Why the same mistakes keep happening
Most college savings 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.
Future College Cost Calculator
Planning from today's sticker price — a 10-year-old's college costs ~1.7× today's at 5.5% inflation.
Assuming general CPI for education costs — the two rates have diverged for forty years.
Ignoring the 4th year: costs keep inflating while the student is enrolled.
529 Plan Growth Calculator
Keeping enrollment-year money in equities — a bad senior-year market should not cut the freshman fund.
Skipping the 529 for a "flexible" taxable account and donating years of tax drag to the IRS.
Waiting for the "right plan" — any low-cost 529 beats years of not-started.
College Savings Target Calculator
Committing to fund 100% without pricing it — the monthly line is the commitment, not the sentiment.
Waiting for the right age to start — the first dollars compound the longest and the earliest years are the cheapest per dollar.
Funding college before retirement accounts — there are loans for tuition and none for retirement.
Student Loan Payment Calculator
Choosing extended terms by payment comfort alone while doubling total interest.
Assuming income-driven forgiveness applies broadly — it requires 20–25 years of qualifying payments.
Borrowing the full offered amount rather than the actual gap — every borrowed dollar compounds from day one.
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.