The first year out on your own goes well, the money is better than the job paid, and then a return gets prepared in the spring and produces a number that does not seem possible. This is the most common financial shock in a first year of self employment and it has nothing to do with the business doing badly.
It happens because two things changed at once and only one of them is obvious.
Why there is a bill at all
When you were employed, income tax was withheld from every paycheck and sent in on your behalf. Nobody does that now. The tax on everything you earned this year is still owed, and it is owed by you, in full, on a schedule you have to manage yourself.
The second change is the one people do not see coming. Social Security and Medicare contributions were split between you and your employer, with each side paying half. Self employed, you pay both halves, which is called self employment tax and which lands on top of the income tax rather than instead of it. That is the single largest reason a first year bill is bigger than anyone expects.
There is a partial offset, in that half of the self employment tax is deductible in computing income tax, and there are deductions available to the self employed that were not available before. Those soften it. They do not remove it.
The four dates
Estimated tax is generally paid four times a year rather than annually. The periods they cover are not equal calendar quarters, which surprises everyone, and the due dates fall in the middle of April, June, September, and the January after the year ends.
Two practical points. The first payment of a year falls at the same time as the previous year's return, so April is a double event for anyone self employed and should be planned as one. And a payment made late accrues an underpayment charge even if the total is eventually paid in full, so the schedule matters independently of the amount.
Most states with an income tax run their own estimated payment system on their own dates, and some cities do as well. Check yours in the first month rather than discovering it in the second year. Take the federal dates from the IRS itself rather than from a summary that may be a year stale, and pay electronically, which is direct and free.
How much to set aside
The honest answer is that nobody can give you a percentage that fits your situation, because it depends on your other income, your filing status, your deductions, your state, and how good the year turns out to be. Anyone who confidently names a single figure is guessing.
What works in practice is a two step approach. In the first year, set aside a substantial share of every payment that arrives, erring high, and adjust once you have real numbers. Erring high is comfortable and erring low is not, because the shortfall is discovered at exactly the moment the money is gone.
From the second year onward there is a much better anchor: what you actually owed last year. The rules provide a route where paying in at least what the prior year required, spread across the four dates, protects you from an underpayment charge even if this year turns out much better. That is worth asking your preparer about specifically, because it converts an unknowable forecast into a known number you can divide by four.
The separate account method
The mechanism that makes this work, and it is unglamorous. Open a second savings account, ideally at a different institution so it is inconvenient to reach. Every time a customer payment lands, move your set aside percentage into it the same day.
The same day is the whole trick. Money left in an operating account gets spent, not through indiscipline but because it is genuinely difficult to look at a healthy balance and remember that a third of it belongs to somebody else. Moving it immediately means the balance you see is the balance you have.
Then pay the four estimated payments out of that account and nothing else. If there is a surplus at the end of the year, that is a good problem and it is the emergency fund the business needed anyway.
If you are already behind
Common, and fixable, and the worst response is to avoid filing. Filing and paying are separate obligations with separate consequences, and the penalty for not filing is generally the harsher of the two. File on time regardless.
If you have missed estimated payments during the year, make the next one and increase it. There is also an alternative approach for someone with a spouse in employment: increasing that spouse's withholding is treated as having been paid evenly across the year, which can repair a shortfall late in a way an estimated payment cannot. That is a genuinely useful trick and it is not widely known.
If the amount owed is more than you can pay, arrangements to pay over time exist and are routine. Ask for one rather than sending nothing.
The year everything changes
Two situations upset any system you build. A year that is far better than the last one, where paying based on the prior year keeps you out of penalty but leaves a large balance due in April. And a year with something unusual in it: selling equipment, a property transaction, a business structure change, or a spouse's income changing significantly.
In both cases the move is the same. A short conversation with a preparer in the fall, while there is still a payment date left and still time to adjust withholding elsewhere. That conversation costs an hour and it is the difference between a manageable April and a memorable one.
What this looks like once it is running
A percentage moved on the day money arrives, four payments made from a dedicated account, and a check in each fall. It becomes invisible within a year, and the April that used to be an event becomes a piece of administration.
Nearly everyone who has been self employed for a while runs some version of this. The ones who do not are the ones for whom every spring is a surprise, and the difference between the two groups is one savings account and a habit that takes ten seconds per payment.