August 30, 2026 7:34 pm
September 15 is one of the most overlooked tax deadlines of the year.
April 15 gets all the attention. But if you own a small business in California and your income is not subject to payroll withholding, the IRS expects a payment from you on September 15 just as surely as it does in April. Miss it and the penalty starts accruing immediately – not at filing. Right now. Here is what is due, who owes it, and how to make sure you are covered.
The U.S. tax system operates on a pay-as-you-go basis. Employees pay as they earn because their employer withholds taxes from every paycheck. Business owners do not have that automatic mechanism, so the IRS requires them to estimate their annual tax liability and prepay it in four installments throughout the year.
Those installments are called quarterly estimated taxes, and they cover federal income tax plus self-employment tax – Social Security and Medicare – for sole proprietors, partners, and S-corp shareholders who take distributions beyond their salary. If you skip them, the IRS does not simply send a bill in April. It charges an underpayment penalty for every quarter you were short, even if you pay the full balance when you file your annual return.
You generally owe quarterly estimated taxes if any of the following apply:
California has a similar threshold for state taxes, set at $500. If you own a business in the Bay Area and your books show consistent monthly revenue, there is a very good chance you fall into this category.
The 2026 federal estimated tax schedule runs on four dates:
Quarter | Period Covered | Federal Due Date |
Q1 | Jan 1 – Mar 31 | April 15, 2026 |
Q2 | Apr 1 – May 31 | June 16, 2026 |
Q3 | Jun 1 – Aug 31 | September 15, 2026 ◀ YOU ARE HERE |
Q4 | Sep 1 – Dec 31 | January 15, 2027 |
California does not follow the same 25% per quarter schedule the IRS uses. The California Franchise Tax Board (FTB) uses a 30/40/0/30 split:
This means if you have already met your Q1 and Q2 California obligations, you owe the FTB nothing on September 15. The IRS payment is still due. California’s is not. September 15 is a federal-only payment date for most California filers – and this catches business owners in two directions. Some assume nothing is due to anyone and miss the federal payment entirely. Others send a California payment out of habit and tie up cash unnecessarily.
There are two methods. Most small business owners use the safer one.
You avoid the underpayment penalty entirely by paying a set amount based on your previous year’s tax return, regardless of what you actually owe this year.
This is called the safe harbor. Even if your 2026 income ends up significantly higher, meeting this threshold protects you from the penalty.
Example: Your 2025 federal tax was $12,000 and your AGI was under $150,000. Your safe harbor is $12,000. Each quarterly payment should be $3,000. You’ve already made Q1 and Q2 payments. Your Q3 payment due September 15 is $3,000.
If you have clean, current books and a clear picture of your 2026 income, you can project your actual tax liability and pay 90% of that across the four quarters. This works when your income this year is materially lower than last year. It only works if your bookkeeping is accurate and up to date – estimating from messy records leads to underpayment and penalties.
The IRS underpayment penalty for 2026 is running at approximately 7% annualized. That is not a flat annual fee applied at filing. It is calculated per quarter, on the amount underpaid, for as long as it was underpaid. The penalty for Q3 starts accruing on September 16 if your payment is late or short.
To put it plainly: you can pay the full balance in April and still owe a penalty for every quarter you were under. Filing an extension does not help either. An extension gives you more time to file your return. It does not extend the estimated tax payment deadlines.
Federal (IRS): IRS Direct Pay at irs.gov is free, connects directly to your bank account, and provides instant payment confirmation. EFTPS (Electronic Federal Tax Payment System) is preferred for businesses making recurring payments. Use Form 1040-ES when submitting.
California FTB: FTB Web Pay at ftb.ca.gov handles state payments. Remember that for most individual filers, no California state payment is due in September under the 30/40/0/30 schedule.
Keep your payment confirmation in every case. These records matter if the IRS ever questions your compliance.
If you elected S-corp status, your salary is subject to payroll withholding. But distributions beyond that salary are not. If you are taking meaningful distributions and your withholding does not cover 90% of your total tax liability, you personally owe estimated taxes on that distribution income.
Many S-corp owners in the Bay Area assume that running payroll handles everything. It does not, if your distributions significantly exceed your W-2 wages. This is one of the most common oversights we see when reviewing books for new clients.
Each quarter is tested independently by the IRS. Missing Q1 and Q2 means penalties have already started accruing for those periods. However, paying Q3 on time on September 15 stops the penalty from growing further on your Q3 obligation. It does not retroactively fix the earlier quarters, but it prevents the situation from compounding.
If you have fallen behind, the right move is to get current as quickly as possible and work with a tax professional to calculate whether any penalty mitigation applies to your specific situation.
Quarterly estimated taxes are only as accurate as the numbers behind them. If your books are disorganized, outdated, or reconciled infrequently, you are essentially guessing at your tax liability. That guessing is how business owners end up with either underpayment penalties or unnecessary overpayments that hurt cash flow.
Clean, current bookkeeping is what makes this calculation straightforward rather than stressful. When your income and expenses are properly categorized and reconciled each month, your estimated tax projection takes minutes, not hours of scrambling before a deadline.
Nuage Digital works with small businesses across Campbell and the Bay Area on exactly this kind of planning. Our team includes an Enrolled Agent and QuickBooks ProAdvisor specialists who keep your books current and your quarterly estimates accurate year-round. If September 15 is coming up fast and you are not certain what you owe, reach out to us at nuage-digital.com/contact-us/ – we will look at your numbers and tell you exactly where you stand.
Possibly. If your side business income plus the gap in W-2 withholding leaves you owing $1,000 or more in federal tax, estimated payments apply to you. The W-2 withholding counts toward your total, but it may not be enough on its own if your business income is meaningful.
The standard equal-quarter approach can overstate what you owed in earlier periods. The IRS annualized income installment method on Form 2210 Schedule AI lets you calculate each quarter's payment based on income actually earned up to that point. This can significantly reduce or eliminate underpayment penalties when income arrives unevenly.
No. The IRS tests each quarter separately. Paying all of your estimated taxes in Q4 means penalties have already been accumulating for Q1, Q2, and Q3, even if your total year-end payment is correct. The quarterly deadlines are real cutoffs, not suggestions.
No. A filing extension only gives you more time to submit your return. It does not extend the payment deadline. If you filed a personal extension in April, your Q3 estimated payment is still due September 15.
Yes. QuickBooks has features that help estimate quarterly tax liabilities based on your profit and loss, and you can set aside funds in a dedicated account. Accurate categorization throughout the year is what makes those projections reliable - garbage in, garbage out. This is why clean monthly reconciliation matters well before any deadline arrives.
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