The Illinois estate tax, computed honestly.

The facts, in one place.

Each of these is computed by the engine on this page from the state's published method and the 2026 figures. Quote them with the year attached.

1. Illinois taxes estates above $4,000,000. The federal estate tax begins at $15,000,000 for deaths in 2026. Between those two lines, Illinois is the only estate tax that applies.
2. A $5,000,000 all-Illinois estate owes the state $285,714 and the federal government $0. That is the state's own published example.
3. Just over the line, each added dollar costs about 28.6 cents of Illinois tax. The ramp eases at about $5.4 million, where the state's credit schedule takes over.
4. Illinois lets a surviving spouse inherit none of a late spouse's unused exclusion. A married couple with $7,000,000 owes Illinois $565,603 at the second death if everything passed outright to the survivor, and $0 if the first death set $4,000,000 aside.
5. For an Illinois decedent with one federal exclusion, the federal tax stays at $0 until the estate reaches about $16.9 million, because the Illinois tax is deducted before the federal tax is figured.
6. Property sited outside Illinois shrinks the bill in proportion. A $5,000,000 estate with half its property in Florida owes $142,857, half the all-Illinois figure, even though Florida has no estate tax.
7. SB 3847 would raise the line to $8,000,000 for deaths from January 1, 2027. As of September 3, 2026: Referred to Senate Assignments on February 6, 2026; a co-sponsor was added June 26, 2026. It has not passed either chamber and is not law.

Key numbers.

The rules the calculator actually uses, each with the year it applies to and where it comes from. Illinois is not a simple bracket table; the marginal rate below is a property of the state's method, not a published rate.

How Illinois counts.

A threshold, not a credit. Below $4,000,000 there is no Illinois tax and no Illinois return. One dollar over, the whole estate enters the computation. That sounds like a cliff, and it is not one, because the computation itself starts gently: the first dollars over the line cost about 28.6 cents each, not the whole estate's worth of tax at once.

The method is the old federal credit. Illinois fixed its tax at what the federal credit for state death taxes would have been under the table in force at the end of 2001. It is applied to the taxable estate less $60,000, and it is interrelated: the Illinois tax is itself deducted before the credit is figured, so the state's calculator loops until the number settles. Then a second rule: if 40% of the amount over the line, after deducting the tax, is lower than that credit, the lower figure is the tax. Just over the line the second rule wins, which is the ramp. Further out the credit table wins, and the marginal rate drifts down to about 13.8% at the top of the table.

Property elsewhere. The tax is computed as if everything were in Illinois and then multiplied by the Illinois share of the gross estate. A Wisconsin lake house does not just escape; it shrinks the Illinois bill in proportion. Which property has an out-of-state tax situs is a legal determination made on the return.

The federal side. The Illinois tax actually paid is deducted from the estate first. Whatever remains above the federal exclusion is taxed at 40%. A surviving spouse may add a late spouse's unused federal exclusion by a timely election; the calculator's second option assumes the full amount was unused. Illinois offers no such carry-over, which is why the married-couple view exists.

What one input cannot do. A real Form 700 starts from the federal Form 706: deductions, adjusted taxable gifts, the marital and charitable deductions, the Illinois QTIP election, and the situs of each asset. This page takes one number, the taxable estate after all of that, and tells you what the two taxes would be on it. It is the arithmetic, not the return.

Worked examples.

Official rows are the Illinois Attorney General's published examples; the engine must reproduce all 4 to the dollar or the site will not build. Derived rows are ours, computed by the same engine from the inputs shown, and pinned so they cannot drift.

A married couple, at the second death.

Derived, Illinois only, under 2026 law. The same combined estate at both deaths, no growth, no spending, nothing to charity. Illinois ports nothing between spouses, so “everything to my spouse” spends the first exclusion; setting $4,000,000 aside at the first death keeps it. An illustration of the mechanism, not advice.

The ramp, drawn.

Illinois estate tax against taxable estate under 2026 law, one all-Illinois estate, no deductions. The table beneath is the same data, with the federal tax for one exclusion alongside. The chart is free to reuse with its attribution; there is a download below.

Methodology.

Enough for a qualified reader to reproduce every figure on this page. Law year 2026; record reviewed September 3, 2026.

