A rate on a quotation is a headline, and three separate things can sit between the headline and what the plant actually pays.

Compounding frequency. $\mathit{EAR} = \left(1 + \dfrac{r}{m}\right)^{m} - 1$ — read aloud E-A-R equals one plus r over m, to the m, minus one. Here $r$ is the nominal annual rate as quoted, a decimal; $m$ is the number of compounding periods in a year, a plain count — 12 monthly, 4 quarterly, 2 semi-annually; and $\mathit{EAR}$ is the effective annual rate, what one year really costs once the intermediate interest starts earning. Divide the headline across the periods, compound through them, subtract the original dollar. 12% compounded monthly is 12.68%. The effective rate is never lower than the nominal one, and the two are equal only when $m = 1$.

Inflation. $r_{\text{real}} = \dfrac{1+i}{1+f} - 1$ — the Fisher equation. $i$ is the nominal rate earned, $f$ the inflation rate over the same period, both decimals, and $r_{\text{real}}$ is what is left in purchasing power. Everyone subtracts in their head — 8% less 3% is 5% — and the honest answer is 4.85%, because the inflation has to be paid on the gain as well as on the capital. The rough version always flatters, and it flatters worst exactly when inflation is high enough to matter.

Time. $n \approx \dfrac{0.72}{i}$, the Rule of 72: divide 72 by the rate written as a PERCENTAGE and you have the doubling time in years. 6% doubles in 12 years; 9% in 8; 12% in 6. It is an approximation to $\ln 2 / \ln(1+i)$ and it is good to a fraction of a year anywhere between about 2% and 20%. Written with the rate as a decimal, as the catalog does, it is 0.72 over $i$ — the same rule, one decimal point apart, and mixing the two forms is a hundredfold error.

The nugget: the Rule of 72 is the fastest sanity check in the whole subject. Any time a discounted figure looks wrong, ask how many doublings fit in the horizon. Thirty years at 8% is more than three doublings, so a sum out there is worth roughly an eighth of its face value today — and if your answer is nowhere near, something upstream is broken.