The Real Interest Rate and the Fisher Equation — The True Interest Shows Only After Subtracting Inflation
If your deposit interest is 4% a year but prices rose 5%, did your money grow or shrink? The key to answering this question is the "real interest rate."
Nominal Rate vs. Real Rate
The nominal rate is the "surface-level interest rate" printed on your account. Numbers like 4% deposit interest or 5% loan interest are nominal rates.
The real rate is the "true interest rate in purchasing-power terms" after subtracting the inflation rate from that. Even if the nominal rate is 4%, if prices rose 5%, it is effectively −1%. In other words, even though you received interest, the real value of your money actually shrank.
What truly matters in investing and saving is not the nominal but the real. Interest that cannot beat inflation is "a loss while sitting still."
The Fisher Equation
What organizes this relationship is the "Fisher equation," named after economist Irving Fisher.
Nominal rate ≈ real rate + expected inflation
In other words, the nominal rate means the "interest you actually want to earn" plus "the inflation you expect to rise ahead." If prices are expected to rise, both borrowers and depositors come to demand a correspondingly higher nominal rate.
The precise equation is (1 + nominal) = (1 + real) × (1 + inflation), but when the numbers are not large, you can understand it well enough with the approximation "nominal ≈ real + inflation."
"Expected inflation" is a forecast about future prices. If actual prices differ from the forecast, the real rate calculated after the fact also differs.
Why the Real Rate Matters to Investors
The real rate is a background condition for asset prices. When the real rate is low or negative, there is less reason to hold cash and deposits, so money tends to move into risk assets and real assets, and when the real rate rises, the opposite pressure arises.
Also, to bond investors, the real rate means the "true return that remains after beating inflation." Inflation-linked government bonds (such as TIPS) are products designed precisely to protect this real rate.
In the end, the real perspective of asking "did my return beat inflation?" is the very yardstick a long-term investor must not miss.
よくある質問
Q. Can the real rate be negative?
Yes. If the inflation rate is higher than the nominal rate, the real rate becomes negative. In this case, even if you keep a deposit, the purchasing power of your money shrinks. It is a phenomenon that often appears in periods of low rates and high inflation.
Q. Where do I check the real rate?
Simply, you can approximate it by subtracting the inflation rate from the nominal rate (deposit or government-bond rate). The real rate reflecting market expectations is also gauged through things like the yield difference (breakeven inflation, BEI) between inflation-linked government bonds (TIPS) and ordinary government bonds.
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