College Cost Calculator Guide
If you're saving for a newborn's college education, you have an 18-year runway — which sounds like plenty of time for compounding to work in your favor. The problem is that the cost you're compounding toward is also growing, and it has historically grown faster than the inflation rate most people default to when they think about "the future." Mixing up those two growth rates is one of the most common ways 529 plans end up short.
Two Different Inflation Rates Are at Work
General inflation — the kind that shows up in cost-of-living raises and retirement projections — has averaged somewhere in the low single digits over long stretches, with a well-known rule of thumb putting it around 2-3% a year outside of unusual periods. College costs have moved differently. Over the past few decades, published tuition and fees at many schools rose at a noticeably faster clip than that, often landing in the mid-single-digits annually before accounting for financial aid discounts. That gap compounds hard over 18 years: at 3% growth, a $25,000-a-year cost becomes about $42,600; at 5.5%, that same cost balloons to roughly $65,600. A savings target built on the wrong assumption isn't off by a rounding error — it can be off by tens of thousands of dollars per year of school.
Why the Runway Length Matters So Much Here
The younger your child is when you start, the more that inflation-rate assumption dominates the outcome — more so than almost any other input in the plan. With only three or four years until enrollment, a 1-2 percentage point difference in assumed cost growth barely moves the total. With a full 18-year runway, that same percentage-point gap compounds across nearly two decades and can change the required monthly contribution by a meaningful multiple. This is the mechanical reason financial planners tend to push college-cost assumptions higher than general CPI the further out the enrollment date sits: a small annual error gets magnified by time in exactly the way compound growth always magnifies small errors, just working against the saver instead of for them. Running the numbers with a rate closer to historical tuition trends, rather than a generic inflation figure, is the more conservative — and usually more realistic — starting point.
Where This Leaves Your Monthly Number
The practical takeaway is to stress-test your plan at more than one growth rate rather than anchoring to a single guess. Try the calculator above with the default 5% college inflation rate, then again at 3% and at 6-7%, and look at how much the required monthly savings figure moves each time. If the spread is wide — which it usually is over an 18-year horizon — it's worth leaning toward the higher end when deciding how much to actually contribute, since underfunding a college account is generally more disruptive to a family's finances than overfunding one (excess 529 funds can often be redirected to another child, a different degree, or under current rules rolled into a retirement account within limits that change periodically). If you're contributing through a 529 or similar account, the compound interest guide is a useful companion for understanding how your contribution rate and investment return interact over time, and the investment calculator can help model contributions that step up gradually rather than staying flat for 18 years. This is general educational information rather than a personalized funding recommendation — a fee-only financial planner can help you weigh 529 contributions against other goals like retirement savings or debt payoff given your actual numbers.