Retirement calculator
Put the dials in your own hands.
The slides walked Steven's retirement math through with fixed assumptions — salary trajectory, savings rate, tax rate, real return. The calculator below lets you change all of them and watch what happens. The math doesn't argue; it just shows you which assumptions you can survive.
Why this calculator
The slide deck only showed two scenarios
The deck gave you 7% and 9% real after-tax returns, two specific savings and spending profiles, and one drawdown shock. Those are illustrative — your actual numbers are different. This page lets you put your own salary, savings, and assumptions in and see the same chart for your life.
If you took only one thing from Section A of the slides, it's this: differences in annual return that look small (7% vs 9%) are not small over 35 years. Use the calculator to feel how true that is for your numbers.
Before you slide the dials
What "real after-tax return" actually means
The single most consequential dial on the calculator is the return assumption. Get this wrong by 2pp and the result is wrong by an order of magnitude over 35 years. The workshop used 7% and 9% as illustrative anchors — those numbers aren't pulled from nowhere, but they're not universal either. Here is the calibration so the dial choice is informed, not optimistic.
S&P 500 long-run, gross nominal
~10%
1928–2024 CAGR with dividends reinvested. Pre-inflation, pre-tax. The number everyone quotes.
Same return, real (after inflation)
~6.9%
Subtract long-run inflation of ~3%. This is the real return in a tax-advantaged wrapper (NISA, iDeCo, IRA, 401k) where there is no annual tax drag on dividends or rebalancing.
Real, after tax (taxable account)
~3.5–5.5%
Once dividends and any realised gains are taxed annually (~20% Japanese capital-gains rate, higher in most US brackets), the real return drops 1.5–3pp. The lower end assumes high turnover; the upper end assumes long holds.
Workshop assumption
7%
The model assumes a 7% real after-tax return — close to the tax-advantaged case. If the calculator is being run for a fully taxable account, dial the return down to ~5% to match reality. If it is for a NISA / iDeCo / similar wrapper, 7% is reasonable.
The Japan-specific note. The new NISA (2024) gives ¥18M of lifetime tax-free room (¥3.6M/yr Tsumitate + Seichou combined) — large enough that for most retirement-planning purposes the "tax-advantaged" column is the right one to plan against. iDeCo adds another tax-deferred layer at the cost of liquidity until age 60. Outside those wrappers, the third column (~3.5–5.5%) is what the math actually produces.
Section 1
Calculator
Earnings & living expenses
Portfolio & passive income
Override any year's living expenses below. Useful for modelling life events — kids, mortgage payoff, late-life healthcare, sabbatical years. Expand the year-by-year detail and type into the Living column for any age — values are in K (USD) or 万 (JPY), with up/down arrows to adjust. The chart and summary recalculate automatically.
Year-by-year detail — click to expand & edit
| Age | Gross income | Tax | Savings | Living | Port. beg | Return (7.0%) | Return override | Cash flow | Port. end |
|---|
All figures in today's dollars — already inflation-adjusted. "Real after-tax return" is what your portfolio actually grows by, net of inflation and after taxes on gains within the account.
Default living expenses during working years = take-home income − savings. With default sliders (gross $80K → $250K, tax 35%, savings 15% of gross), age-30 numbers are: tax $28K, savings $12K, living $40K — matching the slide's setup. Edit any year's living expenses directly in the Living column of the table; the savings / withdrawal cash flow recomputes for that year and propagates forward.
Rough nominal-equivalent at 3% inflation, 0% tax drag (e.g., tax-advantaged accounts): ≈ 10.0% gross nominal.
Section 2
The three rules behind the math
Slide 7 of the deck distilled the math into three rules. The calculator makes each one tactile.
1. Compounding is the key
Convert labour income to compounding capital as fast as possible. The earlier the conversion, the longer time has to do its work. Try the calculator with a savings rate of 5% vs 25%, holding everything else equal, and watch the back-end of the portfolio.
2. Every percent matters
Fees, taxes, and lifestyle inflation are all returns leakage. Each percentage point lost requires a higher gross return to compensate. In the calculator, a 1% increase in tax rate has roughly the same effect as a 1% decrease in real return — they're the same lever from different sides.
