What the headline number means

The tool reports that a plan works in N of 100 futures. That is the share of the simulated futures in which every year from retirement to your target age was funded in full: the income you asked for was paid, from the pots and incomes you entered, without a shortfall in any year. A plan that funds every year but leaves nothing at the end still counts as working; a plan that falls £1 short in one year does not.

It is a statement about the model, not a probability about the world. "Works in 90 of 100 futures" means 90% of the modelled scenarios funded the plan to the end under the assumptions you set. It does not mean you have a 90% chance of a comfortable retirement. Real markets, tax law and your own life will differ from any model.

Two secondary figures sit beside it: the share of futures in which the pension pot itself survives to the target age, and the median outcome, which the story and the year-by-year page describe. Half the simulated futures turned out better than the median path, half worse.

The simulation

1,000 futures by default (you can change the count). Each future is an independent run of the plan from the retirement date to the target age.
Annual steps. Each year: work out the income needed (your target, inflated, adjusted for spending steps, one-offs, care and the survivor case); draw it in the order you chose; tax each person's withdrawals; grow what is left. Withdrawals are taken before growth; growth is applied at the end of the year.
Returns. Each year's investment return is drawn from a normal distribution with the mean and standard deviation you set (defaults 5% and 7.1%). Years are independent: the model has no momentum and no mean reversion.
Inflation. Drawn the same way, independently of returns (defaults 2.5% and 1%). Your spending target rises with it; so do the State Pension and inflation-linked incomes, and, from April 2031, the tax thresholds.
Fixed mode. Switch volatility off and you get one deterministic projection at the means. Useful for checking the arithmetic, useless for judging risk.
Same futures for comparisons. The compare page, the stress tests and the story's alternatives run on a fixed seed, so every column sees the same sequence of market years and the differences between columns are the decision, not luck.

Historical overlays

Three buttons replace the first ten years of every future with the actual sequence that followed a real event: 1973, 2000 and 2008. The figures are nominal UK equity total returns and RPI inflation from the Barclays Equity Gilt Study and the ONS series, and are educational rather than audit-grade. Years eleven onward return to the random model. This is a sequence replay, not a bootstrap: it answers "what if I retired into that decade", not "what does the whole history say".

Tax

Per person, per year. Each of a couple has their own personal allowance and bands. The allowance tapers at £1 for every £2 of income above £100,000 and is gone at £125,140. Scottish rates and bands are an option. Marriage Allowance can be switched on.
Pension withdrawals. By default each withdrawal is treated as UFPLS: 25% tax-free and 75% taxable, until the Lump Sum Allowance of £268,275 is used, after which withdrawals are fully taxable. Tax-free cash taken up front, prior tax-free cash, and a pension already partly in drawdown all reduce the share of the pot that still carries a tax-free quarter; the model tracks that share per person.
Thresholds. Frozen at 2026/27 values to April 2031, as legislated, then rising with inflation. "Frozen throughout" is offered as the pessimistic case.
Other income. The State Pension, defined benefit pensions, rent, annuity income and the taxable part of any other stream are taxed with the withdrawals. ISA withdrawals are tax-free. Sales from a general investment account realise a proportional gain and pay capital gains tax above the exemption.
The stop year. If you choose a stop month, the first year runs from that month to 5 April and the salary already earned in that tax year sits under the plan's withdrawals, so they are taxed at the marginal rate on top of it. The salary itself is not modelled.

Drawing the money

Order. Savings and ISA first, pension first, or fill the basic-rate band from the pension and take the rest from savings. Couples' pension draws are split pro rata by pot size by default, or balanced to use both sets of bands.
Take-home mode. Ask for an after-tax figure and the model solves each year for the gross that delivers it.
Protections. A rolling cash ladder of several years' spending, refilled from the pension inside the basic-rate band and paused after a down year; a spending guardrail that trims income after a bad year, with a floor; a reserve floor the savings are never spent below.
Guaranteed income. Lifetime or fixed-term annuities at illustrative rates, paid to you or into the pension, single or joint life; an immediate needs annuity for care. Rates are illustrative and a real quote will differ.
Life events. Spending steps at set ages, one-off costs and windfalls, the death of one partner (a separate run with one State Pension, the survivor's share of incomes and the pot passed across), and a care window.

State Pension

Paid from each person's State Pension age (66, 67 or 68 by birth year under current law) at the amount you enter, defaulting to the full new State Pension. Uprated by assumed inflation until taken and thereafter, with a triple-lock option; deferral is modelled at the statutory rate. It counts as taxable income and usually uses most of the personal allowance, which is why the years before it arrives are the cheapest years to draw taxable pension money.

Known limits

Normal returns understate the fat tails of real markets. The historical overlays are the stand-in; a bootstrap of historical years is on the list.
Independent years ignore momentum and mean reversion.
All investment buckets earn the same return. A cash ISA is modelled as if it were invested like the rest.
Tax rules are those legislated or announced for 2026/27 and April 2027, and will change.
The care means test, salary, National Insurance, overseas tax and defined benefit transfer values are not modelled.
Emergency tax on a first withdrawal is ignored on purpose, because it is reclaimed.

How it is checked

The engine is a pure function under version control. A set of 30 deterministic golden fixtures pins the full year-by-year output of awkward cases to the pound, so any change in behaviour is deliberate and recorded; around 40 further test files each prove one rule by hand. Readers reconcile their own year-by-year pages against it and against other tools, and their corrections are listed in the changelog. The one-line disclaimer stands: this is a planning tool, not regulated financial advice.

Questions the FAQ does not answer, or a number that looks wrong: use the contact link in the footer. The FAQ covers the everyday questions; the about page says who is behind it.

Methodology