Estimated Quarterly Taxes for Dentists: How to Calculate and Avoid Underpayment Penalties

When you were a W-2 associate, taxes were invisible — your employer withheld them from every paycheck and you forgot about it. The moment you become a practice owner, that changes completely. Nobody withholds for you. The IRS expects payment four times per year, and if you miss or underpay, you owe a penalty on top of the balance due — even if you pay everything when you file in April.

Most dentists get blindsided in their first year of practice ownership. They earn $200,000–$300,000+, file in April, write a massive check, and discover they also owe an underpayment penalty. The solution isn't complicated, but it requires knowing the rules before the deadlines pass — not after.

Who must pay estimated taxes

You are required to pay estimated taxes if you expect to owe at least $1,000 in federal tax after withholding and refundable credits for the year.¹ For dental practice owners, this threshold is almost always met — sometimes before Q1 is even finished.

The obligation applies to:

• Sole proprietors and single-member LLCs (including PLLCs)
• Partners and LLC members in multi-owner practices
• S-corp shareholders who pay themselves below their actual economic income (more on this below)
• 1099 dental associates classified as independent contractors

W-2 employees — including dentists who own an S-corp and pay themselves a full W-2 salary — generally satisfy the requirement through payroll withholding. That's one of the key advantages of the S-corp structure for practice owners.

The 2026 quarterly due dates

Estimated taxes are paid in four installments. "Quarterly" is a misnomer — the periods are uneven and the deadlines don't fall at the end of each calendar quarter.

Missing a deadline — even by one day — starts the underpayment penalty clock for that period. The penalty compounds daily, so Q1 underpayments are more expensive than Q4 ones. You can pay online via IRS Direct Pay or EFTPS (Electronic Federal Tax Payment System); EFTPS is generally better for practice owners who make large, regular payments.

How much to pay: the safe harbor rule

You don't need to predict your income exactly. The IRS provides a safe harbor — if you meet it, no underpayment penalty applies regardless of how much you owe when you file.¹

Most dentists use the prior-year safe harbor because it's simple and predictable. Look at line 24 of your 2025 Form 1040 (total tax). Multiply by 110% if your 2025 AGI exceeded $150,000 (which is likely). Divide by four. Pay that amount by each quarterly deadline.

Example: A dentist had $95,000 in federal tax on her 2025 return. Her 2025 AGI was $380,000. Her 2026 safe harbor is $95,000 × 110% = $104,500, paid as four installments of $26,125 each (due April 15, June 15, September 15, January 15). Whatever she actually owes for 2026 — even if it's $130,000 — no underpayment penalty applies as long as those four payments were made on time.

What the tax bill actually consists of

Practice owners often underestimate their quarterly payments because they forget that self-employment (SE) tax is a large component of the total — not just income tax.

For a sole proprietor or single-member LLC netting $300,000 in 2026:

That's an effective federal rate of 29–34% on $300,000 net income before any tax planning. Add state income tax — often 5–10% for dentists in high-tax states — and the total tax obligation can easily reach $120,000–$140,000 per year. This is the number that needs to be split across four quarterly payments.

How S-corp payroll eliminates the estimated tax problem

Dentists who operate as an S-corp and pay themselves a reasonable W-2 salary solve the estimated tax problem at the source. Payroll taxes are withheld from each paycheck — the S-corp (your practice) deposits them to the IRS on your behalf, just like an employer would. If your W-2 salary adequately covers your expected income tax liability, no quarterly estimated payments are required.

More precisely: the IRS allows you to "annualize" W-2 withholding. If your total W-2 federal withholding for the year covers 100% (or 110%, if prior-year AGI > $150K) of last year's tax, you satisfy the safe harbor — even if your total S-corp pass-through income is much higher. This is why S-corp dentists often instruct their payroll to withhold at the "married, zero allowances" rate or a flat supplemental rate specifically sized to hit the safe harbor threshold.

The S-corp also reduces SE tax: you only owe the employee FICA (7.65%) on your W-2 salary, not on the full practice profit. At $300,000 net income with a $130,000 W-2 salary, SE tax drops from ~$31,500 (sole prop) to ~$9,945 (employee FICA on salary only) — roughly $20,000 in annual savings. See the S-corp calculator for your specific numbers.

