Open a 529 account for a newborn, contribute $3,000 per year — about $250 a month — and stop contributing on the day they graduate high school. Depending on the rate of return you assume, the account will hold somewhere between $85,000 and $145,000 at the end of those eighteen years. That wide band is the whole story. The contributions are almost incidental; the growth assumption is doing most of the work. This post walks through the underlying math and the assumptions that drive it, with no financial-advice angle and no product recommendations.
The formula
A recurring contribution of P per period, earning rate r per period, for n periods, grows to a future value of:
FV = P × (((1 + r)^n − 1) / r)
This is the ordinary-annuity future-value formula from any introductory finance textbook. The contributions happen at the end of each period; if they happen at the beginning of the period ("annuity due"), multiply the result by (1 + r). The difference is modest over long horizons. Our 529 savings growth calculator runs the full formula year by year so you can see how the balance builds.
Why the rate assumption dominates
For an 18-year horizon with $3,000 annual contributions, here is what the formula produces at three common return assumptions:
- 5% annual return: FV ≈ $3,000 × 28.132 ≈ $84,400.
- 7% annual return: FV ≈ $3,000 × 33.999 ≈ $102,000.
- 9% annual return: FV ≈ $3,000 × 41.301 ≈ $123,900.
Total contributions over the 18 years are $54,000 regardless of the rate. The difference between the three outcomes — a spread of about $40,000 — is entirely compound growth. At 9%, the account is roughly 130% contributions and 130% growth; at 5%, the growth is only about 56% of contributions. The rate-of-return assumption is the single largest lever in any long-horizon savings projection, larger even than the contribution size.
Choosing which rate to plug in is harder than it sounds. The S&P 500 has returned about 10% annualised nominal over the last century, but that is nominal (not inflation-adjusted) and includes substantial decade-long stretches well below that average. Target-date and age-based funds inside 529 plans charge fees that subtract 0.1% to 0.6% per year depending on the plan, and they glide toward bonds as the beneficiary approaches college, which reduces the expected return in the later years.
The age-based glide path
Most 529 plans offer an "age-based" or "target-enrollment" default option. The allocation starts aggressive (80% to 100% equities) when the beneficiary is a toddler and gradually shifts toward bonds and cash equivalents as the beneficiary approaches college. By the time the child is a senior in high school, the allocation is often 20% equities and 80% fixed income.
The glide path reduces the risk that a bad stock-market year right before college drains the balance at the worst possible time. It also reduces the long-run expected return. A reasonable blended assumption for an age-based fund over eighteen years is around 6%, which produces about $92,500 under the same contribution schedule. More aggressive self-directed portfolios can plausibly run at 7% to 8% blended if the investor is willing to accept higher sequence-of-returns risk near the end.
The tax advantages, briefly
529 plans are the most tax-advantaged vehicle most U.S. families can access for education savings. At the federal level, contributions are made with after-tax dollars, but all growth inside the account is federal-tax-free and all withdrawals for qualified education expenses are federal-tax-free as well. Qualified expenses include tuition, mandatory fees, required books and supplies, and room and board for at least half-time students, plus up to $10,000 per year toward K-12 tuition.
At the state level, treatment varies sharply. A majority of states that levy income tax offer a state deduction or credit for 529 contributions, sometimes only for contributions to the home state's plan. A handful of states offer no deduction at all. The deduction magnitude, when it exists, is typically between $2,500 and $10,000 per filer per year. None of this is tax advice — state rules change, and anyone relying on a deduction should confirm current terms with their state revenue department or a qualified tax preparer.
The "what if they don't use it" problem
The long-running objection to 529 plans has been that if the beneficiary does not attend college or receives substantial scholarships, the account gets stuck. Withdrawals for non-qualified purposes incur income tax plus a 10% penalty on the earnings portion. That concern has been softened meaningfully by the SECURE 2.0 Act, passed in late 2022 and phased in through 2024.
Under the new rules, a 529 that has been open for at least fifteen years can have its balance rolled over to a Roth IRA in the name of the beneficiary, subject to several guardrails: a lifetime rollover cap of $35,000 per beneficiary, rollovers subject to the annual Roth contribution limit, and rollover contributions only of amounts held in the 529 for at least five years. The effect is that unused 529 funds no longer face a strict penalty-or-forfeiture choice; they can be gradually converted into retirement savings for the beneficiary.
Other escape valves still apply. The account owner can change the beneficiary to another qualifying family member (a sibling, cousin, parent, or the owner themselves) without penalty. Up to $10,000 of a 529 can be used to repay student loans held by the beneficiary or a sibling. Scholarship-equivalent withdrawals can be taken out of the account without the 10% penalty (though ordinary income tax still applies to the earnings portion).
Coordination with financial aid
A 529 owned by a parent is treated favourably on the FAFSA: it is counted as a parental asset at the 5.64% assessment rate, meaning $100,000 in a 529 reduces expected aid eligibility by only about $5,640 at the margin. A 529 owned by a grandparent used to be more complicated, but under the simplified FAFSA that went into effect for the 2024-25 aid year, grandparent-owned 529 distributions no longer count as student income. That change made grandparent-owned 529s meaningfully more useful for coordinated family savings.
None of this guarantees any particular financial aid outcome. The gap between 529 assets and the full cost of a four-year private college is often large even after savings, and understanding the shape of that gap matters for planning. Our financial aid gap calculator shows how savings, grants, and loans typically combine, and our scholarship award estimator gives a rough read on merit-based awards at common institution types.
Why the headline $100,000 number is not a promise
The six-figure projection in the introduction is a product of compounding assumptions, not a guarantee. Market returns over eighteen years can come in well below the long-run average. Inflation erodes the real value of whatever accumulates — $100,000 in 2044 dollars will not pay for the same share of a college year as $100,000 in 2026 dollars. Fees, if unmonitored, can quietly compound against the balance at the same rate that growth compounds for it.
The right way to read a 529 projection is as a budgeting tool, not a forecast. Run the calculation at 5%, at 7%, and at 9%. Look at the spread. If the 5% case covers the share of college costs you need to cover, the plan is robust; if it only works at 9%, the plan is fragile and you probably need to raise contributions, extend the horizon, or plan on loans covering more of the gap. The math is clean; the assumptions are where the real decision lives.