If you’re running a therapy practice, September 15 is one of four dates you want on your calendar every year. It’s the deadline for your third-quarter estimated tax payment to the IRS, and missing it (or underpaying it) can cost you in late fees on top of whatever you already owe.
Estimated taxes confuse a lot of therapists, especially in the first few years of private practice. Nobody sat you down and explained that when you work for yourself, the IRS expects you to pay your taxes four times a year, not just once. This is just how it works for self-employed people, and the system makes more sense once you understand what it’s actually doing.
Here’s what you need to know before September 15.
September 15 Is The Q3 Estimated Tax Deadline For Therapists In Private Practice
The IRS divides the year into four estimated tax periods. Q3 covers income earned from June 1 through August 31, 2026, and the payment is due September 15, 2026. The four deadlines for the 2026 tax year are April 15 (Q1), June 16 (Q2), September 15 (Q3), and January 15, 2027 (Q4). These dates are set by the IRS under IRC Section 6654.
Missing a deadline doesn’t mean you owe double next quarter. It means the IRS charges an underpayment penalty on the amount you were supposed to pay, calculated as an annual interest rate on the shortfall. For 2026, that rate is 8% (the federal short-term rate plus 3 percentage points, updated quarterly by the IRS). It’s not a huge amount, but it’s easy to avoid.
What Are Estimated Taxes And Why Do Therapists Have To Pay Them?
Estimated taxes are how self-employed people prepay their taxes throughout the year. When you worked at an agency or group practice, your employer withheld income tax and payroll taxes from every paycheck automatically. In private practice, nobody does that for you. The IRS still expects to collect as the money comes in, so they set up a quarterly system for self-employed people to send in what they owe on a schedule.
The “estimated” part means you’re calculating what you’ll owe based on what you’ve earned so far, not waiting until you know your exact annual number. You estimate, you pay, and then you true everything up when you file your annual return. If you overpaid, you get a refund. If you underpaid, you pay the difference (plus potentially a penalty).
Estimated taxes come up in almost every conversation we have with therapists who are new to private practice. If you want a fuller overview of how they work and who has to pay them, our guide on what to do about estimated taxes walks through the basics from scratch.
Self-Employment Means No Employer Is Withholding Taxes For You
In a W-2 job, your employer withholds two types of taxes from every paycheck: income tax (federal and state) and payroll tax (also called FICA, which covers Social Security and Medicare). In private practice, you pay both of those yourself, and the payroll tax side is called self-employment tax (SE tax). SE tax is 15.3% on your net self-employment income, up to the Social Security wage base ($176,100 in 2026, per IRS Publication 15).
This is one of the things that surprises therapists most when they first go out on their own. In a salary job, if you made $80,000, you’d see about 7.65% come out of each paycheck for FICA. In private practice, you pay both the employee side and the employer side, so the total is 15.3% before income tax is even factored in. The IRS does let you deduct half of SE tax as an adjustment to income when you file, which partially offsets the hit.
Your estimated payments need to cover both SE tax and income tax. That’s why the calculations can look bigger than people expect.
How Do You Calculate What To Pay By September 15?
There are two methods: the actual method and the safe harbor method. The actual method means estimating your annual income, calculating your approximate tax on that amount, and paying one-quarter each quarter. The safe harbor method is simpler and protects you from underpayment penalties regardless of what you actually owe.
For the actual method, take your gross income for the year so far, subtract your business expenses to get your net profit, then apply SE tax (15.3% on net profit, up to the wage base) and your income tax rate on top. Divide by four and pay that amount per quarter. This requires keeping clean books, which is another reason the mid-year bookkeeping cleanup we wrote about matters for the September payment.
For most therapists we work with, a rough rule of thumb is setting aside 25 to 30% of every deposit for taxes in a dedicated savings account. That range accounts for SE tax plus federal income tax for therapists earning $60,000 to $120,000 in net profit. Your actual number may be higher or lower depending on your state, your deductions, and whether you’ve elected S-corp status.
The Safe Harbor Method Keeps You Out Of Penalty Territory
Safe harbor is the IRS’s built-in protection against underpayment penalties. If you pay at least 100% of what you owed in taxes last year (or 110% if your prior-year income was over $150,000), you won’t be hit with an underpayment penalty regardless of what you owe when you file, even if your income jumped significantly this year.
To use the safe harbor method, look at your prior-year tax return (Form 1040) and find your total tax for the year. Divide that by four and pay that amount each quarter. It’s the simplest calculation, and it fully eliminates the underpayment penalty risk. The downside is that if your income grew a lot this year, you might still owe a lump sum at filing time, but you won’t owe a penalty on top of it.
This is one of the things we help clients figure out when they’re setting up their quarterly payment routine. If you want to run the numbers for your practice specifically, our S-Corp vs PLLC calculator also helps estimate your overall tax picture and how an S-Corp election affects your quarterly payments.
Your Q3 Payment Should Reflect What Your Practice Actually Earned Through July
Even if you’ve been paying the same amount every quarter, it’s worth pausing before September 15 to check whether that number still makes sense. If your caseload grew between April and now, your income is higher than it was when you made your Q2 payment, and your Q3 payment should reflect that.
Pull your year-to-date income and expenses from your bookkeeping software and recalculate using whichever method you’re using. If you’re on safe harbor, your number is the same every quarter, and no recalculation is needed. If you’re using the actual method, a mid-year check keeps the numbers from getting stale.
One thing that catches therapists off guard: Q3 covers income earned June through August, but the payment is due September 15. That means you’re paying before August income is fully in. Factor in your August projection based on your current caseload and session rate when you calculate.
What Happens If You Miss Or Underpay The September 15 Deadline?
Missing the September 15 deadline means the IRS charges an underpayment penalty from that date forward on the amount you were supposed to pay. The penalty is calculated at an annualized interest rate (8% for 2026 as of the most recent IRS update) and it runs until you either pay or file your annual return.
The penalty isn’t catastrophic, but it adds up if you also missed Q1 or Q2 payments. Multiple missed quarters mean multiple penalty calculations running simultaneously. The way the IRS applies these penalties is covered in Form 2210, which your accountant will complete if needed when you file.
If you genuinely can’t pay by September 15, pay what you can. Partial payment reduces the penalty. It doesn’t avoid it entirely, but it’s better than sending nothing.
Setting Aside A Percentage Of Every Deposit Makes The September Payment Easier
The therapists who are least stressed by estimated tax deadlines are the ones who never spend their tax money in the first place. The system that works for most of our clients is simple: every time money comes in from a session, open your bank app and move 25 to 30% of it to a separate savings account labeled “Taxes.” Do it the same day the deposit clears.
By September, the money is sitting there. You log in to the IRS Direct Pay system, enter the amount, pick the Q3 estimated tax payment type, and it’s done. The whole process takes about five minutes once you know your number.
The Profit First system for therapists formalizes this kind of account-based approach.
We wrote about how it works for private practice in our post on Profit First for therapists, which is worth reading if you want a more structured version of the same idea.
If you want help getting your estimated tax payments set up correctly, or you’re not sure whether your current quarterly amounts are right for your practice, we’d be glad to look at it with you.
Use our Let’s Chat page to get started.
TL;DR:
The Q3 estimated tax deadline is September 15, 2026. To avoid penalties, pay at least 100% of last year’s tax divided by four (the safe harbor method), or calculate based on your actual year-to-date earnings. Set aside 25 to 30% of every deposit so the money is ready when the deadline arrives.
