report tax owed on undercollection.6Internal Revenue Service. Questions and Answers for the Additional Medicare Tax
Estimated Payments and Safe Harbors
If you expect to owe either surcharge, factor it into your estimated tax payments during the year. Taxes are supposed to be paid as income is earned, and the year is split into four payment periods.9Internal Revenue Service. Estimated Taxes Waiting until April to settle a large surcharge balance invites an underpayment penalty.
High-income taxpayers have a specific safe harbor. If your prior-year adjusted gross income exceeded $150,000 ($75,000 if married filing separately), you avoid the underpayment penalty by paying at least 110% of your prior year’s total tax through withholding and estimated payments.10Office of the Law Revision Counsel. 26 USC 6654 – Failure by Individual to Pay Estimated Income Tax Paying at least 90% of the current year’s total tax also works. You only need to meet one test.
The 110% rule is the practical choice when your income swings. If last year was normal and this year brings a business sale or big capital gain, paying 110% of last year’s tax locks in the safe harbor no matter what the current bill turns out to be. If income arrives unevenly, the annualized income installment method on Schedule AI of Form 2210 lets you match each installment to the income you actually received that quarter.11Internal Revenue Service. Instructions for Form 2210 That’s the tool for a December stock sale that drives most of your surcharge liability into the final quarter.
Interest on Underpayments
The IRS charges interest on any underpayment, and the rate adjusts quarterly. For the first quarter of 2026 the underpayment interest rate is 7%; for the second quarter it drops to 6%.12Internal Revenue Service. Quarterly Interest Rates Interest runs from the date each installment was due, not from the filing deadline, so a shortfall in your first quarterly payment costs more than one in your fourth. Tracking your investment income and wages against the surcharge thresholds each quarter is the cleanest way to keep the bill predictable.