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When one pays Quarterly Estimated Self Employment Taxes, exactly what are they paying?
From the IRS page on Estimated Taxes (emphasis added): Taxes must be paid as you earn or receive income during the year, either through withholding or estimated tax payments. If the amount of income tax withheld from your salary or pension is not enough, or if you receive income such as interest, dividends, alimony, self-employment income, capital gains, prizes and awards, you may have to make estimated tax payments. If you are in business for yourself, you generally need to make estimated tax payments. Estimated tax is used to pay not only income tax, but other taxes such as self-employment tax and alternative minimum tax. I think that is crystal clear that you're paying income tax as well as self-employment tax. To expand a bit, you seem to be confusing self-employment tax and estimated tax, which are not only two different things, but two different kinds of things.  One is a tax, and the other is just a means of paying your taxes. "Self-employment tax" refers to the Social Security and Medicare taxes that you must pay on your self-employment income.  This is an actual tax that you owe.  If you receive a W-2, half of it is "invisibly" paid by your employer, and half of it is paid by you in the form of visible deductions on your pay stub.  If you're self-employed, you have to pay all of it explicitly. "Estimated tax" does not refer to any actual tax levied on anyone.  A more pedantically correct phrasing would be "estimated tax payment".  Estimated taxes are just payments that you make to the IRS to pay tax you expect to owe.  Whether you have to make such payments depends on how much tax you owe and whether you've paid it by other means.  You may need to pay estimated tax even if you're not self-employed, although this would be unusual.  (It could happen, for instance, if you realized large capital gains over the year.)  You also may be self-employed but not need to pay estimated tax (if, for instance, you also have a W-2 job and you reduce your withholding allowances to have extra tax withheld). That said, if you earn significant income from self-employment, you'll likely have to make estimated tax payments.  These are prepayments of the income tax and Social Security/Medicare taxes you accrue based on your self-employment income. As Pete B. mentioned in his answer, a possible reason that your estiamtes are low is because some taxes have already been withheld from the paychecks you received so far during the year (while you were an employee).  These represent tax payments you've already made; you don't need to pay that money a second time, but you may need to make estimated tax payments for your income going forward.