[spatne]

You're supposed to make 4 quarterly payments. The payment on Sept 15 is the third one, so you've missed two so far.

Basically, you're supposed to do this for any income that doesn't have taxes withheld. The US uses a "pay as you go" tax system, which is why the government withholds taxes from each paycheck. Estimated taxes are a substitute for withholding on income which doesn't come from an employer. If you overpay, you'll get the excess back as part of a refund. If you overpay too much, however, I think there's a penalty.

It doesn't matter if it's your sole source of income, or if you're filing as a pro-- you have to pay estimated taxes on your winnings. If you pay your taxes in one lump sum on tax day, you'll be subject to penalties and probably interest as well.

If you make only the 3rd and 4th payments, but make those payments large enough to cover everything you owe, I have no idea what will happen. Can anyone answer this? For example: if someone goes to Vegas in October and wins $10K, they'd only make one ES payment. How does the IRS differentiate between this person and OP who simply missed payments?