The prior-year safe harbor: certainty for a volatile first year

What the safe harbor actually protects against

A safe harbor is a payment floor that switches off the estimated-tax underpayment penalty. Reach it and the IRS cannot charge that penalty even if you still owe a large balance at filing. It does not reduce your tax — you settle the real number in April. What it buys is protection from the penalty and interest that accrue when quarterly payments fall short.

Estimated tax is the pay-as-you-go system for income that carries no withholding, and the Schedule C profit of a solo practice is exactly that kind of income 1Ref 1Internal Revenue Service (2026).Estimated taxes.The estimated-tax safe-harbor floors (90% current-year, 100%/110% prior-year), the quarterly installment cadence, and the underpayment penalty a solo practice avoids by clearing one.2Ref 2Internal Revenue Service (2026).Self-employed individuals tax center.That a solo clinician files Schedule C and pays tax on that profit through quarterly estimated payments rather than payroll withholding.. Because no employer is holding money back each pay period, the IRS expects you to send it yourself in four installments, and it measures whether you sent enough against one of the safe-harbor floors 1Ref 1Internal Revenue Service (2026).Estimated taxes.The estimated-tax safe-harbor floors (90% current-year, 100%/110% prior-year), the quarterly installment cadence, and the underpayment penalty a solo practice avoids by clearing one..

• The penalty behaves like interest, not a flat fine. It is computed quarter by quarter on the shortfall, so a quarter you skip keeps costing until you catch up.
• Hitting a harbor ends the question. You do not have to be accurate about the whole year — you have to clear a defined floor.

The three safe harbors: 90% now, 100% or 110% then

There are two ways to satisfy the requirement, and one of them has two versions. You can pay 90% of the tax you will owe for the current year, or you can pay a percentage of last year's total tax — 100% for most filers, and 110% if your prior-year adjusted gross income sat above the IRS's higher-income threshold. Any single one of these floors is enough on its own 1Ref 1Internal Revenue Service (2026).Estimated taxes.The estimated-tax safe-harbor floors (90% current-year, 100%/110% prior-year), the quarterly installment cadence, and the underpayment penalty a solo practice avoids by clearing one..

The dollar figure that separates the 100% and 110% tiers is set by the IRS and indexed over time. Confirm the current-year number on the IRS estimated-taxes page rather than relying on a figure you remember from a prior season 1Ref 1Internal Revenue Service (2026).Estimated taxes.The estimated-tax safe-harbor floors (90% current-year, 100%/110% prior-year), the quarterly installment cadence, and the underpayment penalty a solo practice avoids by clearing one.. Whichever harbor is cheapest for you is the one to fund; you are free to pick the smallest of the three.

Why the prior-year harbor fits an unpredictable first year

A first-year practice cannot forecast its own income, so the 90% current-year harbor asks for a number you do not have. The prior-year harbor sidesteps that entirely: it is built on last year's tax, a figure already printed on a filed return. Even a clinician who spent all of last year as a W-2 hospital employee has that number 2Ref 2Internal Revenue Service (2026).Self-employed individuals tax center.That a solo clinician files Schedule C and pays tax on that profit through quarterly estimated payments rather than payroll withholding..

The harbor uses last year's total tax liability — the whole number on the return, not just the balance you paid in April. Two consequences follow for a new practice. First, a low prior-year tax (a residency year, a partial year of work) makes the harbor cheap to clear. Second, the number is fixed the day the prior return is filed, so it does not move as your practice ramps. Keep the prior return where you can reach it; a simple tax-season folder and steady bookkeeping turn the harbor from a guess into a lookup 2Ref 2Internal Revenue Service (2026).Self-employed individuals tax center.That a solo clinician files Schedule C and pays tax on that profit through quarterly estimated payments rather than payroll withholding..

Building the number when you use the current-year harbor

If you go the 90% route, you have to project this year's tax, and that projection starts from net profit, not gross receipts. Every ordinary and necessary practice expense — malpractice premiums, licensure, supervision, continuing education, and mileage between sites — lowers the profit the estimate is built on 3Ref 3Internal Revenue Service (2026).Guide to business expense resources.That ordinary and necessary practice deductions reduce the net profit the current-year (90%) estimate is projected from.. Two deductions matter enough to model before you send a dime.

• The QBI deduction. Up to 20% of qualified business income, though clinicians are a specified service trade or business and the deduction phases out above a taxable-income threshold the IRS sets 4Ref 4Internal Revenue Service (2026).Qualified Business Income Deduction.That the section 199A QBI deduction further lowers taxable income when projecting a current-year estimate, subject to the SSTB phase-out that applies to clinicians..
• Retirement contributions. A solo 401(k) or SEP contribution lowers taxable income for the income-tax half of your estimate, though it does not shrink the self-employment-tax base.

All of this projection is exactly what the prior-year harbor lets you skip. If forecasting a brand-new practice feels like guessing, that is the signal to lean on the prior-year number instead and revisit the math once you have a few quarters of real receipts.

Paying it: four dates and how the money moves

Estimated tax is due in four installments, normally April 15, June 15, September 15, and January 15 of the following year 1Ref 1Internal Revenue Service (2026).Estimated taxes.The estimated-tax safe-harbor floors (90% current-year, 100%/110% prior-year), the quarterly installment cadence, and the underpayment penalty a solo practice avoids by clearing one.. Divide your safe-harbor total by four and send an equal installment each quarter. Pay through the IRS's own channels — IRS Direct Pay from a bank account or the Electronic Federal Tax Payment System — and file the confirmation with your tax records.

When the 15th lands on a weekend or holiday, the deadline shifts to the next business day. Contemporaneous tax retention matters here: keep each payment confirmation with the return it supports, because records-retention rules mean you may need to prove what you paid, and when, years later.

The S-corp wrinkle: withholding counts as paid on time

If you have elected S-corporation taxation, the arithmetic changes. An S corporation pays its shareholder-employee reasonable compensation as W-2 wages before any distribution, and income tax withheld from those wages is treated as paid evenly across the year — even when it was actually withheld in December 5Ref 5Internal Revenue Service (2026).S corporations.That an S-corp shareholder-employee's W-2 withholding is treated as paid across the year and can satisfy the safe harbor, and that reasonable compensation must be paid first.. That withholding can satisfy the safe harbor without four separate estimated payments.

Some practitioners use this deliberately: a late-year bump in payroll withholding backfills an estimated-payment shortfall as if it had been paid on time. Whether the S election earns its administrative cost at all is a math question for your CPA to run against your profit; this page only explains why, once you have elected it, withholding and estimated payments are interchangeable for safe-harbor purposes 5Ref 5Internal Revenue Service (2026).S corporations.That an S-corp shareholder-employee's W-2 withholding is treated as paid across the year and can satisfy the safe harbor, and that reasonable compensation must be paid first..