Estimated Tax Safe Harbor: 100% vs 110% by Sept. 15

By Plain Money Guide · Researched from official sources · Checked 2026-08-17 · Editorial standards

Match last year's tax bill in four equal installments and the penalty math stops mattering.

Paying 100% of your 2025 total tax in four equal installments blocks any IRS underpayment penalty for 2026 — 110% if your 2025 AGI was over $150,000. The third installment is due Tuesday, September 15, 2026.

That single rule is worth more than any income projection, because it lets you settle the September payment from a number you already know instead of guessing what December will look like. The details below are the ones that decide whether you actually land inside the harbor.

Table of Contents

• The "quarters" are not quarters
• The three safe harbors, and how to pick the cheapest
• Two freelancers, same deadline, very different numbers
• The withholding move that repairs a quarter you already missed
• Where people follow the rules and still get a bill
• How to pay by September 15
• FAQ
• Which route to take before Tuesday

The "quarters" are not quarters

The September 15 payment covers income you earned from June 1 through August 31 — three months. The four periods are uneven, and this trips up anyone who assumes an even calendar:

• April 15 payment covers January 1 – March 31 (3 months)
• June 15 payment covers April 1 – May 31 (2 months)
• September 15 payment covers June 1 – August 31 (3 months)
• January 15 payment covers September 1 – December 31 (4 months)

Despite the uneven periods, the safe harbor is still divided into four equal installments. You do not pay more in the four-month period. See the IRS Estimated Taxes page and Form 1040-ES.

The three safe harbors, and how to pick the cheapest

Under IRC §6654, you owe no underpayment penalty if your timely payments hit any one of these. You get to choose the smallest one.

Route	What you must pay for the year	When this one is cheapest
90% of current year	90% of your final 2026 tax	2026 income is lower than 2025
100% of prior year	100% of 2025 total tax (2025 AGI $150,000 or less; $75,000 if married filing separately)	2026 income is higher, and you want certainty today
110% of prior year	110% of 2025 total tax (2025 AGI over $150,000)	Same, for higher earners — the 10% surcharge is the price of certainty
Annualized installments	Varies by period; computed on Schedule AI of Form 2210	Income was lumpy — a big Q4 sale, a seasonal business
De minimis	No penalty at all if your balance after withholding and credits is under $1,000	Small side income

Total tax means line 24 of your 2025 Form 1040 — the whole bill, including self-employment tax. It is not the check you wrote in April. People who got a refund last year often assume their safe harbor is zero. It is not.

Two freelancers, same deadline, very different numbers

Dana had 2025 total tax of $18,000 and 2025 AGI of $172,000. Over the $150,000 line, so her harbor is 110%: $19,800 for the year, or $4,950 per installment. Her 2026 income has doubled, and her actual 2026 tax may end up near $40,000. It does not matter. As long as $4,950 lands by each due date, no penalty accrues — she simply pays the rest by April 15, 2027.

Marcus also had $18,000 of 2025 tax, but a client left and he expects roughly $10,000 of 2026 tax. His 100% harbor would be $18,000 — far more than he owes. The 90% route is $9,000, or $2,250 per installment. He should use the current-year route and keep the $9,000 difference in his own account.

The rule of thumb: income up, use prior year; income down, use current year. The prior-year harbor is a ceiling you can lock in; the current-year harbor is a floor you have to estimate.

The withholding move that repairs a quarter you already missed

This is the most useful asymmetry in the whole system. An estimated tax payment counts on the day you make it. Withholding from a paycheck, pension, or IRA distribution is treated as paid evenly across all four periods, no matter when in the year it was actually withheld (§6654(g)).

So if you skipped April and June, a single September catch-up check does not undo the spring. But bumping withholding on a W-2 job — yours or your spouse's on a joint return — is applied retroactively to every period.

Say you should have paid $4,000 each in April and June and paid nothing. At the 7% underpayment rate, writing one big check on September 15 leaves you owing roughly:

• April installment: $4,000 late 153 days → $4,000 × 7% × 153/365 ≈ $117
• June installment: $4,000 late 92 days → $4,000 × 7% × 92/365 ≈ $71

About $188, plus daily compounding. Withhold that same $8,000 through year-end via line 4(c) of a new Form W-4 instead, and the IRS treats $2,000 as having arrived in each period — the April and June shortfalls largely disappear. Run the numbers in the IRS Tax Withholding Estimator first.

Where people follow the rules and still get a bill

• Right total, wrong timing. The penalty is computed installment by installment. Paying the full year's tax on January 15, 2027 does not erase April, June, and September shortfalls.
• No 2025 return filed. The prior-year harbor requires a full 12-month prior tax year with a return filed. Newly self-employed in 2026 with no 2025 filing? Only the 90% current-year route is open to you.
• Lumpy income, default assumption. Sell stock in November and the IRS assumes that gain was earned evenly all year — so it back-dates a shortfall to April. The fix is Schedule AI of Form 2210, but only if you file it. It is not automatic.
• AGI just over the line. $150,001 of 2025 AGI puts you on the 110% track. On $18,000 of prior-year tax that is $1,800 more across the year, or $450 per installment.

How to pay by September 15

IRS Direct Pay takes a bank debit with no fee — choose "Estimated Tax" and tax year 2026. EFTPS works too but requires enrollment by mail, so it is not a same-day option. Card payments run about 1.75%–2.0% through the IRS processors. A mailed 1040-ES voucher counts as paid on the postmark date. Full detail is in IRS Publication 505.

FAQ

What if I can't afford the full installment by September 15?

Pay what you can on time. The penalty is interest on the unpaid portion only, currently 7% annually — roughly $0.19 per day on every $1,000 short. Partial payment on time beats full payment in October.

Does the safe harbor cover self-employment tax too?

Yes. "Total tax" on line 24 of Form 1040 already includes self-employment tax, so 100% or 110% of that figure covers both income tax and SE tax.

I'm getting a refund for 2026 anyway. Can I skip this?

If your withholding and credits will leave you owing under $1,000 at filing, there is no penalty and nothing to send. Above that, the per-period test still applies even if the year ends in a refund from over-withholding late in the year.

Which route to take before Tuesday

If your 2026 income is up and your 2025 AGI was over $150,000: send 27.5% of your 2025 line-24 total tax (110% ÷ 4) and stop calculating. Dana's $4,950 held regardless of her income doubling — certainty is what the extra 10% buys.

If your 2026 income is down: use 90% of your projected 2026 tax, like Marcus's $2,250. The trade-off is real — if your projection is too low, the 90% harbor fails and interest runs from each due date, while the prior-year harbor never fails. Only take this route if you can defend the estimate.

If you underpaid in April or June: raise W-4 withholding rather than writing a bigger September check. Withholding is deemed paid evenly across all four periods; an estimated payment counts only from the day it lands. That difference was worth about $188 in the example above, and it scales with the shortfall.

If your income arrived in one lump: pay what the annualized method says for this period and plan to file Form 2210 with Schedule AI. Skipping the form means accepting the IRS's even-earnings assumption, which is the more expensive of the two.

This article is general information, not financial, legal, or medical advice. Rules and amounts change — verify with official sources or a licensed professional before acting.