If you're self-employed, run a small business, or earn significant income without tax withholding, the IRS expects you to pay tax as you earn it — not in one lump sum the following April. That's what estimated quarterly taxes are for. Get them wrong in one direction and you owe an underpayment penalty; get them wrong in the other and you've handed the government an interest-free loan of your own cash all year. Neither is ideal, and both are avoidable with a reasonably simple calculation.
Who actually has to pay
Generally, you need to make estimated payments if you expect to owe at least $1,000 in tax for the year after subtracting withholding and refundable credits. This typically applies to:
- Self-employed individuals, freelancers, and independent contractors
- Small business owners (sole proprietors, partners, S-corp shareholders receiving distributions)
- Anyone with significant investment, rental, or other income without withholding
If you're a W-2 employee with enough tax withheld from your paycheck to cover your total liability, you generally don't need to make separate estimated payments — even with a side hustle, as long as total withholding covers you.
The two safe harbors that prevent penalties
The IRS won't penalize you for underpayment if you meet either of these safe harbor tests over the year:
| Safe harbor | What it requires |
|---|---|
| 100% of last year's tax | Pay at least 100% of what you owed last year (110% if last year's adjusted gross income was above a higher threshold) |
| 90% of this year's tax | Pay at least 90% of what you'll actually owe this year |
The 100%-of-last-year method is the easiest to calculate with certainty, since last year's tax liability is already a known number — it's the go-to method for a stable or predictably growing business.
Simple approach for a growing business: if your income is meaningfully higher than last year, using the prior-year safe harbor keeps you penalty-proof even though you'll owe a larger balance at filing time — a reasonable tradeoff if you'd rather keep more cash working in the business during the year than send the IRS an exact running estimate.
How to actually calculate the payment
- Estimate total expected income for the year (business profit, plus any other taxable income).
- Estimate deductions, including the qualified business income deduction if it applies to you.
- Calculate estimated income tax using current tax brackets.
- Add self-employment tax (Social Security and Medicare) if applicable — this is a separate calculation from income tax and often the piece people forget.
- Subtract any withholding from other income sources.
- Divide the remaining amount by four for equal quarterly payments (payments aren't always exactly equal if your income is seasonal — you can use the annualized income method instead).
The due dates to know
| Payment period | Covers income earned |
|---|---|
| Q1 | January – March |
| Q2 | April – May |
| Q3 | June – August |
| Q4 | September – December |
Despite the name, the four "quarters" aren't equal-length calendar quarters — Q2 covers only two months and Q3 covers three, an IRS quirk that catches many first-time filers off guard. Confirm the exact filing dates each year, since they shift slightly around weekends and holidays.
Why overpaying isn't actually "safe"
It's tempting to overestimate and pay extra "just in case," but that cash sits with the IRS interest-free until you file and claim a refund — money that could otherwise cover payroll, inventory, or a cash flow gap in the meantime. A better approach is a mid-year check-in: revisit your estimate around Q2 or Q3 once you have real numbers, rather than guessing once in January and hoping it holds for twelve months.
Bookkeeping is what makes this calculation possible
You can't estimate quarterly taxes accurately without knowing your actual year-to-date profit, which means your books need to be current — not caught up once a year before filing. This is exactly the kind of ongoing tracking covered in our monthly bookkeeping checklist, and it's also worth reading our guide on reading your income statement so you can pull a real profit number in minutes instead of guessing.
Estimated tax calculations get meaningfully harder once your income is uneven month to month, and getting them wrong has real cost either way — penalties on one side, lost cash flow on the other. Get in touch if you'd like a second set of eyes on your numbers before your next due date.
Frequently Asked Questions
Who needs to pay estimated quarterly taxes?
Generally anyone who expects to owe at least $1,000 in tax for the year after withholding, including self-employed individuals, freelancers, and small business owners without paycheck withholding covering their full liability.
What happens if I underpay my estimated taxes?
The IRS can charge an underpayment penalty, calculated based on how much you owed and for how long. You can avoid the penalty by meeting a safe harbor — paying at least 100% of last year's tax liability (110% for higher earners) or 90% of the current year's actual liability.
Why are the estimated tax 'quarters' different lengths?
It's a long-standing IRS quirk — the four payment periods don't align with calendar quarters. Q2 covers only April and May, while Q3 covers June through August. Always check the specific due dates each year rather than assuming standard quarterly spacing.
This article is general information, not personalised tax, legal or accounting advice. Rules and thresholds change — confirm current-year figures with the IRS or a qualified professional before acting. Ask Accounts Buddy if you'd like help applying any of this to your business.