Why Retirement Calculators Give You Wildly Different Target Numbers
Retirement calculators disagree mainly because they use different assumptions for investment return, inflation, retirement length, and whether Social Security or a pension is factored in at all — all genuine unknowns about the future, so reasonable tools using different reasonable assumptions land on meaningfully different target numbers.
Getting a $1.2 million target from one calculator and a $2 million target from another isn't a sign one tool is broken — it's almost always a sign the two tools are quietly assuming different things about the future.
The assumptions that swing the target most
Investment return assumption: a 1–2 percentage point difference in assumed annual return compounds enormously over a multi-decade projection, and reasonable people can reasonably disagree on the right assumption.
Whether Social Security or a pension is included: a tool that ignores other retirement income sources and calculates a target based purely on personal savings will show a meaningfully higher number than one that nets out expected Social Security benefits first.
Assumed retirement length: a target based on a 25-year retirement is smaller than one based on a 35-year retirement (relevant for early retirement planning) at the same annual spending target.
How to make a fair comparison across tools
Rather than trusting a single number, check what each calculator assumes for return rate, inflation, retirement length, and outside income sources — adjusting all of them to match across two tools usually closes most or all of the gap, revealing that the disagreement was about assumptions, not about disagreement on the underlying math.