Savings rate calculator — what it does to your FIRE date

For every savings rate between 5% and 80% the calculator splits your net monthly income into a contribution and an expense figure, sets the target at 25 times annual expenses, and compounds the portfolio forward one month at a time at the monthly root of your expected return until it crosses that target. Sixty years is the cut-off; anything slower is reported as over fifty years rather than as an invented number. The comparison rows underneath run the same arithmetic for four specific steps — 10 to 20, 20 to 30, 30 to 50 and 50 to 70 — and report the years each one buys at the return you have set.

Income, savings rate and spending are all held flat in nominal terms for the whole run, so a pay rise, a mortgage ending or a child arriving change the answer and are not modelled. Nothing here is taxed, and the return is a single fixed rate rather than a sequence, so the date this produces is a central case and not a worst case. The 25 times target is the 4% rule turned around and inherits its limits.

Why does the savings rate matter more than the investment return?

Because it acts on both sides of the equation and the return only acts on one. A percentage point more saved is a percentage point less spent, which raises the monthly contribution and cuts the target by twenty-five times the annual difference at the same time. A percentage point more return only makes the existing contributions grow faster, and over a ten to twenty year accumulation the contributions are still most of the balance. Drag both sliders and watch which one moves the date.

What should I count as income and expenses?

Net take-home pay, after income tax and any pension contribution deducted at source, and the spending that pay actually covers. The reason to use net rather than gross is that the target is built from expenses: if you inflate the income figure with money you never see, the savings rate reads high and the 25 times target reads low, which flatters the date twice over.

Is 25 times annual expenses the right target?

It is a planning convention, not a law. It comes from reading the 4% withdrawal rule backwards, and that rule was derived from United States market history over rolling thirty-year windows. A retirement of fifty years, a portfolio that is not a US-weighted 60/40, or a country that taxes the portfolio annually all argue for a larger multiple. The safe withdrawal rate page lets you vary the rate directly and see what a more conservative number costs in years.

• FIRE calculator with real broker costs
• Safe withdrawal rate calculator — how long the portfolio lasts
• Contributions versus returns — which built your portfolio