A surtax is an extra tax charged on top of an existing tax obligation, usually aimed at a specific type of income or at taxpayers whose earnings pass a set dollar amount. At the federal level, the two you are most likely to encounter are the 3.8% Net Investment Income Tax and the 0.9% Additional Medicare Tax, both of which apply only after your income crosses a statutory threshold. Governments use surtaxes to raise targeted revenue without reworking the broader tax code, and a surtax can be permanent or temporary depending on the law that created it.
How a Surtax Is Calculated
Not every surtax uses the same math, and the method changes how much you actually owe.
The more common approach today applies a percentage to income above a threshold. The Net Investment Income Tax works this way: 3.8% is charged on the lesser of your net investment income or the amount by which your modified adjusted gross income exceeds the statutory line. Only dollars above the threshold are taxed. Someone $10,000 over the line with $10,000 in investment income would owe $380.
The second approach, sometimes called a “tax on a tax,” applies a percentage to your existing tax liability itself. Owe $10,000 in base tax with a 10% surtax and your bill grows by $1,000. This method is less common in current federal law but has appeared historically and in some state and local systems. It reaches a wider group of taxpayers because it multiplies the entire underlying bill rather than singling out income above a cutoff.
The difference matters. A threshold surtax is progressive by design and concentrates its hit on higher earners. A tax-on-a-tax magnifies whatever progressivity already exists in the base tax.
Federal Surtaxes That May Apply to You
Three surtaxes currently operate at the federal level. Two reach individuals; one is aimed at very large corporations.
Net Investment Income Tax
The Net Investment Income Tax charges 3.8% on investment income for individuals, estates, and trusts whose earnings exceed statutory thresholds.1Office of the Law Revision Counsel. 26 USC 1411 – Imposition of Tax Investment income here includes interest, dividends, annuities, royalties, rents, and net gains from selling property, minus any deductions properly tied to that income. Income earned in the ordinary course of an active trade or business generally does not count.
The tax applies to the lesser of two amounts: your net investment income, or the amount by which your modified adjusted gross income exceeds the threshold for your filing status. If your MAGI is $50,000 over the line but you have $20,000 in net investment income, you pay 3.8% on the $20,000.
Additional Medicare Tax
The Additional Medicare Tax adds 0.9% to wages, compensation, and self-employment income above threshold amounts.2Office of the Law Revision Counsel. 26 US Code 3101 – Rate of Tax It sits on top of the standard 1.45% Medicare tax that applies to all earned income. Where the NIIT reaches investment returns, this surtax reaches earned income.
Self-employed individuals face the same 0.9% rate on self-employment income above the same thresholds. If you have both wages and self-employment income, you combine them to see whether you have crossed the line.3eCFR. 26 CFR 1.1401-1 – Tax on Self-Employment Income
Your employer must begin withholding the Additional Medicare Tax once your wages exceed $200,000 in a calendar year, regardless of your filing status.4Internal Revenue Service. Topic No. 560, Additional Medicare Tax That withholding trigger is a flat $200,000 for everyone, but the actual liability threshold for joint filers is $250,000. Married filing jointly with combined wages between those two numbers, and your employer will overwithhold; you claim the excess back on your return.
Corporate Alternative Minimum Tax
The Inflation Reduction Act created a 15% corporate alternative minimum tax that applies to corporations averaging more than $1 billion in annual adjusted financial statement income.5Internal Revenue Service. Corporate Alternative Minimum Tax It uses book income reported on financial statements rather than taxable income under the regular code. The goal is to prevent very large, profitable corporations from using deductions and credits to drop their effective rate below 15%. Individuals do not pay this tax.
Income Thresholds by Filing Status
The two individual surtaxes share the same threshold structure, though they apply to different types of income:
- Married filing jointly: $250,000
- Single or head of household: $200,000
- Married filing separately: $125,000
These thresholds apply to the NIIT based on modified adjusted gross income and to the Additional Medicare Tax based on wages plus self-employment income.6Internal Revenue Service. Topic No. 559, Net Investment Income Tax4Internal Revenue Service. Topic No. 560, Additional Medicare Tax
One detail catches people out: these thresholds are not indexed for inflation.7Internal Revenue Service. Questions and Answers on the Net Investment Income Tax They have sat at the same dollar amounts since both taxes took effect in 2013. Regular income tax brackets shift upward each year with inflation, but the surtax thresholds do not. Inflation alone pushes more taxpayers over the line every year, even when purchasing power has not changed.
