Updated 2026-05-14 · Financial Health · Educational use only ·

Income Replacement Calculator

Capital to replace income.

Calculate the capital needed with our income replacement calculator to sustain a target monthly income over time, adjusted for inflation and returns.

What this tool does

This calculator estimates the starting capital needed to generate a target monthly income over a defined period, accounting for inflation and investment returns. It models how inflation reduces purchasing power over time while investment growth works to offset that erosion. The result shows the lump sum required today to sustain your income goal. The calculation is most sensitive to the investment return rate and the inflation assumption—higher returns lower the capital needed, while higher inflation increases it. A typical use case is planning for retirement income or determining how much to set aside from savings. The calculator assumes consistent monthly withdrawals, a constant investment return, and steady inflation; actual outcomes will vary based on real market performance and spending patterns. Results are for illustration only.

Runs in your browser — the math happens on your device, not our servers. Privacy

Enter Values

Currency($)

Monthly Income Target($)

Years Needed(years)

Inflation

Investment Return

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Formula Used

P V = A \times \frac{1 - ( 1 + r _{r} ) ^{- n}}{r _{r}}

A

Annual income

r_{r}

Real return

n

Years

Spotted something off?

Calculations or display — let us know.

Disclaimer

Results are estimates for educational purposes only. They do not constitute financial advice. Consult a qualified professional before making financial decisions.

How much capital is needed to replace your income for N years? Depends on monthly need, duration, inflation, and investment return. This calculator shows capital required using present value of growing annuity.

4,000/month for 25 years at 2.5% inflation and 4% return: real return 1.46%. Present value capital needed: 998,000. Roughly 25x annual income - matches 4% rule.

Use for life insurance needs or FIRE capital target. Lower return assumption = higher capital needed. Higher inflation assumption = higher capital needed. Conservative inputs give safe target.

A worked example

With the defaults: monthly income target of 4,000, years needed of 25, inflation of 2.5%, investment return of 4%. The tool returns 998,938.49. You can adjust any input and the result updates as you type — no submit button, no reload. That's the real power here: seeing how sensitive the output is to one or two assumptions.

What moves the number most

The result responds to Monthly Income Target, Years Needed, Inflation, and Investment Return. Not every input has equal weight. Adjusting one input at a time toward extreme values shows which ones move the result most.

The formula behind this

Real return = (1+r)/(1+i)-1. Present value of growing annuity at real return. Everything the calculator does is shown in the formula box below, so you can check the math against your own spreadsheet if you want.

Using this as a check-in

Re-run this every three months. A single reading tells you where you stand; four readings tell you whether things are improving. The trend matters more than any individual snapshot.

What this doesn't capture

The result reflects only the inputs you provide and the assumptions built into the formula. It is a simplified model rather than a complete picture, and factors specific to your situation may matter just as much.

Example Scenario

£4,000/mo × 25 years at 2.5% inflation and 4% return = $998,938.49.

Inputs

Monthly Income Target:£4,000

Years Needed:25 years

Inflation:2.5%

Investment Return:4%

Expected Result$998,938.49

This example uses typical values for illustration. Adjust the inputs above to match a specific situation and see how the result changes.

Sources & Methodology

Methodology

The calculator computes the capital required to generate a target monthly income over a specified period. It first derives a real return rate by adjusting the investment return for inflation using the formula (1 + investment return) / (1 + inflation) − 1. This real return represents purchasing power growth after accounting for price increases. The calculator then applies the present value of an annuity formula, treating the monthly income target as a constant real-terms payment stream. The result shows the lump sum needed today to fund those withdrawals. The model assumes a constant real return throughout the period, smooth annuitised withdrawals, and no fees or taxes. It does not model sequence-of-returns risk, market volatility, varying withdrawal patterns, or tax implications.

References

CFA Institute - Present Value

Frequently Asked Questions

Why use real return?

Real return accounts for inflation. At 4% nominal return with 2.5% inflation, real return is only 1.46%. Using nominal understates capital needed because income target erodes in real terms over time.

What happens to the capital estimate if I increase the income period by several years?

A longer period requires more total withdrawals, so the required capital increases, though not in a simple linear way. The annuity formula discounts future withdrawals at the real return rate, meaning additional years near the end of the period contribute less to the total than early years. Extending from 20 to 30 years has a smaller marginal effect than extending from 5 to 15 years, assuming a positive real return.

Why does the calculator not account for taxes or fees?

The model is designed to isolate the mathematical relationship between capital, return, inflation, and income duration without layering in assumptions that vary significantly by individual circumstance, account type, and jurisdiction. In practice, investment management fees and tax on withdrawals or returns would reduce the effective return rate, meaning the true capital required is likely higher than the figure shown. Users can approximate this by entering a lower investment return to reflect after-fee, after-tax expectations.

Can I use this calculator to plan for a period where my spending is expected to change significantly?

The model assumes a constant real monthly income throughout the entire period, so it does not natively handle scenarios with distinct spending phases, such as higher early-retirement spending tapering to a lower amount later. A practical workaround is to run separate calculations for each phase and sum the resulting capital figures, keeping in mind that this approach still ignores sequence-of-returns risk and assumes each phase can be funded independently.

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