The formula
Future value = savings × (1 + r)ⁿ + monthly × ((1 + r)ⁿ − 1) / r, with monthly compounding. The result is then deflated by inflation to show today’s purchasing power.
The 4% rule comes from US historical returns (the Trinity study); a lower withdrawal rate is safer for very long retirements.
Frequently asked questions
What return should I assume?
Many planners use 5–7% a year for a diversified stock-heavy portfolio before inflation.
Does it include pensions?
Subtract expected pension income from your desired income before entering it.
Is this financial advice?
No, it is an estimate.