Skip to content
STI
← Back to Blog

529 vs. High-Yield CD Calculator: The $98,822 Gap for a Two-Kid College Fund

The Question Every Parent With a CD Ladder Eventually Asks

Here's an example that comes up constantly: a two-income household earning $145,000 a year has been parking college money in a high-yield savings account and a rolling CD ladder at 4.50% APY. It feels safe — FDIC insured, no market risk, the balance only goes up. Then someone asks: "Wait, do I owe taxes on that interest every single year, even if I never touch it?"

Yes. According to NerdWallet's breakdown of CD and savings account interest taxation, interest earned in a taxable account is taxed as ordinary income in the year it's credited — whether you withdraw it or not. There's no "I'm saving this for college so it doesn't count" exception. A 529 plan, by contrast, grows tax-free as long as the money eventually pays for qualified education expenses. That single structural difference is worth calculating precisely, not estimating with a gut feeling — and the gap is bigger than most people expect once you run real numbers instead of round ones.

Below is that calculation, using a labeled example family (call them the Ramirezes) with two kids, ages 4 and 7. Your numbers will differ based on your tax bracket, state, and time horizon — but the formula is the part you can reuse today.

The Post-Tax APY Formula (and Why Your Bank Won't Show You This Number)

The core formula is simple:

Post-tax APY = APY × (1 − combined marginal tax rate)

The Ramirezes are in the 22% federal bracket (typical for a married-filing-jointly household around $145,000 in taxable income) and live in Ohio, where income above roughly $26,050 is taxed at a flat 3.5% following the state's 2024 tax reform. Combined marginal rate: 25.5%.

  • CD/HYSA APY: 4.50%
  • Post-tax APY: 4.50% × (1 − 0.255) = 3.35%

That 3.35% is the number that actually compounds their money — not the 4.50% advertised on the bank's homepage. And it's happening against a backdrop where the Bureau of Labor Statistics reported headline CPI up just +0.1% for July 2026 and unemployment holding at 4.1% in August 2026, with payrolls up 162,000 and average hourly earnings up only $0.10. Soft inflation readings like that are exactly what gives the Fed room to keep easing — which means the 4.50%-and-up CD rates many parents locked in over the past couple of years are already drifting lower as new certificates get issued. The after-tax return on cash is getting squeezed from both directions: taxes take a fixed bite, and the pre-tax rate itself isn't holding steady.

A 529 plan invested in a typical age-based growth allocation doesn't dodge market risk, but it does dodge the annual tax bite — and historically has targeted long-run average returns closer to 7% for equity-heavy allocations, with zero federal tax on qualified withdrawals. This is the kind of analysis Nelovanti runs for you automatically, factoring in your actual bracket, state, and allocation — so you're not eyeballing which side of the gap you're on.

Worked Example: $12,000/Year, Two Kids, Two Horizons

The Ramirezes contribute $12,000/year total toward college — $500/month per kid, split across two accounts. Their older child (age 7) has 11 years until college; the younger (age 4) has 14 years. Multi-child coordination matters here because the same annual contribution produces very different outcomes depending on which kid's clock it's running against.

CD/HYSA, post-tax (3.35%)529 plan, pre-tax (7%) + OH deductionGap
11-year horizon (older child)$156,516$191,616$35,100
14-year horizon (younger child)$210,060$273,782$63,722
Combined, both kids$366,576$465,398$98,822

That 529 column includes Ohio's CollegeAdvantage deduction of $4,000 per beneficiary per year, which at a 3.5% state marginal rate saves the Ramirezes about $140/year per kid in state tax — money assumed reinvested at the same 7% growth rate. On its own that's a modest boost. Combined with tax-free compounding over more than a decade, it's the difference between a $366,576 outcome and a $465,398 outcome on the exact same $12,000/year contribution.

If you want to see this same math applied to expense-ratio drag and state deduction stacking in more depth, 529 Plan True Cost walks through a related two-child scenario where a 0.77% fee gap alone drains $45,000.

