Self employment tax is the Social Security and Medicare tax you owe on business earnings, and it can catch new side business owners off guard. The headline rate is 15.3%, but it applies to a portion of your profit, not every dollar. Planning for quarterly payments and setting money aside can make the bill more manageable.
How self employment tax is calculated
At $400 or more in net self employment earnings, you generally owe self employment tax. The $400 threshold is about this tax; it should not be treated as a universal exemption from every other tax or filing rule. The rate is 12.4% for Social Security and 2.9% for Medicare, for a combined 15.3%.
The calculation uses 92.35% of net profit rather than the full amount. For example, $50,000 in profit becomes $46,175 for this calculation. Applying 15.3% produces about $7,065 in self employment tax. Regular federal and, where applicable, state income tax is separate, so the total tax bill will be higher.
Here is how the main rates and thresholds work under the figures discussed for this article:
| Tax or calculation | Rate or threshold | How it applies |
|---|---|---|
| Social Security | 12.4%, on earnings up to $176,100 | Part of the self employment tax rate. Earnings above the stated cap are not subject to this portion. |
| Medicare | 2.9%, with no earnings cap | Applies as part of self employment tax. A single filer earning over $200,000 also pays an additional 0.9% on income above that threshold. |
| Federal income tax | Starts at 10% | Charged separately, with the rate depending on the taxpayer's bracket. |
| State income tax | From almost nothing to over 13% | The amount depends on the state and the taxpayer's circumstances. |
Self employed workers can deduct half of their self employment tax when calculating income tax. That deduction recognizes the employer portion that a W2 worker does not see in a paycheck, but it does not erase the tax itself. Financial planner Carol Bell of Prudential Advisors, who teaches financial planning at New York University, has described that employer share as a cost many employees do not realize their employer covers.
2025 quarterly payments and the penalty risk
For tax year 2025, the listed estimated tax deadlines were April 15, June 16, September 15, and January 15, 2026. If you expect to owe at least $1,000 for the year, the IRS generally expects estimated payments during the year rather than one large payment at filing time.
Missing or underpaying an installment can lead to a penalty calculated using daily interest. The amount can build as time passes, so a new business should not wait until tax season to check whether payments are due. The IRS safe harbor offers a way to reduce the risk of an underpayment penalty when this year's income is hard to predict: pay at least 100% of what you owed last year, divided into four payments.
That method can be useful when sales or client work fluctuate. It relies on last year's tax bill rather than a forecast of this year's profit. Keep in mind that the safe harbor is a payment approach, not a discount on the amount of tax ultimately due.
An honest mistake does not mean automatic arrest. Underpayment can bring penalties and interest, and errors should be corrected, but fear of a mistake is not a reason to ignore notices or put off organizing records. Save payment confirmations and keep a simple calendar of the four due dates.
Business deductions that can lower taxable profit
A deductible business cost needs to be ordinary and necessary for the work. In practical terms, ask whether the expense is common and appropriate for the kind of business you run, then keep evidence of what you bought and why it was used for work. The IRS may ask for supporting records, so keep receipts and invoices instead of relying on memory.
Possible costs include business software, equipment, and the work related share of phone and internet bills. Personal use matters: if a device or service is used for both business and personal reasons, the records should show what portion relates to the business. Do not claim an expense simply because it was paid from a business account.
The home office deduction is available when a specific area is used only for business. One option is $5 per square foot, up to 300 square feet, for a maximum deduction of $1,500. The source figures also describe calculating actual expenses when the work space is larger. Compare the available method with your records before choosing what to claim.
Self employed people who pay for their own health insurance and do not have access to an employer plan may deduct 100% of premiums for themselves, a spouse, and dependents. The deduction for half of self employment tax is separate. These deductions affect taxable income; they are not the same as getting the full amount of an expense back.
How much to reserve from each payment
A practical starting point is to move 25% to 30% of each business payment into a separate savings account for taxes. On a $1,000 project, that means transferring $250 to $300 before using the rest for business costs or personal spending. The account helps keep tax money from blending into everyday cash.
That reserve is meant to cover self employment tax and income tax, but it is only a rule of thumb. The right amount depends on profit, other income, deductions, filing circumstances, and state taxes. Bell has said self employed people should expect roughly 7.65% more of taxable income to go to the government than a W2 employee in a comparable situation, reflecting the employer share normally paid on the employee's behalf.
For a business with $30,000 in earnings, the estimates in the source figures are about $4,240 in self employment tax, plus roughly $2,000 to $3,500 in federal income tax, depending on the bracket. That gives a total reserve of about $6,200 to $7,700, or around 25% of income. State income tax could change the final amount.
Start with three steps: open a separate savings account, transfer a set percentage whenever money comes in, and review the amount before each quarterly deadline. Keep an updated record of income and business costs so you can compare your reserve with the likely bill. If you have several income sources, earnings that change sharply, or repeated tax bills far above or below your estimate, a tax professional can help you set a more suitable payment plan.