Methodology and calculation assumptions
The calculator uses one month-by-month model in real terms. “Nominal” means pounds at the date shown; “today’s money” removes assumed inflation so amounts have comparable buying power. This prevents retirement spending from becoming artificially cheaper over time.
Before retirement
The opening pot and one-off contribution grow for one month, then your and employer contributions are added at the end of each month. Contributions rise annually by the contribution-growth input and are deflated to today’s money.
monthly real return = ((1 + annual return) / (1 + annual fee) / (1 + inflation))^(1/12) − 1
balanceₘ = balanceₘ₋₁ × (1 + real returnₘ) + real contributionₘ
In retirement
At each month end, net spending from the invested pot equals your target less any active State Pension, defined-benefit pension, annuity or other recurring income. Entered incomes are treated as inflation-linked and before tax.
balanceₘ = balanceₘ₋₁ × (1 + real returnₘ) − max(0, target spending − recurring incomeₘ)
The required pot is found by a bounded search for the starting balance that reaches approximately £0 at the end of the chosen retirement period. If the real monthly rate is exactly zero, the month-by-month additions and withdrawals still work without division by zero.
Reverse calculations
Sustainable income is the highest level monthly spending that lasts through the selected period under the central assumptions. Required contribution is the level monthly personal payment that closes the projected pot gap, holding other inputs constant. Viable retirement age tests whole ages up to 80 while keeping the selected retirement duration unchanged.
Worked example
Generated from the same engine: calculating…
What the model excludes
Results are deterministic estimates, not financial advice or a promise. They exclude tax calculations, tax-free lump sums, benefit eligibility, investment volatility and sequence risk, changing asset allocations, contribution tax relief, salary limits, pension-access rules, care costs, irregular withdrawals, non-inflation-linked income and provider-specific charges. The model does not calculate a probability of success.
