An optometry practice that grows from $650,000 to $800,000 in collections in a single year can clear all four quarterly deadlines without a penalty and still find itself writing a $40,000 check to the IRS in April it was not expecting. (Illustrative example.) The safe harbor math worked, technically. The practice paid in 100% of last year's liability (110% if prior-year AGI was over $150,000, which is common at this revenue level). What it did not do was make the decisions that only exist before December 31.

That is the part of quarterly tax planning most practice owners are not actually doing.

What the cost of skipping actually looks like

There are two separate bills that come from not doing this work during the year.

The first is the underpayment penalty. It gets most of the attention. The IRS charges interest on the gap between what you paid in and what you owed, for each quarter the gap existed. The rate changes quarterly. On a $20,000 underpayment carried for nine months, the penalty might be a few hundred to a couple thousand dollars. (Illustrative range; see IRS Topic 306 for the current rate.) It is real money, but it is usually the smaller problem.

The second is the opportunity cost of the decisions you did not make. Equipment you were planning to buy anyway gets pushed to January because no one ran the numbers in October. That is also when retirement plan decisions start to come into focus, because knowing where the year is going is what tells you how much room you actually have to contribute. And the entity structure question, whether your practice setup is extracting more in self-employment taxes than it needs to, tends to stay hypothetical until someone finally models it. April is too late for all of those.

Why optometry practices get hit harder than most

Your Q4 looks nothing like everyone else's Q4

Patients with flexible spending accounts burn through them in November and December. Optical revenue for most practices is noticeably higher in Q4 than in Q2, sometimes by 20 to 30 percent. (Pattern varies by practice; your own prior-year monthly financials tell you the number.) That revenue is taxable income, and it lands in Q4 when your quarterly payment was sized in June or September.

A practice running at $70,000 per month in collections most of the year can see $90,000 or more in December (illustrative). If your Q4 estimated payment was calibrated to the Q1-Q3 run rate, that December surge is not covered. The balance shows up in April, by which point December is four months gone.

Equipment decisions have hard deadlines

You can amend a tax return. You cannot go back and put equipment in service before December 31 after December 31 has passed.

Section 179 expensing lets you deduct the full cost of qualifying equipment in the year it is placed in service, up to the current annual limit set by the IRS. Bonus depreciation works similarly. The amounts and phase-in schedules change; the IRS publishes the current limits at irs.gov. What does not change is that the equipment has to be in your practice, operational, before the year closes.

An optometrist who buys an OCT device in February instead of December is not making a tax mistake. The device probably made sense in February. But an optometrist who needed the device and bought it in February because no one told them in November that buying it in December would have reduced their tax bill by $15,000 to $25,000 (illustrative; depends on the device cost and applicable depreciation rules) - that is a planning gap.

Retirement plan setup is not a December 31 formality

A solo 401(k) plan lets you contribute both as employee and employer and can shelter significantly more income than a SEP-IRA in some situations. The deadlines for setting up a plan and for each kind of contribution are not all the same, and they depend on whether you run the practice as a sole proprietor or an S-corp, which is one more reason to settle the retirement question in October rather than in March. Knowing how much room you have to contribute still requires year-end income numbers, which means running the projection in October gives you the clearest picture before you are also dealing with the return itself. SEP-IRAs remain simpler to set up but often allow less in total contributions; the comparison is worth running before year-end rather than as a March filing-season afterthought.

What quarterly planning actually means in practice

The practical version is not a quarterly call where someone tells you your tax position and you say okay. It is a mid-year projection, typically in September or October, that looks at where the year is actually going. Is income tracking materially ahead of last year? If yes, the Q4 estimated payment probably needs adjusting, and there are year-end decisions to make before December closes that window permanently.

Three months of runway is enough to act on. Running the same exercise in January gives you the numbers but no chance to use them.

For an optometry practice, a useful October projection starts with year-to-date collections broken out by optical and clinical, compared to the same period last year. That tells you right away whether the Q4 estimated payment needs to change. Then it covers equipment: anything already bought, anything still pending before year-end, whether any of it qualifies for full expensing in the current year. Retirement plan contributions are next, specifically how much room remains given current income and what the plan type actually allows. And in an S-corp situation, the owner compensation question gets a look, because the IRS pays attention to whether the salary-to-distribution ratio is reasonable, and adjusting it before December is much cleaner than explaining it in March.

The year-end planning conversation, specifically the October through December one, is where most of the value sits. That is the last chance to run numbers on equipment purchases, retirement contributions, salary adjustments for S-corp owners, and any entity structure questions that affect the full year. None of those are simple transactions. Each one takes a few weeks from decision to execution, which is part of why October is the right time and not November.