How retirement contributions reduce your quarterly bill

Every dollar you contribute to a Solo 401(k), SEP-IRA, or cash balance plan reduces your net taxable income — and therefore the base on which SE tax and income tax are calculated. This has a direct impact on both your annual tax bill and on future estimated tax amounts.

Example: a dentist nets $320,000 from her sole proprietorship. Her preliminary federal tax estimate (before retirement contributions) is ~$105,000. She makes the following contributions:

• Solo 401(k) employee deferral: $24,500
• Solo 401(k) employer profit sharing: ~$47,500
• Cash balance plan: ~$140,000 (age 55)

Total sheltered: ~$212,000. Taxable income drops from $320,000 to roughly $108,000. Federal tax falls from ~$105,000 to ~$27,000 — a reduction of ~$78,000. Her 2027 quarterly estimated payments will be based on the $27,000 amount, not the original $105,000.

The mechanics matter: retirement contributions that reduce taxable income only help the next cycle of estimated payments, not the current year's safe harbor calculation (which is based on last year's return). If you're making large retirement contributions this year for the first time, your 2026 quarterly payments are still tied to your 2025 tax bill — but your 2027 payments will be substantially lower.

Don't forget state estimated taxes

Every state with an income tax has its own estimated tax system — separate deadlines, separate calculations, and a separate underpayment penalty. Most states use the same quarterly schedule as the IRS (April/June/September/January), but some differ. California, for example, uses a 30/40/0/30 weighting rather than equal quarters, and its Q1 due date falls in April while Q2 is April 15 itself — a common source of confusion.

A dentist in New York netting $300,000 owes roughly $26,000–$31,000 in state income tax (8.97% top rate plus NYC surcharge if applicable) in addition to federal. That amount needs to be estimated and paid quarterly to the state, separately from the IRS. The calculations are similar — prior-year safe harbor of 100% of last year's state tax, regardless of income level in most states.

What happens if you underpay

The underpayment penalty under IRC §6654 is calculated as interest on the shortfall for the period it was underpaid.¹ The rate equals the federal short-term interest rate plus 3 percentage points, adjusted quarterly by the IRS. It's not a flat penalty — it accrues daily from the missed due date to the date of payment or the April 15 filing deadline, whichever is earlier.

At current rates, an underpayment of $20,000 for a full year costs roughly $1,000–$1,400 in penalty interest. That's not catastrophic — but it's money that earns no return, and it accumulates on top of your already significant tax liability. More practically, discovering an underpayment at filing adds a cash flow shock on top of the balance due.

You can avoid the penalty entirely by:

• Meeting the prior-year safe harbor (110% of last year's tax if AGI > $150K)
• Meeting the current-year 90% threshold
• Having your withholding (from an S-corp W-2) cover either threshold

A practical system for practice owners

The dentists who handle this well don't recalculate from scratch every quarter. They set up a reliable system early in the year:

1. Pull last year's Form 1040, line 24. That's your total tax. Multiply by 110% (if 2025 AGI > $150K). That's your federal safe harbor amount for 2026.
2. Divide by 4. Set calendar reminders for April 15, June 15, September 15, and January 15.
3. Repeat for your state. Pull last year's state tax, divide by 4, and schedule state payments on the same or nearby dates.
4. Park tax reserves separately. Open a business savings account and transfer 28–35% of each month's net collections into it. Never touch it for operations. Pay from it when estimated taxes are due.
5. Recalibrate mid-year. If your income is running significantly higher or lower than last year, recalculate using the 90%-of-current-year method around September — before the Q3 payment — to see whether increasing or decreasing payments makes sense.

When to get a CPA or advisor involved

The estimated tax calculation is mechanical — you can do it yourself with last year's return and a calculator. But the downstream question — how much can you shelter through retirement contributions, what W-2 salary to pay yourself from your S-corp, how to time bonus depreciation to move income between years — requires more judgment. These decisions can shift your tax bill by tens of thousands of dollars, and they interact with your estimated payment strategy.

A fee-only financial advisor who works with dental practice owners will typically model your estimated tax payments as part of a broader annual tax planning framework, coordinating with your CPA to align retirement contributions, entity structure, and payroll withholding into a cohesive system. The goal is to neither overpay the IRS interest-free all year, nor underpay and face a penalty — but to have your tax reserves working as efficiently as possible while you deploy capital into the practice.