When you work a W-2 job, your employer quietly pays half of your Social Security and Medicare taxes and withholds the other half from your paycheck. When you work for yourself, both halves land on you. That combined bill is self-employment tax, and it is the main reason a freelancer's first tax return often looks so much worse than expected.
The headline rate is 15.3%, but the amount you actually owe is lower than simply multiplying your gross income by 15.3%, thanks to a few built-in adjustments most people never hear about.
What the 15.3% is made of
Self-employment tax is 12.4% for Social Security plus 2.9% for Medicare. The Social Security portion applies only up to an annual wage base that is adjusted each year for inflation; income above that cap is not subject to the 12.4% piece. The Medicare portion has no cap, and higher earners owe an Additional Medicare Tax of 0.9% above certain thresholds.
It is calculated on net earnings, not gross revenue
Self-employment tax applies to your net profit from self-employment, which means your revenue minus your ordinary and necessary business expenses. Every legitimate deduction you claim on your business schedule, like mileage, software, equipment, or a home office, reduces the base that the 15.3% is applied to. That is why good expense tracking matters twice: it lowers both your income tax and your self-employment tax.
The two built-in adjustments
Two adjustments keep self-employed people from being taxed more harshly than employees on the same earnings.
- Only 92.35% of net earnings is subject to self-employment tax, which mirrors the fact that employees are not taxed on the employer half of the payroll tax
- You can deduct half of your self-employment tax as an adjustment to income, which lowers your income tax (though not the self-employment tax itself)
Why it catches people off guard
Self-employment tax is owed even if you owe little or no income tax. A side gig that nets a few thousand dollars can trigger a meaningful bill for someone whose income is otherwise low enough that income tax is near zero. Once net self-employment earnings reach the filing threshold (which is quite low), you generally need to file and pay it.
Plan for it through the year
Because nobody withholds self-employment tax for you, the practical approach is to set aside a percentage of every payment as it arrives and send it in through quarterly estimated payments. Running your expected net profit through a self-employment tax calculator early in the year gives you a realistic number to save toward instead of a surprise in April.
This article is for general education only and is not tax or legal advice. Tax rules vary by jurisdiction, change over time, and depend on your specific situation. Talk to a qualified tax professional before making filing or payment decisions.