Illinois taxes estates much smaller than the federal limit, and its $4,000,000 exclusion works as a threshold rather than a credit, so an estate that goes over it is taxed under a calculation that climbs steeply: $28 of tax at just over $4,000,000 in the state's examples and $285,714 at $5,000,000.
This guide explains the threshold, shows the Attorney General's own examples, covers how the tax is calculated, where to file and pay, and the QTIP and portability rules. It follows the Illinois Attorney General's estate tax fact sheet.
The Illinois Attorney General administers the Illinois Estate Tax. Its fact sheet says the exclusion amount for Illinois purposes is $4,000,000 and that the exemption for federal estate tax purposes is higher. The fact sheet also says the exclusion amount is a taxable threshold and not a credit against tax.
That wording matters. If an estate's gross value exceeds $4,000,000 after inclusion of adjusted taxable gifts, an Illinois Form 700 must be filed, whether or not a federal return is required. Because the exclusion is a threshold and not a credit, an estate over it is not taxed simply on the part above $4,000,000; the interrelated calculation described below applies to the estate. The Attorney General's examples show the effect: $28 of tax for an estate of $3,000,100 with $1,000,000 of adjusted taxable gifts (just over $4,000,000 in total) and $285,714 for a $5,000,000 estate. That is $285,714 on the $1,000,000 above the threshold, about 28.6% of the excess (our arithmetic from the two examples at $5,000,000, not a flat rate that applies to every estate), so the tax rises steeply just above $4,000,000. The federal basic exclusion amount for 2026 is $15,000,000 under Revenue Procedure 2025-32, so many Illinois taxable estates owe no federal tax.
| Estate | Illinois estate tax | Federal estate tax |
|---|---|---|
| $2,000,000 | $0 | $0 |
| $3,000,000 | $0 | $0 |
| $3,000,100 with $1,000,000 of adjusted taxable gifts | $28 | $0 |
| $4,000,000 (all Illinois property) | $0 | $0 |
| $5,000,000 (all Illinois property) | $285,714 | $0 |
| $5,000,000, 50% in Illinois and 50% in Florida | $142,857 | $0 |
| $13,610,000, surviving spouse, Illinois QTIP election of $9,610,000 | $0 | $0 |
These examples are taken from the Attorney General's fact sheet for decedents dying in 2023 and later. The $5,000,000 estate pays $285,714, which is about 5.71% of the estate. The Florida example shows that property outside Illinois reduces the tax in proportion, even though Florida imposes no state estate tax.
The fact sheet says the Illinois Estate Tax is determined using an interrelated calculation. The Attorney General's website provides a calculator for the computation; you enter the amounts from the Form 700 schedule. For both resident and non-resident decedents, a preliminary tax is calculated as if all assets were located in Illinois, then multiplied by the ratio of Illinois assets to total assets. We do not reproduce the calculation here because the Attorney General's calculator is the authority and the figure depends on several inputs.
The note in the fact sheet is explicit: the portability and carry-over of the unused federal exemption to the surviving spouse is inapplicable to the computation and assessment of the Illinois Estate Tax. A couple therefore cannot rely on a surviving spouse inheriting the first spouse's unused exclusion for Illinois purposes.
The Illinois return is Form 700. The original must be filed with the Illinois Attorney General's Office by U.S. mail, courier or hand delivery, with a copy of the federal return. For Cook, DuPage, Lake and McHenry Counties it goes to the Chicago office; other counties file in Springfield. All taxes, interest and penalties are paid directly to the Illinois State Treasurer, not to the Attorney General.
The tax is due nine months after the date of the decedent's death. Extensions are available on application to the Attorney General, and the Attorney General recognizes federal extensions. To seek an Attorney General extension, file Form 700-EXT or a written explanation of why it is impossible or impractical to file and pay on time, within 9 months of the date of death.
For persons dying January 1, 2009 and after, the estate may make an Illinois QTIP election in addition to any federal QTIP election, by checking the election box on a timely filed Illinois return and providing the surviving spouse's Social Security number. The fact sheet's last example shows how a QTIP election can bring the Illinois tax to zero for a large estate with a surviving spouse. Parties to a civil union recognized in Illinois are treated like spouses for the Illinois marital deduction and QTIP elections, and the fact sheet lists the extra documents they must file.
Estates near $4,000,000 are the ones most affected by the threshold, and the tax rises steeply above it, so the valuation and the inclusion of lifetime gifts matter. Talk to a qualified professional if your estate is close. For how other states compare, see our state estate tax guide, and for the federal rules see our federal estate tax exemption guide. For Illinois income and property taxes see our Illinois income tax guide.
Illinois Attorney General: Estate Tax Instruction Fact Sheet, IRS Revenue Procedure 2025-32.
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