A FIRE Calculator for People Going Through Divorce: What to Track

The four levers that drive a FIRE plan — savings rate, starting assets, real return, and target spending — all move at once when a divorce closes. Half the joint nest egg leaves on a QDRO, lifestyle costs barely fall because rent and utilities no longer split two ways, and dual-income compounding stops. Plugging the new numbers into a FIRE calculator usually pushes the date out by a decade, not a few months.^[]

The data confirms the asymmetry. A 2024 Federal Reserve Bank of St. Louis analysis of post-divorce earnings found men in their 30s lose roughly 40% of household income, while women's drop is concentrated more in their 50s and 60s — the so-called "gray divorce" cohort, where Bowling Green NCFMR work pegs women's standard-of-living decline at 45% versus 21% for men.^[] The point isn't that one side wins. It's that any FIRE projection drawn before the decree was signed is now optimistic.

Why the FIRE math resets, even if your job didn't change

A FIRE calculator that doesn't know you're divorced will quietly assume things that no longer hold. Three of those assumptions break the moment the decree is filed.

The first is shared overhead. A married couple with $90,000 in joint annual spending isn't living on $45,000 per person — it's closer to $60,000–$70,000 per person once one spouse is paying solo rent, solo utilities, and a single-person grocery cart. The 30%–50% lifestyle delta is what crushes the SWR target.

The second is the standard deduction and bracket compression. The 2025 single-filer standard deduction ($15,000) is exactly half the MFJ figure ($30,000), but the brackets aren't — single filers hit the 24% federal bracket at $103,350, while MFJ couples don't until $206,700.^[] Same gross income, more tax. The savings-rate side of the FIRE equation tightens.

The third is the legal one-time cost. The U.S. average is around $11,300 in attorney fees per filer, with a $7,000 median; uncontested cases run $1,500–$4,000, and high-conflict cases routinely exceed $20,000. State filing fees range from $70 to $435 depending on jurisdiction. That money comes out of post-tax cash, which means it usually comes out of the brokerage or — if you're particularly unlucky — out of a 401(k) under the QDRO carve-out.

The numbers that change the day the decree is filed

A representative dual-income household earning $200,000 with $90,000 in joint spending, $500,000 invested, and a 35% savings rate is on track for FI in about 15 years at a 5% real return and a 3.5% safe withdrawal rate. The calculation rendered below works through what happens to one of those filers in the year after the decree, holding the rest of the world constant. Headline result: 15 years becomes roughly 25 years. Cutting post-divorce lifestyle from $60,000 to $50,000 and hitting a 30% solo savings rate brings it back to about 18 years — which is the whole reason the recalc is worth doing carefully.

The single biggest swing factor isn't the asset split. It's the lifestyle floor. A QDRO that splits a 401(k) 50/50 is mechanical; the harder work is deciding what to actually live on as a single household.

QDROs, alimony, and Social Security spousal credits

Three federally defined rules sit underneath every post-divorce FIRE projection, and each is its own line on the recalc spreadsheet. None of them are optional in the math, and two of them have unusual carve-outs that work in the FIRE planner's favor if used correctly.

A Qualified Domestic Relations Order is the only mechanism that lets a 401(k), 403(b), or pension be split between spouses. ERISA-governed accounts require it; IRAs do not — IRAs split with the divorce decree alone.^[] The QDRO carries an unusual carve-out: distributions taken by the alternate payee under a QDRO are exempt from the IRC §72(t) 10% early-withdrawal penalty, even if the recipient is under 59½. Ordinary income tax still applies, but the penalty waiver is the main reason QDRO cash is sometimes used to fund post-divorce legal or housing costs without triggering a tax cliff.

Alimony tax treatment changed permanently in 2019. For any divorce or separation agreement executed on or after January 1, 2019, alimony payments are not deductible by the payer and not includable as income for the recipient.^[] Pre-2019 decrees keep the old rules. For the higher earner, this means a FIRE plan that anticipated deductible support is overstating after-tax income by the marginal rate times annual support — usually a four- or low-five-figure error per year.

The Social Security divorced-spouse rule is the under-modeled lever. If the marriage lasted at least 10 years, the lower earner can claim up to 50% of the higher earner's primary insurance amount at full retirement age, even if the ex remarries — provided the lower earner is currently unmarried and at least 62.^[] For a FIRE planner targeting a pre-62 retirement, this is a deferred income stream worth modeling on the asset side of the balance sheet, because it doesn't reduce your ex's benefit and doesn't depend on your post-divorce work history.

What I'd actually track in the first 12 months

The right post-divorce dashboard is shorter than the pre-divorce one, because one earner has fewer levers to pull. I'd track exactly four numbers, monthly:

1. Single-filer savings rate, computed as a 6-month rolling average to damp out the months when legal bills land. The target isn't necessarily 30% — it's whatever is sustainable for 12 months, then re-evaluated.
2. Solo emergency-fund months, defined as cash divided by new monthly expenses, not the old joint figure. Most planners need 6 months of single-household expenses, which is often more dollars than the half-share of the old joint emergency fund.
3. Updated FIRE number and projected date, recomputed from the post-QDRO balances and the post-decree budget. Don't carry forward the old date.
4. Net worth delta versus the post-decree starting line, not against any pre-divorce baseline. The old number is no longer the right denominator, and chasing it slows the rebuild.

The one-page rebuild sequence I'd actually recommend: stabilize a six-month emergency fund first, reset the short-term goal tracker so the next two years of milestones (deposit on a new place, car replacement, kid's activities) are funded out of cash flow, then reopen the FIRE calculator with honest single-filer inputs. The plan that comes out the other side is usually slower than the old one — but it's the first plan in months that's actually true.