Why the same mistakes keep happening
Most retirement planning errors are not arithmetic errors — they are assumption errors. The formula is right, but the input is idealized: a round number, an optimistic rate, a best-case month. Below are the mistakes behind most bad results, grouped by the calculator where they bite.
Retirement Nest Egg Projection
Projecting at the best decade's returns and treating the output as a plan.
Ignoring inflation, then discovering the egg buys half its headline.
Forgetting existing balances — the compounding on money already saved is often half the projection.
4% Rule Withdrawal Calculator
Treating 4% as guaranteed income forever regardless of sequence or horizon.
Forgetting that withdrawals are taxable (traditional accounts) — $4,000/mo pre-tax is not $4,000 of lifestyle.
Counting Social Security at today's dollars without understanding claiming-age trade-offs.
Retirement Savings Needed Calculator
Sizing the egg against gross retirement income while ignoring the pension/Social Security layer already coming.
Choosing the withdrawal rate by optimism instead of horizon.
Treating the monthly number as fixed — it falls with every year of delay added and every raise escalated.
Retirement Age Estimator
Setting the target from headlines ("$1M!") instead of from your spending gap.
Re-running monthly and letting market noise move your behavior.
Ignoring that lower spending needs both a smaller egg AND fewer years to fund it — the double win nobody prices.
The habit that fixes all of them
Write down the assumption you are least sure about every time you run a number. If the answer matters, test it: change that one input by ±20% and see whether the decision flips. If it flips, the assumption — not the math — is your real problem, and it deserves the research time.