How these calculators work

Each simulated month follows the same three steps. First, growth is applied: the balance is multiplied by one twelfth of the annual rate you entered. Second, money moves — a contribution is added, a withdrawal or payment is subtracted, or both. Third, the balance is tested. If it has reached or passed zero, that month is recorded as the depletion point and the simulation stops.

Because growth is applied before money moves, a withdrawal effectively comes out at month end. In real life the timing is messier, and the difference over a long horizon is small — typically a fraction of one percent of the total — but it is worth knowing which convention produced the number you are reading.

Dividing an annual rate by twelve gives a nominal monthly rate, which is the same convention used for mortgage amortisation. It compounds slightly higher than the equivalent effective annual rate: 5% entered as an annual return compounds to roughly 5.12% over a year. Rates quoted by lenders and most fund providers already follow this convention, so entering the headline figure is the correct thing to do.

Depletion is the first month in which the modelled balance would fall to zero or below. The balance is then clamped at zero — no page on this site will ever show you a negative portfolio, because a negative balance is a modelling artefact rather than a meaningful result.

Some scenarios never deplete. When the growth a balance produces in a month is at least as large as the amount coming out of it, the balance is stable or rising and the simulation has no endpoint. In that case the tools show an infinity symbol.

That result is a statement about the arithmetic, not a promise. It means only this: under the fixed rate and the constant withdrawal you entered, the modelled balance is never exhausted. Real returns arrive unevenly, inflation raises what you need to withdraw, and a single poor decade early on can undo the arrangement entirely. Read infinity as "sustainable under these assumptions", and then test assumptions you would not enjoy.

Only the inheritance calculator models inflation directly, by increasing the withdrawal each year and by offering a view of the balance in today's purchasing power. On the other pages, the simplest way to approximate inflation is to enter a real return instead of a nominal one — subtract your inflation assumption from your return assumption and read the result as today's dollars.

Internally the simulation carries full floating-point precision; rounding happens only when a figure is displayed. Durations are reported in whole months or to one decimal place in years, so a projection described as "22 years, 1 month" is the month the balance crosses zero, not a rounded average. Currency is rounded to the nearest dollar for readability, which is why the individual figures in a breakdown can differ by a dollar or two from their total.

Projections are capped at a long but finite horizon — 70 years for drawdown models — so that non-depleting scenarios terminate cleanly. When a result reaches that cap, it is reported as indefinite rather than as a specific number of years.

The simplifications are shared across the site, and they matter more than the arithmetic does:

• Volatility and sequence risk. A steady 5% every year is not the same as an average of 5% with a bad first three years. Early losses while you are withdrawing do disproportionate damage.
• Taxes. Nothing here applies income, capital gains, estate, or withdrawal taxes. Where taxes apply, enter the gross amount you would need to withdraw in order to net your target spending.
• Fees. Fund expenses, platform charges, and advice fees reduce the effective return. Subtract them from the rate you enter.
• Changing behaviour. Real spending is not constant — it rises with a new roof and falls in a quiet year. The models assume a steady pattern.
• Other income and benefits. Pensions, social security, rental income, and part-time work are not modelled; subtract them from your withdrawal to approximate their effect.
• Healthcare and longevity. Later-life care costs and the simple uncertainty of how long a plan needs to last sit outside the model entirely.

Every calculation runs in your browser in JavaScript. Nothing you type is transmitted to a server, stored in an account, or saved between visits, which is also why there is no "save my plan" feature. See the privacy policy for what is and is not collected.