Why there is no single alimony formula
Spousal support exists to address the economic consequences of a marriage ending: one spouse's earning capacity may have been built during it while the other's was set aside, and the standard of living established during a long marriage does not divide cleanly in half. Every state recognises that. Almost none of them reduces it to a formula the way child support has been reduced to one.
Instead, most statutes list factors — the length of the marriage, the standard of living, each spouse's income and earning capacity, contributions to the other's career, age and health, and the time needed to acquire training — and leave the amount to judicial discretion. The result is wide variation between courtrooms on comparable facts, which is what drove the development of guideline formulas in the first place.
Three are represented here. Illinois enacted a statutory guideline at 750 ILCS 5/504(b-1): maintenance is 33⅓% of the payor's net income less 25% of the payee's net income, and the result added to the payee's net income may not exceed 40% of the parties' combined net income. It applies only where combined gross income is under $500,000. The AAML formula, proposed by the American Academy of Matrimonial Lawyers, uses 30% of the payor's gross less 20% of the payee's gross with the same 40% ceiling. Texas is structurally different: eligibility is narrow, and Tex. Fam. Code §8.055 caps maintenance at the lesser of $5,000 a month and 20% of the payor's average monthly gross income.
A guideline is a starting point, not an entitlement. Courts depart from them, parties negotiate around them, and in states with no guideline at all they are still used as a reference in negotiation because both sides recognise the arithmetic.
The formula, the cap, and the duration band
The two-percentage formulas take the shape M = a·Iₚ − b·Iᵣ. The subtraction is what makes them income-gap formulas rather than income-share formulas: as the payee's own income rises, the award falls, and once b·Iᵣ reaches a·Iₚ it reaches zero. On the Illinois percentages that happens when the payee earns 4/3 of the payor's income.
The 40% cap is the part people miss, and it binds more often than the formula does. It says the payee's own income plus the maintenance cannot exceed 40% of the parties' combined income. Written out, M ≤ 0.40(Iₚ + Iᵣ) − Iᵣ. Take a $120,000 payor and a $40,000 payee under the Illinois guideline: the formula gives 40,000 − 10,000 = $30,000, but the cap allows only 0.40 × 160,000 − 40,000 = $24,000. The cap reduces the award by $6,000 a year, and the payee lands on exactly 40% of the combined income — 64,000 of 160,000. Whenever the cap binds, the payee's share output reads exactly 40.00%.
Duration is a separate calculation from amount. Illinois multiplies the length of the marriage by a factor that starts at 0.20 for marriages under five years and rises by 0.04 for each additional year, reaching 0.80 at nineteen years; at twenty years or more the court may order maintenance for a period equal to the length of the marriage or for an indefinite term. A fourteen-year marriage therefore yields 0.20 + 0.04 × (14 − 4) = 0.60, so 14 × 0.60 = 8.4 years. The AAML proposal uses broader bands — roughly half the marriage length in the middle range, three quarters for long marriages, and permanent beyond twenty years. Texas caps duration by statute at five, seven or ten years depending on whether the marriage lasted at least ten, twenty or thirty years.
Worked example: $120,000 and $40,000 after a 14-year marriage
Take a payor with $120,000 of income, a payee with $40,000, a fourteen-year marriage, no child support order, and the Illinois guideline. Both figures are net, as that statute requires.
- Formula amount. ⅓ × 120,000 = 40,000. Less ¼ × 40,000 = 10,000. That gives $30,000 a year.
- Apply the cap. Combined income is 160,000. 40% of that is 64,000. The payee already has 40,000, so the maximum maintenance is 64,000 − 40,000 = $24,000. The cap binds and reduces the award by $6,000.
- Monthly. 24,000 ÷ 12 = $2,000.00.
- Duration. The factor for a 14-year marriage is 0.20 + 0.04 × 10 = 0.60, so 14 × 0.60 = 8.4 years.
- Total. 24,000 × 8.4 = $201,600.
- Check the share. The payee ends on 40,000 + 24,000 = 64,000, and 64,000 ÷ 160,000 = 40.00%, which is exactly where the cap puts them.
Now switch the tax rule. Under the pre-2019 federal treatment the payor deducts the payments, so $2,000 a month costs a payor at a 24% rate 2,000 × 0.76 = $1,520, while a payee at 12% keeps 2,000 × 0.88 = $1,760. The transfer moved $2,000 of income from a 24% bracket to a 12% one and created $240 a month of value out of the tax difference. Under current law, for any instrument executed after 2018, the payor pays $2,000 from after-tax income and the payee receives $2,000 tax-free — that $240 no longer exists, which is why post-2018 settlements are frequently negotiated to lower nominal amounts than pre-2019 ones on the same incomes.