Why the 529 Number Isn't Guaranteed — and the CD Number Basically Is

This is the honest trade-off, and it matters more than the headline gap: the CD/HYSA path is close to a locked-in outcome. Barring a bank failure below FDIC limits, the Ramirezes will get their 3.35% post-tax return, full stop. The 529's 7% is a long-run average, not a promise — a bad market stretch in years 9-11 (right when the older child needs the money) could leave the account well below $191,616, or even below the CD path in a genuinely bad sequence-of-returns scenario.

There's also a penalty asymmetry worth knowing cold: money in a CD can be spent on anything, no questions asked. Money withdrawn from a 529 for a non-qualified expense gets hit with ordinary income tax on the earnings portion plus a 10% federal penalty. If there's real uncertainty about whether either kid will need — or want — a traditional four-year degree, that illiquidity is a genuine cost, not a footnote. Nobody should read a $98,822 projected gap and treat it as a guaranteed outcome; it's the expected-value case under stated assumptions, and the further out the horizon, the wider the range of actual results gets in both directions.

The Savings Rate Question: How Much Should Even Go to College at All

Before optimizing where the college money goes, it's worth checking whether the amount itself makes sense. NerdWallet defines savings rate as the percentage of income set aside for savings across all goals — retirement, emergency fund, and college combined, not just one bucket in isolation. At $145,000 household income, the Ramirezes' $12,000/year college contribution alone is about 8.3% of gross income. If they're also hitting 15% into retirement and building an emergency fund, the total savings rate could be pushing 25-30% — worth confirming that's sustainable before locking in an 11- or 14-year commitment.

This is also where a lot of families stall out entirely. NerdWallet's research on financial planning confidence found that a large share of Americans don't feel confident building a financial plan at all — which is a big part of why "just keep it in the CD, it's simple" wins by default even when the math says otherwise. A formula you can actually run beats a feeling you can't verify. You can model this for your specific bracket, state, and contribution amount at Nelovanti rather than guessing at the combined rate or approximating the deduction.

If the contribution-versus-other-priorities question is the one keeping you up at night, 529 Contribution Calculator: The 50/30/20 Rule walks through exactly how to split a fixed savings rate across two kids.

Multi-Child Coordination: Why the Same Formula Gives Different Answers Per Kid

Notice in the table above that the gap in dollars nearly doubles between the 11-year and 14-year horizons on identical $12,000/year contributions. That's compounding doing what compounding does — the extra three years give the 7% side more room to separate from the 3.35% side. This is the core reason multi-child portfolios need per-kid math, not a single blended number: a family with a 4-year-old and a 16-year-old isn't running one calculation, they're running two very different ones, and the older child's account probably needs a more conservative glide path regardless of which vehicle you choose, simply because there's less time to recover from a bad sequence.

For a deeper look at how state plan selection interacts with this timing question across siblings, Home State 529 vs. Utah My529 vs. High-Fee Plans is a useful companion read, and if the state deduction piece specifically is unclear, 529 Plan State Tax Deductions breaks down how much that's commonly worth on its own.

Your 5-Step Calculation

To run this for your own household:

  1. Find your combined marginal rate. Federal bracket + state marginal rate (check if your state even taxes income — nine don't).
  2. Compute post-tax APY. Your CD/HYSA rate × (1 − combined marginal rate).
  3. Estimate your 529's expected return based on your actual investment allocation (age-based, static equity, conservative) — not a generic 7% unless that matches your glide path.
  4. Add your state's 529 deduction value, if any, as an annual reinvested amount at your expected growth rate.
  5. Run the annuity formula separately for each child's actual horizon — FV = PMT × ((1+r)^n − 1) / r — because a shared contribution schedule still produces different dollar outcomes per kid.

The Ramirezes' $98,822 combined gap is specific to their bracket, their state, their contribution amount, and their kids' ages. Change any one input — a higher tax bracket, a no-income-tax state, a more conservative 529 allocation — and the number moves, sometimes by tens of thousands of dollars. That's exactly why this isn't a one-size-fits-all answer: it's a formula that needs your actual numbers plugged in, not someone else's example treated as a rule of thumb.

You can run your household's specific version of this calculation — combined tax rate, state deduction, per-child horizon, and allocation — at Nelovanti instead of building the spreadsheet from scratch.

Sources

Ready to optimize your 529 plan?

Optimize Your 529 Plan Free