Do you have a small business? A side hustle that's finally making real money? Are you an independent contractor, freelancer, or self-employed? If you nodded to any of those — and you don't quite know how to calculate quarterly taxes, or you're just hoping it works itself out at filing time — this one's for you.
Ask around about quarterly estimated taxes, and you'll probably hear the same advice: "Take last year's tax bill, divide it by four, and send that amount each quarter." That's one of the IRS-approved methods for making estimated tax payments, and for some business owners, it's perfectly reasonable. But it's not always the most accurate approach. If your income has increased, your business has grown, you have dependents, or your tax situation has changed, relying on last year's tax bill could mean paying too much—or not enough. That's because your quarterly taxes aren't based on profit alone. Self-employment tax, deductions, credits, filing status, and other factors all affect what you'll actually owe. Two business owners with the same profit can end up with very different quarterly tax payments.
The good news is you don't have to guess. Once you understand the pieces that go into your estimate, calculating your quarterly tax payment becomes much more straightforward—and that's exactly what we'll walk through next.
Here's what actually goes into the number, and why two business owners with the same profit can owe completely different quarterly amounts.
The Two Ways to Stay Penalty-Free
You can avoid an underpayment penalty entirely by paying the smaller of two amounts over the year:
- 90% of this year's actual tax — accurate, but requires a real estimate of income you haven't fully earned yet
- 100% of last year's tax (110% if last year's AGI was over $150,000) — guaranteed safe no matter how this year turns out, even in a big growth year
Most people default to the "divide last year by four" method because it's simple — and for a stable income, that's fine. But if your business grew significantly this year, the 90%-of-current-year method can mean paying more than the prior-year method requires. Running both numbers, rather than assuming, is the difference between a rough guess and an actual plan.
The Part Most Guides Skip: Self-Employment Tax
A lot of quarterly-tax explainers focus entirely on income tax brackets and forget that self-employed income also owes Social Security and Medicare tax — 15.3% on 92.35% of your net profit, with the Social Security portion capped at the 2026 wage base of $184,500. This isn't a separate, optional payment — it's baked into the same quarterly number, and skipping it in your mental math is how people end up shocked at how large their actual required payment is.
Where Dependents Actually Change the Number
Here's a piece that's easy to miss entirely: the Child Tax Credit and Credit for Other Dependents reduce your federal income tax — not your self-employment tax — which means the effect is real but partial. On that same $100,000 profit example, adding two qualifying children under 17 changes the picture meaningfully:
That's nearly $1,000 less per quarter, just from two child tax credits at $2,200 each. The credit phases out above $200,000 AGI (single) or $400,000 AGI (married filing jointly), reduced $50 for every $1,000 over the threshold — so it matters most for small business owners in the middle income range, exactly where a lot of solo operators and small S Corps actually sit.
2026 Due Dates — And Why the "Quarters" Aren't Equal
| Payment | Due Date | Covers Income From |
|---|---|---|
| Q1 | April 15, 2026 | January – March |
| Q2 | June 15, 2026 | April – May |
| Q3 | September 15, 2026 | June – August |
| Q4 | January 15, 2027 | September – December |
Notice Q2 covers only two months while Q4 covers four — the IRS's "quarters" were never actual calendar quarters. Most filers still split their required payment into four equal installments regardless, which is what the safe harbor rules assume unless you use the more complex Annualized Income Installment Method for genuinely seasonal income.
If You Also Have a W-2 Job
One option that gets overlooked: if you (or a spouse) also have W-2 income, increasing withholding on that job can cover your self-employment tax gap entirely — no separate quarterly checks required. Withholding is treated as paid evenly throughout the year regardless of when it's actually withheld, which makes it a genuinely simpler alternative to quarterly payments for some households.
The Bottom Line
"Divide last year by four" isn't wrong, but it's one option among a few, and it's not always the cheaper or more accurate one. The actual math includes self-employment tax, whichever safe harbor method is genuinely lower for your situation, and any dependents that reduce what you owe. Running your specific numbers, rather than a rule of thumb, is the difference between a plan and a guess.
Frequently Asked Questions
What happens if I miss a quarterly payment deadline?
The IRS calculates the underpayment penalty separately for each quarter, so paying the full amount late in Q4 doesn't erase a shortfall from Q1 — each quarter is judged on its own.
Which safe harbor method should I use if my income varies a lot?
Run both — 90% of this year's actual tax and 100% (or 110% for higher earners) of last year's tax — and pay whichever is lower. In a growth year, last year's number is often the safer, lower target.
Does self-employment tax apply on top of income tax, or instead of it?
On top of it. The 15.3% self-employment tax (Social Security and Medicare) is calculated on 92.35% of net profit and owed in addition to regular federal income tax — it's not an either/or.
How much do dependents actually reduce my quarterly payment?
Each qualifying child under 17 can reduce federal income tax by up to $2,200 through the Child Tax Credit, which lowers your total tax and therefore your per-quarter payment — though it only offsets income tax, not the self-employment tax portion.
Do I need to make four equal payments, or can they vary by quarter?
Most filers split their safe harbor total into four equal installments. If your income is genuinely seasonal, the Annualized Income Installment Method allows uneven payments that match when the income was actually earned — but it's more complex to calculate correctly.