Inside a Kessavo run
Five guided steps. One extraordinarily detailed answer.
The form takes about 2 minutes. The model then runs month by month using the tax, Social Security, and Medicare surcharge rules Kessavo currently models, hundreds of times over. Here's what's actually happening while you wait for the results to load.
What you actually answer
The guided setup covers timing, Social Security, savings, spending, state, and coverage, with optional fields you can refine later.
Household
Birth year (and your spouse's, if you're planning as a couple).
Social Security
Your benefit — from your statement, or a single full-retirement-age estimate.
Balances
Taxable, tax-deferred, and Roth account balances.
Coverage
State of residence and your health insurance or Medicare cost.
Then, behind the scenes
Everything below runs on those dozen fields — nothing here is a separate input you have to fill out. This is what "shows its work" actually means in terms of raw computation, not just formulas you can read on the Methodology page.
Scale & granularity
129,600
simulated months, one full run
Worked example for a 65-year-old: 36 years to age 100, 432 months per trial, across 300 Monte Carlo trials. A 60-year-old's plan runs longer; a 70-year-old's shorter — the depth holds at any age.
300 trials, not one projection
Each trial draws a fresh random annual return for every year, then simulates the full plan against it — not a single fixed growth-rate line.
Resolved monthly, in a fixed order
Every month checks all three balances, applies that year's RMD if one is owed, and carries the result into the next month.
Tax & legal logic
7 federal brackets + capital gains, stacked
Ordinary income runs through all 7 current federal brackets — different thresholds for single filers and married couples, not one table doubled. Long-term capital gains from taxable withdrawals stack on top at their own rates.
RMD divisor, shrinking on schedule
Applies from age 73 or 75, by birth year, per SECURE 2.0 — not a flat rule.
6 IRMAA tiers, checked per person
Tiers shown are the single-filer thresholds. A couple where both spouses are 65+ sees both premiums and both surcharges stack, checked against the higher married-filing-jointly thresholds — roughly double the single-filer amounts, except the top tier ($750,000 joint vs. $500,000 single).
Timing, by design
A one-year lag, on purpose
This year's income sets next year's tax and IRMAA bill — the same lag mechanism the real IRMAA system uses (two years, simplified to one here for clarity, and said so on the Methodology page).
Built to expire on schedule
The age-65 senior deduction phases out above $75,000 of income ($150,000 for a couple) and is scheduled to disappear after tax year 2028 — the model applies and expires it on that exact calendar.
26 states with real handling
Run in real, not nominal, dollars
The whole projection is deflated to today's-dollar terms as it runs, so the current tax brackets apply consistently across the years — rather than re-guessing where inflation will move them.
The numbers
engine snapshot reviewed August 7, 2026
| Monte Carlo trials per run | 300 |
| Simulation granularity | Monthly, current age → age 100 |
| Example: age 65, a full run | 432 mo × 300 trials = 129,600 |
| Federal tax brackets | 7 (single-filer) / 7 (married filing jointly — different thresholds, not doubled) |
| IRMAA surcharge tiers | 6, checked per person, per year |
| States with curated bracket/exemption logic | 26 (13 curated + 9 no-tax + 4 exempt) |
| RMD divisor range | 26.5 (age 73) → 6.4 (age 100) |
| Tax / IRMAA timing | One-year lag, by design |
A note on the number that leads this page: 129,600 is a worked example for a 65-year-old, not a universal constant — total simulated months scale with how many years remain to age 100. Every other figure above is fixed and applies to every plan.
See it run on your own numbers through the same guided setup.
Start your planOr read every formula behind these numbers on Methodology.