Q3 estimated tax deadline September 2026: Accountant's preparation checklist

Published Friday 10 July 2026

Table of contents

• When are estimated taxes due for Q3?
• How to protect clients from estimated tax penalties
• Your Q3 estimated tax preparation checklist
• How estimated taxes can help you build advisory relationships
• Simplify tax management with the Xero Partner Program
• FAQs on estimated tax deadlines

Key takeaways

• The Q3 estimated tax payment covers income earned from June 1 through August 31 and is due September 15, 2026. Estimated taxes are calculated using Form 1040-ES and 1040-ES (NR) for non-residents.
• You can use the safe harbor method or the current year method to calculate estimated taxes for clients. To avoid underpayment penalties, clients either need to pay 90% of their estimated total bill for the current year, or 100% of last year's tax due in total.
• Check state-specific requirements for clients, and remind them when payments are due to avoid penalties. More conversations with clients about tax planning can open up opportunities for advisory services.

When are estimated taxes due for Q3?

The Q3 estimated tax deadline is September 15, 2026, which falls on a Tuesday this year. The payment is for income earned between June 1 – August 31.

Estimated taxes are calculated using Form 1040-ES and 1040-ES (NR). Your clients must submit estimated taxes to the IRS if their income is not subject to withholding. The payments cover income tax, self-employment tax, and alternative minimum tax.

• January 1 – March 31: Due April 15, 2026
• April 1 – May 31: Due June 15, 2026
• June 1 – August 31: Due September 15, 2026
• September 1 – December 31: Due January 15, 2027

Which clients need to make estimated payments

Any client who expects to owe $1,000 or more in tax after subtracting withholding and credits is required to make estimated payments. In practice, this means you should flag self-employed clients, freelancers, landlords with rental income, and investors with significant capital gains or dividend income each quarter.

Estimated taxes schedule for the rest of 2026

The remaining estimated tax deadline is January 15, 2027, for the period September 1 – December 31. The Q1 and Q2 estimated tax deadlines were April 15 and June 15, for the periods January 1 – March 31 and April 1 – May 31.

Not paying enough tax can result in underpayment penalties for your clients, so they'll need your help creating accurate estimates for the year ahead.

How to protect clients from estimated tax penalties

Avoiding estimated tax penalties is all about calculating amounts accurately. You have two options calculating quarterly estimated taxes for clients:

• Safe harbor method
• Current year method

The safe harbor method involves using your client's tax return from the previous year. You divide the total tax that was due last year into four equal quarterly tax payments. By paying 100% of the tax that was due last year, your clients can avoid an underpayment penalty.

If your client earns over $150,000, they need to pay 110% of the tax due last year. This method is simple to apply, but, if a client's income has grown significantly this year, they may need to pay a large lump sum of the remaining tax when their return is submitted.

The current year method involves working with estimates, since your clients won't know their total earnings for the year ahead. You start by estimating their adjusted income for the year, including earnings from self-employment, dividends, and interest. Then, you make adjustments to this amount for things like student loan payments and health savings account contributions.

On the 1040-ES form, you calculate expected deductions, taxes, and credits, before applying the current year tax rates to work out their estimated bill. If you use the current year method, the IRS requires that at least 90% of the total tax is paid through estimated payments. If your clients don't reach this threshold, they'll face underpayment penalties.

Keep in mind that clients who expect to owe less than $1,000 after withholding and credits aren't required to make estimated payments at all. For clients with income that fluctuates throughout the year, consider the annualized income installment method using Form 2210 Schedule AI, which bases each quarterly payment on income actually received during that period rather than dividing the annual estimate equally.

Your Q3 estimated tax preparation checklist

Follow these steps to prepare your clients and practice for the Q3 estimated tax deadline:

1. Check state-specific filing deadlines and requirements to see if you need to do any additional filing for clients.
2. Gather client records for the quarter and calculate their total income.
3. Apply deductions to their income, such as business expenses and tax credits.
4. Use Form 1040-ES to calculate their adjusted gross income and tax liability. Previous estimates made for Q1 or Q2 could be inaccurate if your clients' income increased unexpectedly.
5. Once you've calculated clients' tax liability, subtract payments already made in Q1 and Q2 and any taxes withheld by an employer, or for a pension.
6. Double check that clients are paying at least a quarter of last year's tax bill, or that their Q3 payment will contribute to 90% of their estimated bill this year.
7. Send clients reminders to pay by September 15. Tell them the exact amount due, and give them plenty of notice to pay.

How estimated taxes can help you build advisory relationships

Planning for taxes opens up conversations with clients about cash flow, growth, and how their business is tracking, all of which you can help with as their trusted advisor. By calculating their estimated taxes, you'll already be paying close attention to their records and financial position.

You'll spot opportunities for growth by reviewing their records, or patterns and trends that could be addressed. Offering advisory services gives you more ways to help clients, by using the same financial records that go into compliance work. The information you use to produce estimates could also be used to help clients improve their financial performance or close cash flow gaps.

Simplify tax management with the Xero Partner Program

Helping your clients with estimated taxes requires complete oversight of their financial records. With Xero HQ, you can see all of your clients in one place and drill down into their live financial records for easier tax estimations.

Xero Practice Manager lets you assign tasks to the team and follow up with clients for missing data, so you can manage your client work and practice work from a single platform.

Join the partner program

FAQs on estimated tax deadlines

Here are answers to frequently asked questions about quarterly estimated taxes.

What is the 110% rule for estimated tax payments?

If a client using the safe harbor method earned more than $150,000 in the previous tax year, they must pay 110% of last year's total tax bill through quarterly estimated payments. Falling short of this threshold triggers the underpayment penalty even if the client's current-year income has dropped. Clients below $150,000 only need to cover 100% of their prior year's liability.

How do you avoid the penalty for underpayment of estimated tax?

The safe harbor and current year methods outlined above are the two main approaches to avoiding the penalty. If a client does trigger the penalty, you can use Form 2210 to calculate the exact amount owed or request a waiver from the IRS in cases of casualty, disaster, or other unusual circumstances.

Do state estimated tax deadlines match federal deadlines?

State estimated tax deadlines don't always match federal deadlines. Some states follow the same quarterly schedule, while others have different due dates or requirements. Check the specific state deadlines that apply to each of your clients before filing to make sure they're in compliance with both state and federal rules.

What happens if a client misses an estimated tax payment?

If a client misses an estimated tax payment, the IRS may charge an underpayment penalty calculated as interest on the unpaid amount from the due date until the payment is made. The penalty applies separately to each missed or underpaid quarter. Clients who miss a deadline should make the payment as soon as possible to minimize the interest that accrues, and you can use Form 2210 to calculate or request a waiver of the penalty in certain circumstances.

What do you do when a client refuses to pay quarterly estimates?

Make sure clients are clear that their quarterly estimates contribute to their total tax bill, and that paying each one is essential for avoiding underpayment penalties. If clients are struggling to pay quarterly taxes for financial reasons, direct them towards a payment plan with the IRS. This will help them avoid future penalties.