Forecast calculators

A forecast is not a prediction. It is a projection — a what-if applied to a set of assumptions, run forward across a multi-decade horizon. The point is not to be right about the year-30 number; the point is to see how that number changes when each assumption changes, so you can identify the decisions that matter most and the assumptions you cannot afford to be wrong about.

These calculators take your starting position, your contribution schedule, and your return assumptions, and produce a year-by-year projection of net worth, portfolio balance, or retirement readiness. The output is a curve, not a point. Stress-test the curve at lower returns, higher inflation, and earlier-than-expected retirement to find the decisions that survive multiple futures.

All maths uses 20-digit decimal precision. We make no jurisdiction-specific assumptions about retirement-account tax treatment, withdrawal rules, or pension entitlements — you supply the local numbers. The output is unbranded; no fund affiliations, no advisor pitches, just the long-arithmetic projection.

Projecting your financial future

Net-worth projection is the most useful long-horizon financial calculator most people never run. It takes your current assets and liabilities, your annual savings rate, your expected returns, and a horizon, and produces a year-by-year projection of net worth — the difference between what you own and what you owe.

The most striking output of any long-term projection is the back-loaded nature of compound growth. A net-worth curve that looks flat for the first 10 years often inflects sharply in years 15–25 and accelerates further from there. The first decade is mostly contributions; the last decade is mostly compounding on prior contributions. This is why "starting late" is so painful and "starting early" is so disproportionately powerful.

Worked example: a household saving $20,000/year at 6% real return reaches roughly $270,000 after 10 years, $730,000 after 20 years, and $1.6 million after 30 years. The first decade adds $270,000; the third decade alone adds about $870,000 — almost as much as the first two combined. None of those figures relies on a higher savings rate or a higher return; they all come from the same $20,000/year applied for longer. Time, not heroism, is the lever.

The net-worth-projection calculator runs the curve and lets you toggle a single input at a time — savings rate, return assumption, time horizon — to see the sensitivity. Most people are surprised by how much the savings rate dominates in the first 15 years and how much the return rate dominates from year 15 onward. Both matter; their relative importance shifts over time.

Early retirement planning

Financial Independence, Retire Early (FIRE) is the most quantified branch of personal finance. The core arithmetic is the 4% rule (or its more conservative variants): a portfolio is 'enough' when 4% of its value covers your annual expenses. By inversion, the FIRE number is 25 × annual expenses (1 / 0.04 = 25). At 3.5% withdrawal, the multiple is roughly 28.5x; at 3%, 33x.

The FIRE-calculator runs the projection forward: given your current portfolio, your savings rate, and your expense level, when does the portfolio cross the FIRE threshold? The answer is sensitive to all three inputs, but disproportionately sensitive to the savings rate. A household saving 20% of income retires roughly 15 years later than a household saving 50% of income, even at the same return assumption — because high-savings households both contribute more and have lower expenses, which lowers the target.

The savings-rate effect is the single most counter-intuitive lever in retirement planning. Consider a household earning $100,000/year, saving 10% ($10,000) at a 5% real return. Their FIRE number — 25 × annual expenses of $90,000 — is $2.25 million, reachable in roughly 51 years. The same household saving 30% ($30,000) has a FIRE number of $1.75 million (25 × $70,000) reachable in about 28 years. Tripling the savings rate roughly halves the time to FIRE; doubling it from 30% to 60% cuts the time roughly in half again. The compounding effect on both the contribution and the lower target is why aggressive savers retire decades earlier on the same gross income.

Sequence-of-returns risk is the single biggest gap between a FIRE projection and a FIRE reality. A retiree drawing down a portfolio in the first few years of a market downturn can run out of money even if the long-run average return is strong, because early losses compound against the withdrawal rate. The FIRE calculator should always be paired with stress-testing at lower-than-average returns — particularly in the early withdrawal years.

