How Inflation Changes Your Coast FIRE Number

A coast target depends on real growth, not nominal growth alone. Higher inflation reduces the purchasing power of a fixed investment return and raises the amount needed today.

By Steven, Founder · Published November 12, 2026 · Updated November 12, 2026

Short answer

Real return equals (1 + nominal return) ÷ (1 + inflation) − 1.
Keep the retirement target and growth rate in the same dollar convention.
A small annual assumption difference can materially change a decades-long coast threshold.

Keep the target fixed in purchasing power

Assume a 40-year-old wants $1.5 million in today's purchasing power at 65 and plans no further contributions or withdrawals. The target might come from a separate spending analysis; this article does not establish that $1.5 million is enough. The Coast FIRE guide covers the overall milestone; here we isolate inflation sensitivity.

If the target is expressed in today's dollars, discount it using a real return. If expressed in future nominal dollars, use nominal growth and first inflate the target consistently. Mixing today's spending target with nominal growth understates the amount needed now.

Convert a 6% nominal assumption correctly

Hold assumed nominal annual growth at 6%, after fees, and vary annual inflation. At 2% inflation, real growth is 1.06 ÷ 1.02 − 1, about 3.92%. At 3% inflation it is about 2.91%; at 4% it is about 1.92%. Subtracting inflation gives a quick approximation, not the exact compound-growth rate.

The current coast amount is $1.5 million divided by (1 + real return)^25. That gives approximately $573,388 at 2% inflation, $731,771 at 3%, and $931,704 at 4%. The nominal return never changed; the purchasing-power assumption did. These are constant-rate illustrations, not market forecasts.

Why a single threshold can be fragile

A person with $750,000 would appear above the middle scenario's threshold and below the higher-inflation threshold. Declaring coasting permanently achieved from one point estimate hides that sensitivity. Future returns and inflation can both differ from their assumptions.

Use the Coast FIRE calculator to compare a base case and less favorable real growth. Make sure fees are not subtracted twice if the return input is already net of costs. Taxes on eventual withdrawals also belong in the spending-target calculation, not in an unexplained adjustment to the rate.

The remaining years matter too

At a fixed 4% real return, a $1.5 million target 20 years away requires about $684,580 now. That differs from a 25-year horizon even with the same target. Reaching a birthday without making new contributions does not automatically preserve the old target if actual returns have been lower than assumed.

Recalculate using the current balance and actual years remaining. Do not count an emergency reserve as coast capital if it will fund current bills. Coasting assumes the portfolio remains invested; withdrawals during a career break change the calculation.

Plan what you would change if the margin shrinks

Possible responses include resuming contributions, lowering the future spending target, delaying full retirement or maintaining some work income later. Each has a different effect on the household. Write the response down before treating a threshold as permission to stop saving indefinitely.

The sample household demo helps connect today's saving decision with later spending and income. These examples are educational, assume smooth compounding and exclude market sequence effects; they do not establish financial independence.

Same nominal growth, different coast amounts

Target / horizon: $1,500,000 in today's dollars / 25 years
Nominal annual growth: 6% after fees
Annual inflation: 2% / 3% / 4%

Current targets: about $573,388 / $731,771 / $931,704

For each scenario, divide $1.5 million by (1.06 ÷ (1 + inflation))^25. No contributions or withdrawals occur during the horizon.

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Methodology

See how Meridary checks calculator figures, tests the planning engine, states model limits, and records corrections on the validation methodology page.

Sources

Compound Interest Calculator — U.S. Securities and Exchange Commission, Investor.gov
Compound-growth inputs include starting investment, recurring monthly contributions, time, estimated rate, rate sensitivity and compounding frequency. Article results are separately calculated illustrations.
Accessed October 25, 2026
Determining Withdrawal Rates Using Historical Data (1994; 2004 reprint) — William P. Bengen, Journal of Financial Planning / Financial Planning Association
Historical U.S. stock/Treasury retirement tests, initial-balance withdrawal percentages followed by inflation adjustments, rebalancing, and the inadequacy of average returns for drawdown planning.
Accessed October 25, 2026

This is an educational estimate, not tax or financial advice. Figures reflect the law year shown and the inputs you provide. Consult a qualified professional before acting.