Why Your Compound Interest Result Looks Too High (or Too Low)
The most common cause is entering an annual interest rate but selecting monthly compounding without realizing the calculator expects a per-period rate, or the reverse — always confirm whether a rate field expects an annual rate (with compounding frequency handled separately) or a rate already matched to the compounding period.
A compound interest result that looks dramatically too high or too low almost always traces back to one specific input mismatch, not a flaw in the underlying formula.
Annual rate vs. per-period rate confusion
Most calculators expect an annual interest rate and handle the conversion to monthly or daily compounding internally — entering a rate that's already been divided by 12 into a field expecting the full annual rate produces a result that's far too low, since the rate gets divided again internally.
Forgetting that returns aren't guaranteed or constant
A compound interest projection assumes the same fixed rate every single period — real investment returns vary year to year, sometimes significantly, even if they average out to something close to the assumed rate over a long horizon. A projection is a best-estimate model, not a guarantee, especially for shorter time horizons where a few bad years can meaningfully lag the projection.
Nominal vs. real (inflation-adjusted) results
A standard compound growth calculation shows the nominal future balance — its actual dollar amount, not adjusted for inflation. A large-looking future number can represent meaningfully less real purchasing power once inflation over a long time horizon is factored in, which is worth remembering when a result looks unexpectedly large.