Real vs nominal returns: the assumption that makes or breaks a FIRE calculation

Every FIRE and Coast FIRE calculator asks for an expected return. It is the single most influential input, and it is also the one most often entered incorrectly. The problem is not usually optimism — it is mixing two kinds of numbers. This guide explains the difference between nominal and real returns, how to convert between them, and why our calculators work entirely in today’s dollars.

Nominal and real, defined

A nominal return is the growth you see on a statement: if $10,000 becomes $10,700 in a year, the nominal return is 7%. A real return is what is left after inflation. If prices rose 2.5% that year, your $10,700 buys about what $10,439 bought a year earlier, so the real return is about 4.39%.

The exact conversion is:

real return = (1 + nominal return) ÷ (1 + inflation) − 1

Subtracting inflation from the nominal return (7% − 2.5% = 4.5%) is a common approximation and is close enough at low rates, but the division is exact.

Why it matters for FIRE

FIRE calculations combine two kinds of input: spending and returns. If you enter retirement spending in today’s dollars — “I want to live on $40,000 a year, like now” — then the portfolio must also be measured in today’s dollars, which means using a real return. If you enter a nominal return, you must also inflate your spending to the dollars of your retirement year.

Mixing them flatters the result badly. Consider $100,000 invested for 30 years:

• At 7% nominal it grows to about $761,226 in future dollars.
• With 2.5% inflation, prices roughly double over 30 years (1.025³⁰ ≈ 2.098), so that is about $362,909 in today’s dollars.
• Growing at the equivalent real return of 4.39% gives the same $362,909 directly.

If you compared $761,226 with a FIRE number expressed in today’s dollars, you would think you were more than twice as far along as you are.

The Coast FIRE example

For a $1,000,000 FIRE number in today’s dollars and 30 years to retirement:

• Using 7% (treated as real): Coast FIRE number ≈ $131,367.
• Using 5% real: ≈ $231,377.

If 7% was really a nominal assumption with 2% inflation, the real return is about 4.9%, and the honest Coast FIRE number is close to the 5% figure — about $100,000 higher than the mistaken one. That is the difference between “I can stop saving now” and “I need several more years”.

Why our calculators use today’s dollars

Thinking in today’s dollars is easier. You know what $40,000 buys now; you do not intuitively know what $84,000 will buy in 30 years (which is roughly what $40,000 becomes after 30 years of 2.5% inflation). So every CoastMath calculator asks for spending in today’s dollars and a real return. The results — FIRE numbers, Coast FIRE numbers, balances — are all in today’s purchasing power.

Choosing a real return

There is no correct number, and nobody knows future returns. Reasonable practice is to:

• Test a range. Run your plan at 3%, 5% and 7% real to see how sensitive it is.
• Consider your allocation. A stock-heavy portfolio has historically produced higher but more volatile returns than one with more bonds or cash. Your assumption should reflect what you actually hold.
• Subtract costs. Fund expense ratios and advisory fees come straight off returns. A 1% annual fee on a 5% real return leaves 4%.
• Be wary of recent history. Strong recent returns tempt people to extrapolate. Forward-looking research, such as the capital-market assumptions behind Morningstar’s withdrawal-rate work, often differs from trailing averages.

Inflation

The U.S. Bureau of Labor Statistics publishes the Consumer Price Index, the most common inflation measure. Your personal inflation rate can differ: housing, health care and education have at times risen faster than the overall index. If you expect your costs to rise faster than CPI, use a lower real return or a higher spending estimate.

Rules of thumb that help

Rule of 72. Investor.gov explains that dividing 72 by the annual return estimates the years for money to double. At 5% real, about 14.4 years; at 7% nominal, about 10.3 years. If a projection shows faster doubling than the rule suggests, check whether it uses nominal returns.

Compound interest calculators. The SEC’s Investor.gov compound interest calculator lets you check growth independently. If you enter a real return there, the result is in today’s dollars.

Withdrawal rates are already real

The classic 4% rule withdraws 4% in the first year and then raises the dollar amount with inflation each year. That means the rule is defined in real terms. It fits naturally with today’s-dollar spending and real returns. Morningstar’s safe-withdrawal research uses the same structure — fixed real withdrawals — in its base case.

Taxes on returns

In taxable accounts, taxes on dividends and realised gains reduce the return you keep. In tax-advantaged accounts, taxes apply on withdrawal instead (for traditional accounts) or not at all for qualified Roth withdrawals. A simple approach: use a pre-tax real return for growth and include expected taxes in retirement spending.

A checklist before you trust a projection

1. Is spending in today’s dollars? Then the return must be real.
2. Have fees been subtracted from the return?
3. Have you tested at least one pessimistic return?
4. Are taxes included in spending?
5. Does the result pass a Rule of 72 sanity check?

Common mistakes we see

Copying a headline return. Long-run stock-market averages are usually quoted in nominal terms. Using one directly in a today’s-dollar calculator overstates growth by the inflation rate every year.

Inflating spending twice. Some spreadsheets inflate spending to future dollars and then also use a real return. That understates progress and can push people to over-save.

Forgetting that withdrawals also rise with prices. A plan that withdraws a flat nominal amount will see its purchasing power fall every year. The classic withdrawal-rate research raises withdrawals with inflation, which is another reason to keep everything real.

Summary

Use real returns with today’s-dollar spending, or nominal returns with future-dollar spending — never a mix. Convert with (1 + nominal) ÷ (1 + inflation) − 1, subtract fees, and test a range. All CoastMath calculators work in today’s dollars with real returns for this reason. This guide is educational, not investment advice.