The Q4 timing problem specific to optometry

Most industries have a relatively predictable Q4. Optometry does not, and that asymmetry is worth naming directly.

Frame and lens orders often increase in Q4 as practices stock up for the FSA rush. Exam volume tends to climb as patients use their vision benefits before they lapse. Insurance payment batches can lag the visits by 30 to 60 days, which means some of the Q4 clinical revenue does not hit your bank until January. The optical revenue, by contrast, tends to be collected at point of sale.

That mix matters for tax planning because the optical gross profit lands in the current tax year when you sell the frame, not when you ordered the inventory. A strong November in optical sales means taxable income you may not have seen coming when you sized your September payment.

The practice owner who reviews financials quarterly, or whose accountant does, will see the surge building in October. The one running on annual tax prep does not find out until March.

When annual tax prep is honestly enough

Not every practice needs this. If your collections have been stable for two or three years, your revenue is mostly insurance-based with no significant optical surge, you have no major capital plans, and your entity structure has been settled for a while, the prior-year safe harbor will keep the penalty away and April will not surprise you much.

The practice this matters most for is growing, has variable optical revenue, has an equipment decision pending, or is at the point in scale where entity structure is worth reconsidering. Adding an associate OD or planning a dispensary expansion will change the tax picture enough that last year's numbers are a poor guide to this year's liability, and those are exactly the decisions that benefit from a mid-year projection rather than a March reckoning.

If any of that sounds like where your practice is right now, the Q4 window is the one that matters most, and it is open for about another 90 days.

Our KPI white paper for optometry practice owners covers the monthly numbers worth watching, including the ones that tell you whether your Q4 is tracking above or below prior year. If you want to talk through your current tax position before year-end, the tax planning and advisory services page explains how that conversation works at Caro & Associates.

More context on the financial metrics worth tracking in your practice: Optometry Practice KPI Benchmarks.

The KPI white paper and additional resources for optometry practice owners are on the Caro & Associates optometrists page.

Questions owners ask about this

What is the IRS penalty for not paying enough quarterly taxes?

The IRS assesses an underpayment penalty based on the federal short-term interest rate plus 3 percentage points, applied to the underpaid amount for each period it was underpaid. The rate adjusts quarterly. In most cases, you avoid the penalty entirely by paying at least 100% of last year's tax liability, or 110% if your prior year adjusted gross income exceeded $150,000. Check IRS Topic 306 for the current rate and the Form 1040-ES instructions for the current year's due dates.

When are quarterly estimated tax payments due for optometrists?

The general due dates are April 15, June 15, September 15, and January 15 of the following year. These shift when they fall on weekends or federal holidays. Note that the fourth quarter payment covers income from September through December, so that is where a strong Q4 optical season tends to hit hardest. The IRS Form 1040-ES instructions have the exact dates for the current year.

Do S-corp optometry practice owners still need to pay quarterly estimates?

Yes, though the mechanics differ from a sole proprietor or PLLC. Your W-2 salary from the S-corp is subject to payroll withholding, which counts toward your tax payments. But the S-corp distributions passed through to your personal return are not withheld against. If those distributions are significant, you typically still need quarterly estimates on that income. Run a projection in Q1 or early Q2 so you are not guessing in January.

My practice income varies a lot year to year. Can I just use last year's tax as my target?

The safe harbor approach, paying 100% or 110% of last year's liability, protects you from the underpayment penalty even if you owe a lot more in April. That is a legitimate strategy, and for some practices it is the right one. The downside is that a year where your income grew significantly will produce a large April balance you may not have planned cash for. A mid-year projection in September or October usually costs an hour and answers whether the safe harbor is still the right choice.

What can I actually do in Q4 to reduce my tax bill?

Several things, depending on your practice structure and income level. Equipment you were already planning to buy can often be placed in service before December 31 and expensed under Section 179 or bonus depreciation rules, taking the deduction in the current year rather than depreciating it over several years. Retirement plan contributions are another lever, but the deadlines differ by plan type, by type of contribution, and by whether you are a sole proprietor or an S-corp, and knowing how much to contribute still requires year-end income numbers, so running the calculation in October or November gives you the full picture. If your practice is structured as a PLLC and your income suggests an S-corp election might reduce your tax burden, timing matters for that decision too. None of these are last-minute tax tricks; they are timing decisions that require knowing where the year is going before the decisions are locked in.