The plan runs year by year until every person is past their “plan to age”. For each year:

1. Grow every pot by its own growth rate, then take off its fees.
2. If a pot’s “sell at age” matches its owner’s age this year, move its balance into your chosen surplus pot. Selling an investment account, or property that is not your main home, is charged Capital Gains Tax on the gain.
3. Add income and one-off “in” events; subtract expenses and one-off “out” events. Each item’s age range is checked against its owner’s age (Joint items use Person 1).
4. Run each debt through twelve months: add that month’s interest (the yearly rate divided by twelve), then take the monthly payment plus any extra. A debt stops costing anything once it reaches zero. The year’s payments are added to money out. Debt payments do not rise with inflation.
5. Pay each pot’s scheduled payments into it, and subtract the total from the year’s flow. A payment into a pension pot reduces the owner’s taxable income (tax relief).
6. Work out each person’s tax: Income Tax on their taxable income — pay, pensions, and interest on taxable savings (not ISAs or pensions) — plus National Insurance on Employment income if they’re below their State Pension age, plus Capital Gains Tax on anything sold. Subtract the total from the year’s flow.
7. A surplus goes into your chosen pot; a shortfall is taken from the drawdown pots in order. Taking money from a pension pot is taxable (25% tax-free until that person’s tax-free cash allowance, £268,275 today, is used up; the rest taxed as income), so the app withdraws a bit extra to cover the tax. Cash and ISA withdrawals are tax-free.
8. Move anything marked “rises with inflation” forward for next year.

Spendable = pots ticked for drawdown that can be reached that year — a pension counts from its access age. Closing = all pots. “Money runs out” is when Spendable drops below zero, or when a year’s shortfall could only be met from a pension that cannot be reached yet. All money is whole pence, rounded after each pot’s growth (half a penny up).

Timing and simplifications. All money moves at the end of each year: pots grow for the whole year first, so money you take out has had a full year’s growth (slightly generous), money you pay in earns nothing until the next year (slightly cautious), and fees are charged on money about to be spent. Household money is pooled: spare cash and shortfalls are shared across everyone’s pots. Tax relief on “from taxed pay” pension payments is added whatever you earn — in real life it is limited to your earnings, or £3,600 a year if you have none. Debt payments stay the same every year. The State Pension rises each year with the inflation rate you set; in real life the triple lock raises it by the highest of inflation, earnings growth or 2.5%, so it may grow faster. The first year is counted as a whole year — a full year of pay, spending and growth — even if part of it has already gone.

Tax is simplified. England / Wales / Northern Ireland rates only (not Scotland). Tax thresholds follow the setting in Assumptions — by default frozen until April 2031; after that, as the law says, the Personal Allowance, the basic-rate band and the £5,000 starting rate for savings rise with inflation (rounded up as the law rounds them), while the £100,000 and £125,140 points stay fixed. Interest on taxable savings is taxed, with the savings allowances; gains are taxed when a pot is sold. Dividends are not modelled, and money drawn from an investment account before it is sold is not charged. Self-employment uses the employee NI rates; payments into pensions are checked against the £60,000 annual allowance, and the £10,000 limit once pension money has been drawn, as warnings only (no tax charge is worked out). Money going into ISAs is checked the same way, as a warning only, against the £20,000 a year each person can put in; the plan still treats it all as tax-free. Income marked Joint under Other taxable or Tax-free counts half to each person, as a joint pot does; a salary or pension marked Joint counts as Person 1’s. The tax figures are those for the tax year shown at the foot of this page, and are updated each April.