A contribution tax is a tax charged at the moment money is put into a government benefit program or a tax-advantaged account, rather than when that money is earned, spent, or later withdrawn. In the United States, the term covers two very different things: the payroll taxes under the Federal Insurance Contributions Act that fund Social Security and Medicare, and the penalty tax the IRS charges when you put too much money into a retirement or savings account. In Australia, it refers specifically to the flat 15% tax on money going into a superannuation fund. The common thread is timing. The tax hits on the way in.
FICA Payroll Contributions
For most American workers, the everyday contribution tax is FICA. Your employer withholds 6.2% of your gross wages for Social Security (formally Old-Age, Survivors, and Disability Insurance) and 1.45% for Medicare hospital insurance.1Office of the Law Revision Counsel. 26 USC Chapter 21 – Federal Insurance Contributions Act Your employer pays a matching 7.65% on top of that, so the combined cost tied to each worker is 15.3%.2Office of the Law Revision Counsel. 26 USC 3111 – Employer FICA Tax
These are called contributions rather than income taxes because the money is legally restricted. It flows into trust funds dedicated to retirement benefits, disability payments, survivor benefits, and hospital coverage after age 65, and paying in is what builds your eligibility for those programs later.3Social Security Administration. Social Security Programs in the United States
The Social Security Wage Cap and the Extra Medicare Tax
The 6.2% Social Security portion doesn’t apply to every dollar. For 2026, only the first $184,500 of wages is subject to it. Once your year-to-date pay crosses that line, the Social Security withholding stops for the rest of the year. A worker earning at or above the cap contributes $11,439 in 2026, and the employer contributes the same amount.4Social Security Administration. Contribution and Benefit Base
Medicare works differently. The 1.45% tax applies to all wages, with no ceiling. High earners also owe an Additional Medicare Tax of 0.9% on wages above $200,000 for single filers, $250,000 for married couples filing jointly, or $125,000 for married individuals filing separately. Your employer begins withholding the extra 0.9% once your pay in a calendar year passes $200,000, regardless of your filing status; if your true threshold is higher or lower, you settle up when you file.5Internal Revenue Service. Additional Medicare Tax
What Self-Employed Workers Pay
If you work for yourself, you owe both halves. The self-employment tax rate is 15.3%, made up of 12.4% for Social Security and 2.9% for Medicare.6Office of the Law Revision Counsel. 26 USC 1401 – Rate of Tax You calculate it on Schedule SE and file it with your Form 1040. You can deduct half of the self-employment tax when computing your adjusted gross income, which reduces your income tax bill somewhat.7Internal Revenue Service. Self-Employment Tax (Social Security and Medicare Taxes) The $184,500 Social Security cap and the 0.9% Additional Medicare Tax apply to self-employment income too.4Social Security Administration. Contribution and Benefit Base
The Other Contribution Tax: Excess Retirement Contributions
The second American use of the term shows up around retirement and savings accounts. Money you put into a 401(k) or IRA isn’t taxed on the way in the way Social Security wages are. What the tax code does instead is set annual ceilings and impose a penalty tax on anything above them. For 2026, the elective deferral limit for 401(k), 403(b), and 457 plans is $24,500, and the IRA limit is $7,500, with catch-up amounts for savers age 50 and older.8Internal Revenue Service. 401(k) Limit Increases to $24,500 for 2026, IRA Limit Increases to $7,500
Go over any of those limits and the excess is hit with a 6% excise tax for every year it sits in the account. The tax applies to IRAs, health savings accounts, Coverdell education savings accounts, and ABLE accounts.9Office of the Law Revision Counsel. 26 USC 4973 – Tax on Excess Contributions to Certain Tax-Favored Accounts and Annuities The 6% keeps applying every year the excess remains, so a small over-contribution can compound if ignored.
The fix is straightforward. Withdraw the excess amount, along with any earnings it generated, before your tax-filing deadline (including extensions), and the 6% penalty doesn’t apply for that year. You report the penalty, when it does apply, on Form 5329 with your annual return.10Internal Revenue Service. About Form 5329, Additional Taxes on Qualified Plans (Including IRAs) and Other Tax-Favored Accounts
One rule change worth flagging if you’re a higher earner: beginning in 2026, if your FICA wages in the prior year exceeded $150,000, any catch-up contribution to your 401(k) must go into a Roth account with after-tax dollars, so you lose the upfront deduction on that piece. If your employer’s plan has no Roth option, you can’t make catch-up contributions at all.8Internal Revenue Service. 401(k) Limit Increases to $24,500 for 2026, IRA Limit Increases to $7,500
How the Term Is Used in Australia
The phrase “contribution tax” is used most literally in Australia, where the government taxes money going into superannuation (retirement) accounts directly. Concessional contributions, which include employer contributions and salary-sacrifice amounts, face a flat 15% tax before they’re invested. That rate is designed to sit below most workers’ marginal income tax rate, which is the point of the concession.
High earners owe an additional 15% under Division 293 tax, which applies when combined income and concessional contributions exceed $250,000. On the portion above that threshold, the effective contribution tax rate doubles to 30%.11Australian Taxation Office. Division 293 Tax on Concessional Contributions by High-Income Earners These rules sit under the Income Tax Assessment Act 1997.12Australian Law Reform Commission. Superannuation Contributions If you’re a U.S. worker, none of the Australian rules touch you; the U.S. simply doesn’t tax 401(k) or traditional IRA contributions the way Australia taxes super contributions.
Where You’ll See It on Your Paperwork
Most employees never pay FICA directly. Your employer withholds it from each paycheck and remits both halves to the IRS, and the amounts appear in Boxes 4 (Social Security) and 6 (Medicare) of your W-2 at year’s end.13Internal Revenue Service. Retirement Topics – Contributions If you’re self-employed, the self-employment tax lives on Schedule SE and is generally paid across the year through quarterly estimated tax payments rather than withheld.7Internal Revenue Service. Self-Employment Tax (Social Security and Medicare Taxes) The 6% excess-contribution penalty, when it applies, is self-reported on Form 5329.10Internal Revenue Service. About Form 5329, Additional Taxes on Qualified Plans (Including IRAs) and Other Tax-Favored Accounts