If you made money on the side this year, drove for a delivery app, sold crafts online, or picked up freelance work, the IRS is expecting a payment from you on September 15.
Miss it and the penalty math starts working against you immediately.
Here's the part that trips people up: there's no automatic withholding on most side income.
When you work a regular job, taxes come out of every paycheck before you ever see the money.
When you're self-employed, you're responsible for sending it in yourself, four times a year.
That quarterly schedule is what confuses everyone.
The payments aren't spaced evenly across the calendar.
The due dates are April 15, June 15, September 15, and January 15 of the following year.
So if you're just realizing you owe something now, you may already be behind on two earlier deadlines.
Generally, if you expect to owe $1,000 or more when you file your return, the IRS wants quarterly payments.
That threshold catches more people than you'd think.
A $6,000 side gig, a big freelance project, even decent investment income can push you over it.
It's calculated as interest on the amount you underpaid, and it accrues daily.
Right now that rate is steep compared to the past decade, so dragging your feet costs more than it used to.
The IRS charges it automatically, so there's no warning letter first.
Salaried workers have an easier escape hatch.
You can ask your employer to withhold extra from each paycheck by adjusting your W-4, and the IRS treats that withholding as if it were paid evenly throughout the year.
That can wipe out an underpayment penalty even if you didn't send a single quarterly check.
Self-employed folks have a similar option through their own estimated payments, but the timing rules are stricter.
If you catch up late in the year, you can still reduce the damage by paying more than you owe in the final quarter, though it won't erase everything.
One number worth knowing: the safe harbor.
If you pay at least 90% of this year's tax bill, or 100% of last year's (110% if your income was above $150,000), you generally avoid the penalty entirely.
For a lot of people, matching last year's total is the simplest way to stay out of trouble.
If money is tight, the IRS does offer a few relief valves.
You can request a payment plan online, and short-term plans under 180 days are often free to set up.
Penalties can sometimes be reduced if you have a reasonable cause, but you have to ask.
Set aside roughly 25% to 30% of every freelance or side payment the moment it lands.
A separate savings account makes this painless and keeps you from scrambling when a deadline shows up.
Our take: the quarterly system punishes people who earn irregular income, and it does so quietly.
But treating taxes as a running expense instead of a spring surprise is the single cheapest money habit a self-employed worker can build.
Final Thoughts
Set the money aside now, and September stops being a crisis.