How to read the result
Treat the monthly figure as an anchor for negotiation rather than a prediction. In a guideline state a court is expected to apply the formula or to explain in writing why it did not; in a non-guideline state the same number carries no authority at all but still tells both sides what a comparable jurisdiction thinks the income gap is worth.
Check whether the 40% cap is binding before arguing about percentages, because when it is, the percentages have stopped mattering. Above the cap, changing the formula from 30/20 to 33⅓/25 changes nothing at all — both are cut back to the same ceiling. What moves the answer then is the income figures themselves.
Watch the income basis. The Illinois guideline works from net income, the AAML formula from gross, and mixing them produces an answer that is wrong by roughly the tax rate. If you enter gross figures into the Illinois setting, the result is materially overstated.
Several things no formula captures, and any of them can dominate. Imputed income: a court may attribute earning capacity to a voluntarily unemployed or underemployed spouse rather than using actual income. The property division interacts with support directly — a spouse who receives income-producing assets needs less maintenance. Health and age can convert a term award into an indefinite one. Remarriage or cohabitation generally terminates maintenance. And a modification requires a substantial change in circumstances unless the award was made non-modifiable by agreement, which is a trade parties make deliberately.
Illinois duration factor by length of marriage
| Years married | Factor | Years of maintenance |
|---|---|---|
| 3 | 0.20 | 0.60 |
| 5 | 0.24 | 1.20 |
| 8 | 0.36 | 2.88 |
| 10 | 0.44 | 4.40 |
| 12 | 0.52 | 6.24 |
| 14 | 0.60 | 8.40 |
| 16 | 0.68 | 10.88 |
| 19 | 0.80 | 15.20 |
| 20 or more | — | Length of the marriage, or indefinite |
Each term is the marriage length multiplied by the factor on the same row: 14 × 0.60 = 8.40. The factor climbs faster than the term for short marriages and the two converge as the marriage lengthens.
What the guideline does not decide
- Eligibility. In several states, including Texas, most spouses do not qualify for maintenance at all without a long marriage, a disability, or family violence. The formula answers the amount question only after eligibility is established.
- Which income counts. Bonuses, equity compensation, business distributions and perquisites are all fought over, and a formula applied to a disputed income figure inherits the dispute.
- Imputed income. A court may use earning capacity instead of actual earnings where a spouse is voluntarily unemployed or underemployed.
- The interaction with child support. States order the two calculations differently, and the ordering changes both figures. This calculator deducts an existing child support order from the payor's income before running the formula.
- Property and retirement division. A larger share of the marital estate can substitute for maintenance; a pension divided by a qualified order changes the payee's long-term position.
- Termination events. Death, remarriage and in most states cohabitation end the obligation, and none of them appear in the total shown.
The tax rule changed in 2019
For divorce or separation instruments executed after 31 December 2018, alimony is not deductible by the payor and not included in the payee's income — the Tax Cuts and Jobs Act repealed the deduction. Instruments executed before that date keep the old treatment unless they are modified and the modification expressly adopts the new rule. This is not a small drafting detail: under the old rule a transfer from a higher bracket to a lower one created value that both parties could share, and that value no longer exists.
Where support sits in the wider settlement
Maintenance is one of four numbers in a divorce, and they trade against each other. Child support is calculated under its own state guideline and generally takes priority; the child support estimate calculator handles it. Property division allocates the marital estate and can substitute directly for support — see the marital property division calculator. Retirement assets earned during the marriage are divided by a qualified domestic relations order using a coverture fraction, which the QDRO coverture fraction calculator computes. And where one spouse keeps the house, the buyout is its own calculation — the divorce house buyout calculator.
Those trades are real. A payee who takes a larger share of liquid assets in exchange for shorter maintenance converts an uncertain income stream into certain capital; a payor who keeps a business in exchange for a longer support term does the opposite. Comparing them properly means discounting the support stream to present value rather than adding the payments up — $2,000 a month for 8.4 years is $201,600 nominal but less than that in today's money, and the same discounting logic as the structured settlement present value calculator applies.
Finally, security matters as much as amount. A support obligation is only worth what it is actually paid, so agreements commonly require life insurance on the payor's life, and enforcement mechanisms differ sharply between jurisdictions. That is a drafting question, not an arithmetic one, and it belongs to the lawyer rather than to this page.