3. Losing money is catastrophic, not just painful
A -30% year takes a +43% recovery just to break even. A -50% year takes +100%. And the lost time is unrecoverable. Use the calculator's drawdown-shock input to see how vulnerable a plan is at different ages — late shocks (age 55+) are far harder to recover from than early ones.
Section 3
What matters most
If the calculator's nine sliders all moved by 10%, only three of them would move the outcome by much. Ranked by impact, holding the other dials at their workshop defaults:
1. Real after-tax return (dominates everything)
A 1pp change in real return moves the age-65 portfolio by roughly 30–40% in the workshop scenario, because the return compounds for 35 years. The dial labelled "Real after-tax return" is the only one whose direction is asymmetric — over-estimating it has worse consequences than under-estimating it, because the plan looks fine on paper and falls short in life. Be calibrated, not optimistic.
2. Savings rate (large under 30, small over 50)
Doubling the savings rate from 15% to 30% in the working years roughly doubles the peak portfolio. But the impact tapers — once retirement is within ten years, raising the savings rate has only marginal effect because there isn't enough time to compound. Savings-rate decisions made in your 20s and early 30s are worth multiples of the same decisions made in your 40s.
3. Retirement age (the hidden multiplier)
Working an extra five years past the workshop default of 45 doesn't just add five years of saving — it removes five years of drawdown and gives the portfolio five more years of compounding. The combined effect on the age-65 portfolio is typically larger than a 2pp return increase. Conversely, retiring five years earlier than planned is harsh: you lose the saving years and add five years of drawdown.
4. Lifestyle (matters, but less than people fear)
Annual expenses scale the size of portfolio you need linearly, but compounding is unforgiving in only one direction — too-high expenses deplete the portfolio late, when there's no time to recover. The calculator's editable per-year expenses column makes this tactile: a "lumpy" retirement (high spend years 50–65, lower 65+) is much easier to fund than a constant high spend.
5. Starting portfolio (helpful only when young)
Going from $0 to $100K of starting capital at age 30 produces a meaningfully larger age-65 portfolio (more compounding runway). The same $100K starting at age 50 produces only a small bump — there isn't enough time for it to multiply. Inheritance, bonuses, equity windfalls early in life are disproportionately valuable; late in life they mainly reduce drawdown.
Below the threshold
Tax rate, starting salary, and salary growth are real dials but their leverage is small compared to the five above. Salary growth in particular feels important emotionally but is dominated by what fraction of it gets saved versus consumed.
Section 4
Common pitfalls
The math reveals four mistakes that recur in retirement-planning conversations. Each is easy to make and expensive to discover late.
1. Conflating nominal with real
The calculator uses real (inflation-adjusted) dollars throughout. "I'll have $5M at 65" is meaningless without specifying whether that's nominal or real. A nominal $5M in 35 years, at 3% inflation, has roughly the purchasing power of $1.8M today. Always plan in real dollars; the headline-grabbing nominal figure is a friend to no one.
2. Conflating pre-tax with after-tax
This is the most consequential pitfall and the one the calibration block above is built to prevent. The S&P 500's "10% nominal" return is gross — pre-inflation, pre-tax. A retirement plan run with that number is overstated by 3–6pp depending on the tax wrapper. In a fully taxable account at typical rates, the real after-tax return is closer to 4–5%, not 7%. The plan that works at 7% doesn't necessarily work at 5%.
3. Ignoring sequence-of-returns risk
The average return over 35 years doesn't tell you whether the bad years are clustered at the start, middle, or end of retirement. A -30% year at age 35 (working) is recoverable in saving years. A -30% year at age 65 (drawing down) can cut decades off the plan, because you're selling assets at the bottom to fund living expenses. The calculator currently uses constant returns; mentally add a worst-case scenario where the bad year lands when the portfolio is largest.
4. Planning for the median outcome
Retirement plans built around expected returns have a 50% chance of being too optimistic. The Buffett rule — "don't lose money" — applies to long-horizon planning too. Plan for the 25th-percentile case (worse-than-expected returns, worse-than-expected longevity, worse-than-expected expense shocks); enjoy the upside if it happens. The asymmetry: too much money in old age is a minor inconvenience; too little is catastrophic.