The U.S. tax system is pay-as-you-go. Employees satisfy this through withholding. The self-employed, business owners, and anyone with substantial income not subject to withholding must satisfy it through quarterly estimated payments. Get them right and April is uneventful; get them wrong and you face underpayment penalties, interest, and a cash-flow shock.
Who Must Pay
Generally, you must make estimated payments if you expect to owe at least $1,000 after withholding and credits, and your withholding will be less than the smaller of 90% of this year's tax or 100% of last year's (110% if prior-year AGI exceeded $150,000). This sweeps in sole proprietors, partners, S-corp shareholders, freelancers, investors, and landlords.
The Safe Harbor Rules
| Safe Harbor | Requirement |
|---|---|
| 90% of current year | Pay at least 90% of what you'll owe this year |
| 100% of prior year | Pay 100% of last year's tax (AGI ≤ $150,000) |
| 110% of prior year | Pay 110% of last year's tax (AGI > $150,000) |
The most important concept
Meet a safe harbor and the underpayment penalty is avoided entirely — regardless of how large your eventual balance is. Pay 100% (or 110%) of last year's tax in four equal installments and you're protected even if your income rises sharply. You'll still owe the balance in April, but no penalty.
The Payment Schedule
| Payment | Income Period | Due Date |
|---|---|---|
| Q1 | Jan 1 – Mar 31 | April 15 |
| Q2 | Apr 1 – May 31 | June 15 |
| Q3 | Jun 1 – Aug 31 | September 15 |
| Q4 | Sep 1 – Dec 31 | January 15 (following year) |
Note the uneven periods — Q2 covers two months and Q4 covers four. Treating them as equal three-month quarters can cause an underpayment in an early period even if the annual total is correct.
How to Calculate
- Safe-harbor method (simplest): prior-year tax × 100% (or 110%) ÷ 4 — guarantees no penalty regardless of current-year income
- Current-year projection: estimate this year's tax and pay 90% across the installments — avoids overpaying but requires accurate forecasting
- Annualized income method (Form 2210, Schedule AI): pay based on income actually earned each period — best for seasonal or lumpy income
Don't forget SE tax
Estimated payments must cover not just income tax but also self-employment tax — 15.3% on net earnings up to the wage base, plus 2.9% Medicare above it. Owners who budget only for income tax routinely come up short.
SMAART Tax Team
CPAs & Enrolled Agents, SMAART Tax
