Inflation-adjusted return calculator

A nominal return is what the statement says. A real return is what the money will buy. This turns one into the other with the Fisher equation, and shows what a lump sum is actually worth in today's dollars.

How the math works

The exact relationship between a nominal return, inflation, and the real return is the Fisher equation. It divides the growth of your money by the growth of prices.

Where nominal is the stated return and inflation is the annual change in prices, both as decimals. The tempting shortcut is to just subtract — real ≈ nominal − inflation — and it lands close. But it is always a little high whenever inflation is positive, because it skips the interaction between the two rates.

Take the S&P 500's long-run nominal return of 10.2% and the post-WWII inflation average of 3.3%. Subtraction says 6.9%. The Fisher equation says 6.68%. The gap is about a fifth of a percentage point — small in one year, and not small compounded across a working life.

For the lump-sum mode, the calculator grows your money at the nominal rate, then deflates the ending balance back to today's dollars: real value = principal × (1 + nominal)years ÷ (1 + inflation)years. That is the same thing as compounding the real rate directly — the two agree to the penny.

Worked example

Put $10,000 into a broad-market index fund and leave it for 30 years. Model the nominal return at the S&P long-run figure of 10.2% and inflation at the post-WWII average of 3.3%.

The sticker balance after 30 years is $184,267. That is the number a plain compound-interest calculator shows, and it is not wrong — it is just in future dollars. Deflate it back to what it buys today and the real balance is $69,573. Barely 38% of the sticker balance survives inflation over three decades.

Both numbers are true. The nominal figure is what lands in the account; the real figure is what it buys. A retirement plan built on the sticker number is planning to be a third poorer than it thinks.

When this calculator is wrong

The real return above is a pre-tax number, and taxes are where inflation does its quietest damage. The tax code charges you on the nominal gain, not the real one. Inflation inflates the number you are taxed on, so it pushes your real tax rate above the rate on the table.

Run the S&P example one year at a time. A $10,000 position returning 10.2% shows a nominal gain of $1,020. At a 22% marginal rate that is $224.40 in tax. But the real, inflation-adjusted gain was only 6.68% — and after that tax the real return drops to about 4.51%. The tax took roughly 32.5% of your real gain, not 22%. The 10-point gap is pure inflation, taxed as if it were profit.

Two more ways the plain number misleads:

• A single average inflation rate is not the real history. Deriving a real return from one average nominal figure and one average inflation figure will not match the true historical real return. The S&P's long-run real return, measured directly year by year, is 7.0% — not the 6.68% the Fisher equation gives from the two averages, and not the 6.9% the subtraction shortcut gives. Inflation varied every year; averaging it first throws away that detail.
• It assumes one steady rate for both. Real markets deliver their returns unevenly, and inflation spikes and stalls. Two portfolios with the same average real return can end at very different places depending on the order the good and bad years arrive — sequence-of-returns risk that a single-rate model can't see.
• It says nothing about which dollars you're comparing. A "real" figure is only meaningful against a stated base year. This calculator uses today as the base; a projection someone else made last year is anchored to last year's dollars.

What to do with the result

Use the real number, not the nominal one, for any goal more than a few years out. The practical rule: when you see a long-horizon projection quoted in raw dollars, mentally deflate it. At 3.3% inflation, money roughly halves in purchasing power every 21 years or so, which is why a $184,267 future balance is really a $69,573 balance in the dollars you spend today.

For a target stated in today's dollars, flip the question: to earn a 4% real return while inflation runs at 3.3%, you need a nominal return of about 7.43%. If the account you're holding can't plausibly clear that — a savings product at the FDIC average of 0.41% can't come close — the math says the money is losing ground in real terms, however positive the statement looks.

Common questions

What's the difference between nominal and real return?

Nominal return is the raw percentage gain before inflation. Real return subtracts inflation's effect, so it measures the change in what your money can actually buy. A 10.2% nominal return at 3.3% inflation is a 6.68% real return.

Why not just subtract inflation from the return?

You can, and it's close. But nominal − inflation always overstates the real return a little when inflation is positive, because it ignores the interaction between the two rates. The Fisher equation, (1 + nominal) / (1 + inflation) − 1, is the exact version. The gap is about 0.2 points on the S&P example — enough to matter over a long horizon.

Does inflation affect the taxes I pay on investment gains?

Yes, and not in your favor. Tax is charged on the nominal gain, so part of what you're taxed on is just inflation, not real profit. At 22% statutory tax and 3.3% inflation, the tax can eat roughly a third of your real gain rather than 22% of it. Tax-advantaged accounts like a Roth IRA or 401(k) sidestep this.

What inflation rate should I use?

For a long horizon, the post-WWII U.S. average of 3.3% is a reasonable default; the Federal Reserve's stated long-run target is 2.0%. For a short horizon, use the current reading — inflation is far more volatile year to year than its long-run average suggests.

Is a real return ever negative when the nominal return is positive?

Often. Any time inflation runs higher than your nominal return, the real return is negative — your balance grew in dollars but shrank in purchasing power. A savings account paying 0.41% during a year of 3.3% inflation loses almost 3% in real terms.