1. Input. One taxable estate in whole dollars: the federal taxable estate after deductions, with adjusted taxable gifts included as Illinois requires. Nothing passing to a spouse or charity unless it has already been deducted from the number you enter. Optional: the share of the gross estate with an Illinois tax situs, as a percentage.
2. Illinois, step one. If the estate is at or below the exclusion, the tax is $0. Otherwise apply the § 2011 credit table as of December 31, 2001 to the estate less $60,000 less the tax itself, iterating until successive results differ by under a dollar. This is the Attorney General's first loop, ported line for line.
3. Illinois, step two. If 40% of the estate less the tax less the exclusion is smaller than the step one result, replace it with that interrelated 40% figure, again iterated to convergence. This is the second loop, and the source of the 28.6-cent ramp.
4. Apportionment. Multiply the whole-estate figure by the Illinois share (35 ILCS 405/3). The state's own example, half in Florida, halves the tax.
5. Federal. Deduct the Illinois tax actually paid (26 U.S.C. § 2058). Subtract the selected exclusion: $15,000,000 for one person, or $30,000,000 where a late spouse's full unused exclusion was ported. Tax any positive remainder at 40%; because both totals sit in the top bracket of § 2001(c), the bracket terms cancel and this is exact.
6. Rounding. Illinois and federal results are rounded to whole dollars once, at the end; nothing is rounded mid-loop. The Attorney General's calculator shows $0 for the $0.29 that one dollar over the line produces; this engine agrees.
7. The married-couple view. Treats the entered amount as a couple's combined estate and computes the Illinois tax at the second death two ways: on the whole amount with one exclusion, and on the amount less what the first death set aside (the smaller of the exclusion and the estate). Same total at both deaths, no growth, no spending, no charitable or other deductions, federal side omitted. It shows the mechanism, not a plan.
8. The proposed-law setting. Replaces the exclusion with the $8,000,000 figure in SB 3847 as introduced and leaves the computation otherwise unchanged. That is an assumption of ours: the bill would also add portability and later indexing, which this setting does not model. It is labeled “not law” wherever it appears.
9. Validation. Every published example in the state's fact sheet, including the apportionment example, is a build-time test with zero tolerance; so are the derived rows above, the couple illustration, and the proposed-law cases. Any engine change that moves a pinned result fails the build until the expectation is consciously updated and logged below.
10. Not modeled. Deductions of any kind, the Illinois QTIP election, generation-skipping transfers, installment payment, interest and penalties, non-resident decedents, and any change in law after September 3, 2026. These are educational estimates, not a filing and not advice.

Sources.

Primary sources only for anything that changes a number. Retrieved is when we fetched the text; verified is the last day someone re-read it against this page.

1. 1. Illinois Attorney General, Estate Tax Calculator (online application; computation file CalcHtm.js). The interrelated Illinois computation this engine ports line for line: the § 2011 credit loop and the 40%-over-the-exclusion loop. Retrieved June 11, 2026; verified September 3, 2026.
2. 2. Illinois Attorney General, Important Notice Regarding Illinois Estate Tax and Fact Sheet. The $4,000,000 threshold, the published worked examples, the statement that federal portability does not apply to Illinois, and the out-of-state apportionment example. Retrieved June 11, 2026; verified September 3, 2026.
3. 3. Illinois General Assembly, 35 ILCS 405/2 — Illinois Estate and Generation-Skipping Transfer Tax Act, definitions. The exclusion amount as a threshold, and the state tax credit frozen at the federal § 2011 table in force on December 31, 2001. Retrieved June 11, 2026; verified September 3, 2026.
4. 4. Illinois General Assembly, 35 ILCS 405/3 — Illinois estate tax, apportionment for property outside Illinois. The reduction of a resident's tax by the share of the estate that has no Illinois tax situs. Retrieved June 11, 2026; verified September 3, 2026.
5. 5. United States Code (Cornell LII), 26 U.S.C. § 2011 — Credit for State death taxes (table as of 2001). The bracket table the Illinois computation is built on. Retrieved June 11, 2026; verified September 3, 2026.
6. 6. United States Code (Cornell LII), 26 U.S.C. § 2058 — Deduction for State death taxes. Deducting the Illinois tax actually paid before the federal tax is computed. Retrieved June 11, 2026; verified September 3, 2026.
7. 7. United States Code (Cornell LII), 26 U.S.C. § 2001(c) — Federal estate tax rate schedule. The 40% rate on every dollar above the basic exclusion. Retrieved June 11, 2026; verified September 3, 2026.
8. 8. Internal Revenue Service, IRS releases tax inflation adjustments for tax year 2026. The $15,000,000 federal basic exclusion for 2026 deaths. Retrieved June 11, 2026; verified September 3, 2026.
9. 9. Internal Revenue Service, Rev. Proc. 2022-32 — Portability election relief. That a surviving spouse may add a deceased spouse's UNUSED federal exclusion by a timely election; the ported amount is the unused portion only. Retrieved June 11, 2026; verified September 3, 2026.
10. 10. Illinois General Assembly, SB 3847 bill status, 104th General Assembly. The pending-bill note and the proposed-law setting in the calculator. Retrieved September 3, 2026; verified September 3, 2026.

Planning context, and the human explanation of all of this, is in The four-million-dollar line, which renders this same calculator.

Revision history.

Material changes only: data, formula, assumptions, or scope. Copy edits are not logged.

September 3, 2026

Reference layer added: key numbers, official and derived examples rendered from the engine, methodology, sources, and this history. Calculator gained an out-of-state share, a married-couple second-death view, and a clearly labeled proposed-law setting for SB 3847. SB 3847 status re-checked; still in Senate Assignments, not law. Sources re-verified. No change to any enacted-law result.

June 11, 2026

Added the federal portability option (one exclusion, or two with a late spouse's full unused exclusion) and the explanation of why the federal tax stays $0 until roughly $16.9 million for an Illinois decedent.

June 11, 2026

First release. Illinois computation ported from the Attorney General's calculator and validated against the state's published examples; federal estimate applies the § 2058 deduction and the 2026 $15,000,000 exclusion.

Canonical address: https://consideratecapital.com/tools/illinois-estate-tax-calculator