Educational information, not individual financial advice.
Key Takeaways
If you have W-2 income with appropriate withholding, you usually don't think about quarterly estimated taxes — your employer does the work. The moment you have meaningful income WITHOUT withholding (self-employment, large taxable investment income, RMDs, large Roth conversions), you're on the hook for quarterly payments.
The IRS rule: if you expect to owe $1,000 or more in federal tax beyond what's been withheld for the year, you must pay estimated taxes during the year. Otherwise the IRS treats your underpayment as a loan to you — and charges interest.
Common triggers:
You avoid the underpayment penalty entirely if you pay (across the year) the lesser of:
The prior-year safe harbor is the easier one for most filers. You know exactly what you owed last year — pay 100% (or 110% for high earners), divided into 4 equal payments, and you're protected.
The 110% rule for high earners catches most consultants, doctors, and other high-income SE filers off guard. Using "100% of prior year" when you should have paid 110% triggers the underpayment penalty on the gap.
| Quarter | Income period | Payment due |
|---|---|---|
| Q1 | Jan–Mar | April 15 |
| Q2 | Apr–May | June 15 |
| Q3 | Jun–Aug | September 15 |
| Q4 | Sep–Dec | January 15 of next year |
The dates are NOT evenly spaced — Q2 only covers 2 months, Q3 covers 3 months, Q4 covers 4 months. The IRS calls them "quarters" but they're not actually quarters. Mark these dates, all four — missing one is the most common cause of penalty even for filers with the right total annual payment.
The penalty isn't a flat fee. It's interest on the underpayment, computed daily, at:
Short-term Treasury rate + 3 percentage points
In 2026 Q1 that's roughly 8% annualized. The penalty applies from the date the payment was due to the date it was actually paid (or April 15 of the following year, whichever is earlier).
So a $5,000 underpayment from Q1 left unpaid for the full year would accrue ~$400 in penalty. Not crushing for a one-off, but compounds quickly if you're chronically underpaying.
Three approaches, in increasing accuracy:
1. Annualized prior-year safe harbor. Take last year's total federal tax owed. Divide by 4. Pay each quarter. Done.
2. Quarterly current-year projection. Estimate each quarter what your YTD income will produce in tax, subtract YTD withholding + previous estimates, pay the difference.
3. Annualized income method. IRS Form 2210 lets you compute a separate safe harbor for each quarter based on income earned through that point. Useful if your income is heavily back-loaded (e.g., you get most consulting revenue in Q4).
Withholding counts as paid evenly across the year, regardless of when it was actually withheld. So if you under-pay all year and then catch up by jacking up December withholding (e.g., from an RMD), the IRS treats the December withholding as if it had been paid evenly January–December. Useful for last-minute fixes.
Estimated payments DON'T get the same treatment. A $20k January 15 estimated payment is credited as a Q4 (prior year) payment if marked as such, OR as a Q1 (current year) Q1 payment if you mistakenly mark it for the new year. Mark carefully.
State quarterly estimates are separate and have their own dates (often slightly different from federal). Check your state's rules.
The /taxes page surfaces a "Quarterly estimated taxes" Pro panel — placeholder pending wiring of the engine's safe-harbor calculator. The full quarterly schedule + lesser-of-90%-current vs 110%-prior calculation is on the roadmap as a Phase-E follow-up. For now, the federal-total line on the /taxes page tells you the annual liability — divide by 4 (or use last year's tax × 100/110%) for a starting estimate.
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