How the retirement savings calculator works
The projection, honestly framed
Balance grows at (balance × monthly return) + contribution, repeated for every month to your retirement date. The formula is exact; the return input is a forecast — which is why the calculator pairs the nominal egg with its real value and a 4%-rule preview instead of pretending the headline number is a promise.
Which return for a 25-year horizon
A stock-heavy portfolio has historically returned ~10% nominal / ~7% real; bonds materially less. Planning at 6–7% nominal for a diversified long-horizon portfolio is defensible and slightly conservative. What is not defensible: planning at 10% and calling the extra cushion "realistic optimism."
The contribution lever is bigger than people think
Between two 30-year savers at 7%, the one contributing $1,200/month ends with ~$1.5M vs ~$735k at $600 — contribution rate moves the outcome 2×, while plausible return tweaks (6.5 vs 7.5%) move it ~1.2×. Savings rate is the lever you control; this is the calculation that proves it.