How a Surtax Stacks on Top of Your Regular Rate
A surtax does not replace your regular federal income tax. It piles on top of it. For 2026, the top ordinary income tax rate is 37%, which applies to single filers earning above $640,600 and joint filers above $768,700.8Internal Revenue Service. IRS Releases Tax Inflation Adjustments for Tax Year 2026, Including Amendments From the One, Big, Beautiful Bill
A top-bracket taxpayer with significant investment income can face a combined federal marginal rate of 40.8% on that investment income (37% ordinary plus 3.8% NIIT). Someone with high wages in the same bracket faces 37.9% on earned income above the threshold (37% plus 0.9%). State income tax comes on top of that.
The stacking also matters at lower brackets. A single filer in the 24% bracket with MAGI just above $200,000 and some net investment income can see a combined 27.8% rate on the investment portion. Surtaxes are flat rates and do not graduate the way the underlying brackets do.
Reporting, Withholding, and Estimated Payments
Each individual surtax has its own IRS form. You report the Net Investment Income Tax on Form 8960, attached to your return when your modified adjusted gross income exceeds the applicable threshold.9IRS. 2025 Instructions for Form 8960 – Net Investment Income Tax The Additional Medicare Tax goes on Form 8959, which reconciles what you owe against what your employer withheld. You must file Form 8959 if any single W-2 shows Medicare wages above $200,000 in box 5, or if your combined Medicare wages and self-employment income exceed the threshold for your filing status.10Internal Revenue Service. Instructions for Form 8959
If you expect to owe either surtax, factor it into your quarterly estimated payments. The IRS treats surtax liabilities like any other federal tax when checking whether you paid enough during the year.11Internal Revenue Service. Net Investment Income Tax The general safe harbor is to pay at least 90% of the current year’s total tax through withholding and estimated payments, or 100% of last year’s tax (110% if your adjusted gross income exceeded $150,000).12Internal Revenue Service. Pay As You Go, So You Won’t Owe: A Guide to Withholding, Estimated Taxes and Ways to Avoid the Estimated Tax Penalty Total tax includes NIIT and Additional Medicare Tax. People with a large one-year capital gain get tripped up here, because the surtax bill often does not surface until they prepare the return.
W-2 employees near the threshold have partial protection, since employers automatically withhold the Additional Medicare Tax above $200,000. There is no employer withholding for the NIIT. If your investment income is substantial, estimated payments are the only way to stay current.
State and Local Surtaxes
Surtaxes are not only federal. State and local governments layer their own additional charges to fund specific projects or programs. Local sales tax add-ons are among the most visible: a city or county may charge an extra fraction of a percent on retail sales to finance transit, a stadium, or a school district. These surcharges typically run from a fraction of a percent up to around 2% above the base state rate, and the amount varies widely by jurisdiction.
Several states also impose surtaxes on high-income earners, adding a percentage on top of the standard state income tax rates once income crosses a state-defined line. The rates and triggers differ significantly from state to state, so check your own state’s rules if your income is in the upper brackets.
What Happens if You Underpay
Surtaxes do not carry a special penalty regime of their own. Owe a surtax and pay it late, and the ordinary IRS failure-to-pay penalty applies: 0.5% of the unpaid amount for each month or partial month the balance is outstanding, capping at 25%.13Internal Revenue Service. Failure to Pay Penalty If you set up an approved payment plan, the monthly rate drops to 0.25%.
Interest also accrues on unpaid balances, including on the penalty itself. The IRS sets the underpayment interest rate at the federal short-term rate plus three percentage points, compounded daily.14Office of the Law Revision Counsel. 26 US Code 6621 – Determination of Rate of Interest Because the short-term rate moves, the interest charge shifts from quarter to quarter. Any surtax you owe is part of your tax bill, and leaving it unpaid triggers the same escalating consequences as any other underpayment.