Expected Inflation and the Fisher Effect

Lenders care about what they can actually buy with the money they get back, not the number printed on the loan. So before quoting a rate, a lender starts with the real return they want, then adds on however much inflation they expect over the life of the loan. That sum is the nominal rate: nominal rate = real rate + expected inflation. If inflation expectations rise, lenders quote a higher nominal rate immediately, even though nothing about the real economy changed, because they are protecting the same real payoff against a currency that's expected to buy less later.

The catch is that most loans fix the nominal rate at signing, based on expected inflation, but the real rate actually earned depends on inflation that actually happens. Real rate paid = nominal rate minus actual inflation. If actual inflation matches what was expected, both sides get exactly the real rate they planned on. If actual inflation comes in higher than expected, the fixed payments are worth less in real terms than either side planned, so the borrower pays back less purchasing power than promised and the lender loses ground. If inflation comes in lower than expected, the reverse happens: the lender ends up earning a higher real return than planned, and the borrower pays more in real terms than they bargained for.

This is why a high nominal rate alone tells you nothing about how expensive borrowing really is. A 15 percent nominal rate during a 14 percent inflation stretch is cheap money in real terms; a 5 percent nominal rate during 1 percent inflation is not. This connects directly to the money market and loanable funds market next door: the money market pins down the nominal rate through money supply and demand, while the loanable funds market works in real terms, and the Fisher equation is the bridge that keeps the two consistent.