The assumptions that matter most

Every long-horizon projection rests on three primary assumptions: the return rate, the inflation rate, and the savings rate. Each has a wide plausible range, and small differences in any one of them compound into large differences over a 30-year horizon. Recognising which assumption your plan is most sensitive to is the difference between a projection that informs decisions and one that flatters them.

A 1% difference in expected real return — say 6% vs 7% — produces roughly a 30% difference in terminal balance after 30 years. The mechanism is straightforward: 1.07³⁰ ≈ 7.6 vs 1.06³⁰ ≈ 5.7, a multiplier gap of about 33%. This is why anyone projecting at 9–10% real returns based on a single bull-market decade should reverse the calculation: at 4% real, a plan that needs 7% to work simply doesn't.

Inflation is the silent erosion. A 1% difference in long-run inflation — 3% vs 4% — produces about a 25% difference in real (purchasing-power) terminal value. Nominal projections look great until you deflate them: a $2 million portfolio in 30 years at 3% inflation is worth roughly $820,000 in today's purchasing power; at 4% inflation, it is worth about $620,000. Always project in real terms or, at minimum, deflate the nominal terminal number before comparing it to today's expenses.

The savings rate has a different shape of sensitivity: it dominates the first 10–15 years, then matters less than the return rate for the rest. Doubling the savings rate (from 10% to 20%) doubles the year-10 balance and halves the time to most milestones. Doubling it again (20% to 40%) has a smaller proportional effect because returns on the larger early balance are doing more of the work. The right way to read these three sensitivities is to identify which one your plan can least afford to be wrong about — and stress-test that one hardest.

The general rule for any long-horizon assumption: if the plan only works at the optimistic value, it does not work. A retirement plan that survives at 4% real returns is more robust than one that requires 7%. A FIRE plan that hits the target even with a 5-year delay in starting is more robust than one that depends on early-career discipline that may not survive life events. The forecast calculators in this pillar are designed for this kind of multi-scenario stress-testing — treat the point estimate as a hypothesis, not a prediction.

Monte Carlo vs deterministic projections

A deterministic projection assumes every year delivers the same return — say 6% real, every year, for 30 years. The output is a single smooth curve that ends at a single number. It is easy to build, easy to understand, and almost always wrong about the path the actual portfolio takes. Real returns are volatile; the smooth curve is the geometric average of a sequence that bounces above and below it every year.

A Monte Carlo projection runs the same horizon thousands of times, each time drawing the year's return from a probability distribution rather than a single rate. The output is not one number but a distribution — the 10th-percentile outcome, the median, the 90th-percentile outcome — and a 'success rate', the share of simulations in which the portfolio lasts the full retirement horizon. A 95% success rate means 5% of simulated futures ran out of money before the end; an 80% success rate means 20% did.

The crucial gap between the two is sequence-of-returns risk in retirement. A deterministic projection at 6% real return can show a portfolio lasting 40 years comfortably. A Monte Carlo projection of the same portfolio with a 6% mean and realistic volatility might show only an 80% success rate — because in 20% of simulated paths, an early bear market combined with withdrawals depletes the portfolio before recovery. The mean return is the same; the path matters because withdrawals lock in losses that future gains cannot fully recover.

Deterministic projections are most useful in the accumulation phase, where contributions buffer against bad years and the long horizon smooths most volatility out. They are misleading in the withdrawal phase, where a bad first decade can permanently impair a portfolio that the long-run average says should have been fine. The standard professional advice — use a 3–3.5% withdrawal rate rather than the headline 4% — exists specifically to compensate for this risk.

The forecast calculators in this pillar are deterministic by design — they project at a single return rate so the sensitivity to each assumption stays visible. To approximate Monte Carlo behaviour without the simulation, run the deterministic projection at three return rates: optimistic (the long-run average plus 1–2%), base (the long-run average), and pessimistic (the long-run average minus 2–3%). If the plan succeeds at the pessimistic rate, it is robust to most realistic sequences. If it only works at the optimistic rate, you are betting the plan against a path that historically only happened in roughly the top quintile of decades.