The real raise formula
real raise = (1 + raise%) ÷ (1 + inflation%) − 1
Subtracting inflation from your raise is close enough for everyday numbers, but the division form is exact and matters when either number is large. The calculator uses the exact form.
A worked example
A $70,000 salary with a 4% raise becomes $72,800. If inflation is running 3%, the real raise is (1.04 ÷ 1.03) − 1 = 0.97% — your new salary buys about $680 more per year than the old one did, not $2,800. With 5% inflation, the same "raise" is a real cut of about 0.95%: the bigger paycheck buys less than last year's smaller one.
Using this number in a negotiation
"My raise is below inflation" is a concrete, unemotional argument that HR departments understand — you're asking to not be paid less, which is an easier case than asking to be paid more. Bring the CPI figure, show the real-raise math, and anchor your ask at inflation plus the merit percentage they'd normally give.