# Quarterly Estimated Tax: Avoiding the IRS Underpayment Penalty

Last updated: 26 April 2026

US self-employed individuals, freelancers, partners, and S-corp shareholders must pay federal estimated tax quarterly to avoid the Section 6654 underpayment penalty. The safe harbor rule: pay either 90% of current year tax or 100% of prior year tax (110% if prior AGI exceeded $150,000), split across four installments due 15 April, 15 June, 15 September, and 15 January.

The US tax system is "pay as you go" — the IRS expects to receive tax throughout the year, not just on 15 April. W-2 employees have it withheld automatically. Self-employed people, partners and S-corp shareholders have to send it themselves. Miss it and the underpayment penalty kicks in.

This guide covers the safe-harbor rules that let you avoid penalties even when your current-year income is uncertain.

## When estimated tax applies

You generally must pay estimated tax if you expect to owe $1,000 or more after withholding and credits. This includes:

- Self-employed Schedule C filers and partnerships
- S-corp shareholders with K-1 income beyond W-2 wages
- Landlords with rental income
- Investors with dividends, interest and capital gains
- Anyone whose W-2 withholding doesn't cover their tax bill

W-2 employees with no other income usually don't need estimated tax — withholding handles it.

## The four "quarters" (which aren't equal)

The IRS calls them "quarterly" but they're not equal-length quarters:

| Installment | Period covered | Due |
|-------------|---------------|-----|
| Q1 | 1 January – 31 March (3 months) | 15 April |
| Q2 | 1 April – 31 May (2 months) | 15 June |
| Q3 | 1 June – 31 August (3 months) | 15 September |
| Q4 | 1 September – 31 December (4 months) | 15 January (next year) |

If a deadline falls on a weekend or federal holiday, it shifts to the next business day. (Note: Emancipation Day in DC sometimes pushes 15 April or 15 June to a Monday.)

Crucially: the penalty is calculated **per quarter**, not annually. Paying the full year's tax on 15 April doesn't avoid the Q2/Q3/Q4 penalty windows. You actually need to pay each quarter on time.

## Safe harbor — the rule that saves you

Section 6654 grants safe harbor if you pay at least the LESSER of:

1. **90% of current year tax** (your actual liability)
2. **100% of prior year tax** (110% if prior year AGI > $150,000)

Most taxpayers use option 2 — it's predictable. You know your prior year tax exactly; pay 100% of that across four equal installments and you cannot owe an underpayment penalty regardless of how much your income spikes.

Worked example: 2023 federal tax was $40,000.

- AGI 2023 was $130,000 (under $150k threshold) → use 100% safe harbor
- Pay $10,000 each quarter in 2024 ($40,000 / 4)
- Even if 2024 income doubles and tax hits $80k, no underpayment penalty
- Balance ($80k − $40k = $40k) due 15 April 2025 with the return

For high earners (prior AGI > $150k), bump to 110% — pay $44,000 spread across the year.

## How much to pay

If you choose to estimate current year (option 1), the math:

1. Project annual federal income tax + SE tax
2. Subtract any federal income tax withholding (from W-2 spouse, etc.)
3. Divide remaining by 4

Example: $150,000 projected SE income, no other withholding.

- Estimated SE tax: ~$21,000
- Estimated federal income tax: ~$26,000 (single, standard deduction)
- Total: ~$47,000
- Per quarter: $11,750

If income is uneven through the year (most freelancers), use the **annualized income installment method** (Form 2210 Schedule AI) to match payments to actual income. More work, but avoids the trap of paying high-Q1 estimate when Q1 income is light.

## How to pay

Three main payment channels:

### IRS Direct Pay (recommended for most)

- Free, no signup
- ACH bank transfer
- Confirmation email
- Limit: $10m per transaction
- Paid online: irs.gov/payments

### EFTPS (Electronic Federal Tax Payment System)

- Free, requires PIN signup (5-7 business days)
- Better for businesses paying multiple types (payroll, estimated, etc.)
- Schedule payments up to a year ahead
- ACH bank transfer

### Form 1040-ES voucher + check

- Old-school but still works
- Mail with check to IRS address (state-dependent)
- Allow 5-7 days mail time
- Use Pay.gov for credit/debit (with fee)

Avoid credit card payments unless you're chasing rewards — fees of 1.85%–2.25% wipe out most reward value.

