General calculation approach
Calculations run locally in your browser. Money is shown in the selected currency, but changing currency does not perform an exchange-rate conversion. Results are rounded for display while calculations use unrounded values.
Growth and interest
Growth calculators generally convert an annual rate to a monthly rate, apply it to the current balance and add monthly contributions at the end of the period. Where a calculator explicitly uses annual periods, this is stated on its page.
Compound growth
Future value is based on the current balance multiplied by one plus the periodic rate for each period. Regular payments are added according to the timing described by the calculator.
Present value
A future amount is discounted by dividing it by one plus the periodic discount rate raised to the number of periods.
Real return
The exact inflation-adjusted return is calculated as (1 + nominal return) ÷ (1 + inflation) − 1.
Loans and mortgages
Repayment calculations use the standard amortising-payment formula. Rates are assumed constant, payments occur monthly and overpayments reduce principal immediately. The reduce-term-or-payment comparison applies the lump sum first, then either keeps the original amortising payment until payoff or recalculates a lower payment across the unchanged remaining term. The interest-only overpayment calculator instead holds the original balance constant for its baseline, then reduces capital by the entered lump sum and monthly capital payments. It compares the interest charged on those balances and shows any capital still due at the term end. Real lender figures can differ because of daily interest, fees, rounding, changing rates, payment recalculation policies and early-repayment conditions. MoneyHelper explains the role of mortgage repayments and lender terms in its mortgage calculator guidance.
Pensions and tax
The pension relief tool illustrates a relief-at-source arrangement. GOV.UK states that a provider normally claims basic-rate relief at 20%, while eligible taxpayers paying above that rate may need to claim additional relief. Scheme method, earnings, allowances and Scottish rates can change the outcome. See the current GOV.UK pension tax relief guidance.
Retirement and FIRE
Withdrawal-rate tools are scenarios, not safe-income guarantees. Smooth annual returns do not capture market volatility or sequence-of-returns risk. FIRE targets divide annual spending by the selected withdrawal rate; Coast FIRE then discounts the future target to today using the selected real return.
Limitations
- Returns, inflation and interest rates may change.
- Tax rules and personal circumstances vary.
- Most tools exclude fees, tax or market volatility unless those inputs are shown.
- Results do not account for every product term or legal restriction.
Methodology last reviewed: August 2026.