Guide

How to file your Illinois sales tax return (ST-1)

Registering is step one. After that comes a routine: file a return and pay on a schedule. This guide covers how often, when, how, and what late costs.

General information only, not legal or tax advice. I am not a lawyer, a CPA or an accountant. Flipfiled does not prepare tax returns or give legal advice. This page explains what government sources say. Rules, rates and fees change, so confirm with the agency. The dollar examples are made up to show the math.

The short version

• After you register, you file a Form ST-1 (Sales and Use Tax and E911 Surcharge Return) for every reporting period. IDOR assigns how often.
• The return and the payment are due on or before the 20th day of the month after the period ends. If the 20th falls on a weekend or holiday, the deadline moves to the next business day.
• You file and pay online through MyTax Illinois (opens in a new tab). Some taxpayers are required to e-file.
• Filing late costs money, even when little or no tax is due. See the penalty section below.

Before you start: what you need

1. An Illinois sales tax account. If you have not registered yet, see the registration guide. Register after you have formed your business and have an EIN; see the order of steps.
2. A MyTax Illinois login linked to that account.
3. Your sales records for the period: total sales, sales for resale (if any), and taxable sales. A good record system makes this a 20-minute job.
4. The correct tax rate for your location. See how much sales tax to charge in Chicago.

How often do I file?

You do not pick your schedule. IDOR sets it from your initial registration and your annual liability. The ST-1 instructions say the Department may notify you of a change based on your average monthly liability:

The state’s administrative rule on quarterly returns also says the Department may authorize quarterly filing when average monthly liability is $200 or less, and may change your schedule and must notify you. If your average monthly liability is $20,000 or more, IDOR says you must make quarter-monthly payments (on the 7th, 15th, 22nd and last day of the month).

Due dates

If the 20th is a weekend or holiday, the due date is the next business day. Sources: ST-1 instructions (opens in a new tab); Illinois Administrative Code, Title 86, Part 130 (opens in a new tab); IDOR: Retailers’ Occupation Tax (opens in a new tab).

Step by step: filing a return

1. Log in to MyTax Illinois (opens in a new tab).
2. Pick the return and period. Choose the ST-1 for the period that just ended.
3. Enter your sales. The return asks for your gross receipts and your deductions, such as sales for resale. MyTax Illinois fills in combined tax rates based on the locations you registered. Check that they match the current Chicago rate.
4. Handle prior-rate sales. If you were paid after the August 1, 2026 rate change for a sale taxed at the older rate, those go on Lines 8a and 8b, per IDOR’s bulletin.
5. Take the vendor’s discount if you qualify. Line 10 of the ST-1 gives a discount of 1.75% of the tax due or $5 per calendar year, whichever is greater. If you must e-file and have no waiver, you qualify only by e-filing and paying on time. If you are not required to e-file, you qualify by mailing or e-filing and paying by the due date.
6. Submit and pay electronically. Save the confirmation.
7. File your records. Keep your work papers and the confirmation with your business records.

The ST-1 has other schedules for things most resellers do not use, such as E911 surcharge details. IDOR’s instructions walk through each line. Source: ST-1 instructions (R-01/24) (opens in a new tab). The instructions are also on the free forms page.

What if I had no sales this period?

The ST-1 instructions I read do not directly address a period with no sales. IDOR’s penalty publication says that a second-tier failure-to-file penalty “is assessed even if there is no tax due,” which shows IDOR expects a return for each period even when nothing is owed. Treat a quiet month like any other: file a return showing no sales rather than skipping it. If you stopped selling altogether, ask IDOR how to close or change your account.

What does filing or paying late cost?

IDOR’s Publication 103 lists penalties and interest. These are the rules, not a prediction for your case:

• Late filing. A first-tier penalty equal to the lesser of $250 or 2% of the tax required to be shown on the return. If you still have not filed 30 days after IDOR sends a notice of nonfiling, an additional penalty applies: the greater of $250 or 2% of the tax, capped at $5,000.
• Late payment. 2% of the unpaid tax if the payment is 1 to 30 days late; 10% if it is 31 or more days late. Higher rates (15% and 20%) apply in some audit situations.
• Interest. Simple interest, calculated daily, starting the day after the due date. The rate is reviewed January 1 and July 1.
• Vendor’s discount. You may lose it if you do not meet the filing and payment conditions above.

An invented example: your quarterly tax due is $315.00.

Interest, any filing penalty and the lost vendor’s discount would come on top. The bigger risk is letting returns pile up. If you are behind, read what to do if you are behind on sales tax. Source: IDOR Publication 103 (revised September 2025) (opens in a new tab).

Habits that make filing easy

• Put the due dates on your calendar the day you register.
• Keep sales tax you collect in a place you will not spend it.
• Reconcile your sales system to your marketplace reports before you file.
• Save a copy of every return and confirmation.

I am not a CPA or an attorney

This page is general information, not tax or legal advice. Flipfiled does not prepare tax returns. I can help with the clerical side of registering and with reminders. For questions about what to report, ask IDOR or a CPA. Checked Oct 30, 2026 against the ST-1 instructions (R-01/24), Publication 103 (September 2025) and IDOR’s ROT page.