If you made money on the side this year — driving, freelancing, selling online, or collecting rent — there's a decent chance you owe the IRS a payment right now, and a lot of people don't find out until it's too late.
The third-quarter estimated tax deadline lands on September 15.
Miss it, and the IRS can tack on a penalty plus interest, even if you're getting a refund overall when you file next spring.
When you work a regular job, your employer quietly withholds taxes from every paycheck.
When you work for yourself, nobody does that for you.
You're the withholding department now, and the IRS expects payments four times a year: April, June, September, and January.
If you expect to owe at least $1,000 for the year after subtracting withholding and credits, the IRS generally wants quarterly payments.
That catches freelancers, gig workers, small business owners, independent contractors, and retirees pulling from investments or a 401(k).
It also catches people who took a side hustle seriously for the first time this year.
The tricky math is figuring out how much.
A common shortcut is to pay roughly what you owed last year, divided into four chunks.
Another is to aim for 90% of what you'll owe this year, which is harder to guess.
If your income jumped in 2025, that first method can leave you short.
You can pay through IRS Direct Pay, your IRS online account, or the Electronic Federal Tax Payment System.
Debit and credit card payments are accepted through processors, though they charge a convenience fee that eats into the total.
Setting up an automatic monthly transfer to a savings account labeled "taxes" is the move many accountants push, because it turns a scary lump sum into a boring habit.
There's also a safety valve worth knowing about.
If you had a job with withholding for part of the year, you can ask your employer for a new W-4 and have extra tax pulled from your remaining paychecks.
That can cover a side-income shortfall without writing a separate check.
Just don't wait until December, because there isn't enough payroll left to fix it.
One more thing: the IRS has been steadily raising penalty rates, and they compound daily.
A missed payment that feels small in September can look ugly by April.
If you genuinely can't pay the full amount, pay something and file on time.
The failure-to-pay penalty is far gentler than the failure-to-file penalty, and payment plans exist.
If your situation is messy — multiple income streams, a business, big deductions — a few hundred dollars spent with a tax pro now often beats a surprise bill later.
At minimum, log into your IRS online account and check what you've already paid this year before September 15 slips past.
My take: the quarterly system punishes people who work for themselves simply because nobody withholds for them, and that's a real flaw in how the code is built.
But knowing the dates and paying something, even imperfectly, is far cheaper than ignoring it.
Final Thoughts
Put September 15 and January 15 in your phone right now.