A 10-Year Marriage, $120K vs $40K Incomes: What Illinois's Guideline Formula Says
33.33% of the higher earner's net income minus 25% of the lower earner's — Illinois's actual statutory formula, one of the few states with a published one. $120K vs $40K over 10 years works out to roughly $24K/year for about 4.4 years. See your own numbers against the same formula.
One of the only states with an actual formula
We ran a 10-year marriage through the calculator: a $120,000 net-income earner and a $40,000 net-income earner. Illinois's guideline formula — 33.33% of the higher earner's net income minus 25% of the lower earner's — works out to a raw $30,000/year, but the statute caps the payee's total income (their own plus the maintenance) at 40% of the couple's combined net income, which pulls the actual guideline amount down to $24,000/year. The duration table then applies: a 10-year marriage lands in the bracket giving 44% of the marriage length, or about 4.4 years of maintenance.
Most states don't have anything this concrete — alimony is typically a multi-factor judicial determination weighing the marriage's standard of living, each spouse's earning capacity, age, and health, with no published formula to plug numbers into. Illinois's formula gets used as a reference point across many "how is alimony calculated" discussions precisely because it's one of the few places with an actual number to point to.
How the math works
Raw amount = 33.33% × payor's net income − 25% × payee's net income, floored at 0. Capped amount = min(raw amount, 40% of combined net income − payee's net income), also floored at 0. Duration follows the statute's bracket table: 20% of the marriage length under 5 years, then a new 1-year bracket starting at 24% for a 5-6 year marriage and increasing 4 percentage points per additional year-bracket, capping at 80% for the 19-20 year bracket. At 20+ years, the court may instead order permanent maintenance — this tool flags that rather than guessing a number.
Source: 750 ILCS 5/504 (Illinois Marriage and Dissolution of Marriage Act). The statutory formula applies to couples with combined gross income up to $500,000.
Math runs locally. Inputs never leave your browser.Source on github.
Where this calculation doesn't apply
- You're not in Illinois.This is one state's codified formula, shown purely as a reference point for what a guideline-based approach looks like — it has no legal force outside Illinois, and even similar-sounding "guideline" states often use different percentages or caps.
- Combined gross income exceeds $500,000.Above that threshold, Illinois's own statute no longer prescribes the guideline formula — maintenance reverts to full judicial discretion.
- There are grounds for deviation.Even within Illinois, a judge can deviate from the guideline for factors like health, earning capacity, or the standard of living during the marriage — none of which this formula weighs.
- Short marriages with minimal income gap.The guideline amount and duration both shrink fast at the low end — for a short marriage with similar incomes, the estimate may be close to $0 either way.
What to actually do
- Use this as a starting reference point for the conversation, not a prediction of any actual court order in your state.
- Look up your own state's approach — many states publish nothing like a formula, so a family-law attorney's read on local judicial norms matters more than any calculator.
- Combine this with the Cost of Divorce tool for the fuller financial picture — that tool explicitly excludes alimony, this one fills that gap.
- If negotiating a settlement, treat this range as a floor for discussion, not a target — actual awards depend heavily on factors this formula doesn't weigh.
- Talk to a family-law attorney in your own state before relying on any number here.