Real Return Calculator (inflation & Fisher equation)

Understanding the real return calculator — concept and principle

A nominal return tells you how much your money grew; a real return tells you how much your purchasing power grew after inflation. This calculator converts a nominal return into an after-tax real return with the exact Fisher equation, compares it with the quick subtraction shortcut, and shows how much 100 invested would be worth in today's money after a number of years.

How it is calculated

• After-tax nominal return: n_at = nominal × (1 − tax rate on gains).
• Exact real return (Fisher): r = (1 + n_at) ÷ (1 + inflation) − 1.
• Approximation: r ≈ n_at − inflation.
• Real value of 100 after y years: 100 × (1 + r)^y.

Rates are entered as annual percentages; the tax rate here is a simple flat rate you supply.

The principle

Irving Fisher described how nominal rates combine expected inflation with a real rate. If prices rise by inflation π, one unit of money buys 1 ÷ (1 + π) as much next year. Growing your money by (1 + n) and dividing by (1 + π) gives real growth. Subtracting inflation ignores the cross term n × π, so the shortcut overstates the real return, and the error grows when inflation and returns are high. Taxes apply to nominal gains, which is why tax hurts more in inflationary times.

Worked example

Default inputs: nominal return 7%, inflation 3%, tax 0%, 10 years.

1. After-tax nominal: 7% × (1 − 0) = 7%.
2. Real return: 1.07 ÷ 1.03 − 1 = 3.883% (shown as 3.88%).
3. Approximation: 7% − 3% = 4%, overstating by about 0.12 points.
4. Real value of 100 after 10 years: 100 × 1.03883¹⁰ = 146.4.

In nominal terms 100 would have grown to about 196.7, but prices would have risen about 34%.

How to read the result

A positive real return means your purchasing power is growing. Cash and short-term deposits often earn close to zero real return, while long-run stock returns have historically been higher but uneven. Use real returns when planning goals in today's money, such as retirement income, so that inflation is already accounted for.

Common mistakes

• Subtracting inflation when rates are high.
• Using current inflation as if it were constant for decades.
• Applying tax to the real return instead of the nominal gain.
• Comparing a real return with a goal stated in future, nominal dollars.

Frequently asked questions

What is 7% nominal with 3% inflation in real terms?

About 3.88% using the exact Fisher equation.

Why not just subtract inflation?

Subtracting ignores how inflation compounds against your return; the error grows with higher rates.

Can real returns be negative?

Yes, whenever inflation exceeds your after-tax nominal return.

Sources: Fisher, I., The Theory of Interest (1930) · Bodie, Kane and Marcus, Investments (real versus nominal rates)