If you made money on the side this year, from DoorDash runs to freelance design work to a rented-out spare room, there is a decent chance the IRS expects a payment from you by September 15, and a lot of people miss it.
The reason is simple and easy to overlook.
When you work for an employer, taxes come out of every check automatically.
When you work for yourself, nobody withholds anything.
You are responsible for sending the government estimated payments four times a year, and the third one lands in mid-September.
Skip it and the tax bill does not disappear.
The penalty for underpaying is currently running around 7% to 8% annually, compounded daily, according to IRS rules.
That is roughly in line with what a credit card charges, except this debt does not come with a rewards program.
It just quietly adds up until April, when many self-employed workers open their return and discover they owe thousands more than they set aside.
The people most exposed are not just full-time freelancers.
They are gig drivers, Etsy sellers, consultants, tutors, and anyone who picked up a second income stream in the past year.
A common trap: a worker takes a new remote contract job and fills out a W-4 as if it were a normal salaried position, then gets blindsided when the 1099 arrives with no withholding.
There is also a lesser-known safe harbor that can protect you.
If you pay in at least 100% of what you owed last year, the IRS generally will not hit you with the underpayment penalty, even if your income jumped.
For higher earners, that threshold rises to 110%.
It is one of the few places in the tax code where paying last year's number can beat paying this year's.
Figuring out the right amount is not guesswork if you do a little math.
Take your expected annual profit, subtract your deductions, apply your marginal rate plus the 15.3% self-employment tax, then divide by four.
If that sounds tedious, most tax software and a free IRS worksheet will do it for you in a few minutes.
The deadline matters more than the amount for most people.
Paying something, even an estimate that is slightly low, beats paying nothing at all.
Partial payments reduce the penalty because the penalty is calculated on the shortfall, not the full obligation.
Waiting until April to pay everything is the single most expensive way to handle it.
One more thing worth checking: if you also have a regular job, you can raise your withholding there instead of mailing quarterly checks.
The IRS treats withholding as if it were paid evenly throughout the year, which can wipe out penalties that quarterly payments would not fix. **Our take:** The estimated tax system is not designed to catch people, but it absolutely catches people who ignore it.
Setting aside 25% to 30% of every freelance payment in a separate account is boring, unglamorous, and probably the single best financial habit a self-employed worker can build.
Final Thoughts
Do it now, before the September deadline sneaks past.