## What happens if you miss

The Section 6654 underpayment penalty is essentially interest on the shortfall, calculated quarter by quarter:

- Penalty rate: IRS short-term rate + 3% (~8% for FY24, varies by quarter)
- Calculated from due date to date paid (or 15 April next year, whichever first)
- Tier-based: hit safe harbor for some quarters, miss for others — penalty applies to the missed quarters only

Practical impact: missing a $10,000 quarterly payment by 6 months → roughly $400 penalty. Annoying but not catastrophic. Missing all four quarters of $10,000 → roughly $2,000+. Painful.

## State estimated tax

Federal is just one layer. Most states with income tax have their own estimated tax requirements:

- **California**: aligned with federal but with different installment percentages (30%/40%/0%/30%)
- **New York**: matches federal dates
- **No-tax states (Texas, Florida, Wyoming, etc.)**: nothing extra
- **City taxes (NYC, San Francisco, Philadelphia)**: separate quarterly obligations

Check your state-level rules. AccountsOS covers federal at launch; for state estimated tax, work with a state CPA or use state revenue department portals directly.

## The annualized income method (for uneven income)

Most freelancers don't earn evenly. If you earn $20k in Q1, $30k in Q2, $50k in Q3 and $50k in Q4, paying ¼ of $47k each quarter is wrong — Q1's $11,750 estimate is way more than the tax actually owed for that period.

The **annualized income installment method** lets you compute estimated tax based on income actually earned through each due date. Q1 covers 1 January–31 March → pay tax on income through 31 March. Q2 covers through 31 May → pay tax on income through 31 May (less Q1 paid). And so on.

It's complex (Form 2210 Schedule AI is dense), but eliminates over-payment in early quarters and provides a defense against penalty if income is genuinely lumpy.

AccountsOS automates the annualized income method by tracking actual income through each due date and proposing the right Q1/Q2/Q3/Q4 amounts.

## Common mistakes

- **Skipping Q1 because tax day already passed and they "haven't earned anything yet."** Use 100% safe harbor.
- **Paying the full year on 15 April.** Late for Q2/Q3/Q4 — penalty calculated per quarter.
- **Forgetting to count K-1 distribution income.** Both general partners and S-corp shareholders.
- **Miscalculating SE tax.** Don't just budget income tax bracket — add 15.3% SE tax for non-wage income.
- **Wrong tax year on payment.** Selecting "2024 estimated" when meant "2023 balance" mis-applies the payment.
- **Believing that more withholding will fix it.** Withholding is treated as paid evenly across the year, so increasing W-2 withholding in December can retroactively cure earlier-quarter shortfalls — a useful trick spouses sometimes use.

## How AccountsOS handles estimated tax

AccountsOS is live in the US and runs estimated tax planning natively:

- Tracks current year income (Schedule C, K-1, W-2) in real time
- Computes both 100% prior year safe harbor AND annualized current year estimate
- Recommends the lower per-quarter payment
- Sends reminders 14 and 7 days before each due date
- Generates IRS Direct Pay flow with pre-filled amounts
- Tracks payments made and reconciles to year-end Form 1040

[Try AccountsOS free](/signup) or read about [AccountsOS in the United States](/us).

## Related reading

- [C-corp vs S-corp vs LLC: how to choose](/blog/c-corp-vs-s-corp-vs-llc-us)
- [Self-employment tax explained](/blog/self-employment-tax-15-percent)
- [S-corp election: when does it pay off?](/blog/s-corp-election-when-it-pays)
- [QSBS Section 1202: $10m tax-free for founders](/blog/qsbs-section-1202-explained)