529 Contribution Calculator: The $111,661 Gap Between a 7% 529 and a 4% Savings Account for Two Kids
The Question Every Parent With a Barclays or Amex Tab Open Is Asking
Here's a scenario that's playing out in a lot of browser tabs right now: a parent has NerdWallet's Barclays savings rate comparison open in one tab and the American Express savings rate comparison open in another, trying to decide whether it's even worth bothering with a 529 plan. Both accounts are FDIC-insured, both are liquid, and both pay a "good" rate — so why lock money into a 529 with investment risk and spending restrictions?
It's a fair question. But it's the wrong comparison unless you run the actual numbers for your kids, your timeline, and your tax bracket. Below is the formula, a worked example for two kids, and the math on where a savings-account strategy quietly falls short.
The 529 Contribution Formula, Step by Step
The calculation has three parts, and it's the same one behind our 529 Plan Formula breakdown:
Step 1 — Project the future cost. Future Cost = Current 4-Year Cost × (1 + tuition inflation rate)ⁿ, where n = years until enrollment.
Step 2 — Solve for the required monthly contribution. Monthly Payment = Future Cost × r ÷ ((1 + r)ᵐ − 1), where r = monthly return rate and m = number of months until enrollment.
Step 3 — Repeat per child, then sum. Each kid has a different n and m, so each kid needs its own line in the spreadsheet before you add them together. This is the step most parents skip — they average the two kids' timelines instead of calculating each one separately, which understates what the younger child actually needs.
Worked Example: Two Kids, 16 and 13 Years to Go
Here's an example — not a universal answer, just a labeled scenario to show how the formula behaves. Assume a current 4-year total cost (tuition, fees, room and board) of $100,000, tuition inflation of 5% a year, and a 529 blended growth rate of 7% a year (a typical age-based equity/bond glide path).
| Kid A (age 2, 16 yrs out) | Kid B (age 5, 13 yrs out) | |
|---|---|---|
| Future 4-year cost at 5% inflation | $218,288 | $188,565 |
| Required monthly 529 contribution at 7% | $620 | $744 |
| Combined monthly target | $1,364/month |
That $1,364/month combined figure is the number a lot of two-kid households are actually staring down right now. Your inputs — current cost basis, inflation assumption, kids' ages, expected return — will move this number a lot. This is exactly the kind of side-by-side calculation Nelovanti runs for your specific family, so you're not rebuilding this spreadsheet from scratch every time your assumptions change.
Where the $111,661 Gap Comes From
Now here's the part that matters for the Barclays-vs-Amex question. Say instead of a 529, this family puts the same effort into a high-yield taxable savings account, using an illustrative rate of 4.00% APY (roughly in the range NerdWallet describes for both banks) and a 24% federal marginal tax bracket. After-tax yield drops to about 3.04% — taxable interest is the hidden cost round-number calculators ignore.
Run the same $620 and $744 monthly contributions through a 3.04% after-tax rate instead of 7%, and here's what happens by the time each kid enrolls:
| Kid A target | Kid B target | |
|---|---|---|
| Future cost needed | $218,288 | $188,565 |
| Value of same contribution in savings account | $153,034 | $142,158 |
| Shortfall | $65,254 | $46,407 |
| Combined shortfall | $111,661 |
That's the gap: $111,661 across two kids if you calibrate your monthly contribution to a 529's growth assumptions but actually park the money in a taxable savings account instead. Alternatively, to hit the same targets purely through a savings account, you'd need to contribute $1,871/month combined instead of $1,364 — about $507 more every month, indefinitely, just to offset the lower after-tax return.
This lines up with the pattern in our 529 vs. High-Yield CD Calculator breakdown and the 529 vs. Taxable CD analysis — different products, same mechanism: taxable interest and a lower nominal rate compound against you exactly when compounding is supposed to be working for you.
Why Barclays and Amex Don't Close the Gap on Their Own
Two details from the NerdWallet comparisons matter here, and they cut in opposite directions for different families:
Barclays' top rate has a floor most families won't clear. NerdWallet notes Barclays' highest advertised rate is reserved for balances over $250,000. A family actively saving toward two kids' college funds is, almost by definition, not sitting on a quarter-million dollars in cash — they're building toward a target, not starting from one. That means most 529-alternative savers land in a lower tier than the headline rate suggests, which widens the $111,661 gap rather than closing it.
Amex is solid, but NerdWallet is explicit that it's not the highest rate available. That's a reasonable trade if you value Amex's app and customer service, but "good, not highest" on a taxable account compounding for 13-16 years is a meaningfully different number than "good, not highest" on a 529 growing tax-free. The rate gap and the tax gap stack on top of each other.
Neither bank is doing anything wrong here — these are genuinely competitive savings accounts. The problem is using a savings account to do a 529's job over a multi-year horizon. This is where running your own numbers at Nelovanti matters more than comparing headline APYs, because the real comparison is after-tax yield over your actual timeline, not the number in the ad.
What September 2026's Economic Data Means for Your Assumptions
The BLS's latest indicators give useful context for the inflation and income assumptions in this formula. Headline CPI rose just +0.1% in July 2026, and the unemployment rate sat at 4.1% in August 2026, with payroll employment up +162,000 and average hourly earnings up +$0.10. That's a benign, stable labor market print — good news for your ability to sustain a $1,364/month contribution without income disruption.
But don't let a 0.1% monthly CPI print lull you into using a low inflation assumption for tuition. College costs have historically outpaced headline CPI by roughly double, which is exactly why the worked example above uses 5% tuition inflation, not 1.2% annualized CPI. Our 2026 inflation and 529 savings target analysis walks through why using CPI as your tuition-inflation proxy is one of the most common formula errors parents make.
There's also a budget-competition angle. Mortgage rates ticked up this week — NerdWallet's September 9 mortgage rate update points to markets reacting to geopolitical tension pushing rates higher. If you're weighing an extra mortgage principal payment against that $1,364/month 529 target, the math shifts with every rate move. We've run that exact trade-off in 529 vs. Extra Mortgage Payments, and it's worth checking where current rates put you before you split the difference.
Multi-Child Portfolio Coordination: Adjusting the Formula
The formula above treats each kid independently, which is correct — but coordination still matters in two ways:
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Glide path staggering. Kid A, 16 years out, can carry more equity exposure early on than Kid B at 13 years out. If both accounts use the same age-based portfolio at the plan level, that's usually handled automatically, but if you're building a custom allocation, weight the younger-timeline kid's account more conservatively sooner.
-
Contribution rebalancing as college approaches. As Kid B gets closer to enrollment, that account should shift toward stability while Kid A's account still has room to grow. Recalculating both PMT figures annually — not just setting them once — catches drift from actual market returns versus your 7% assumption.
When the Savings Account Actually Wins
To be fair to Barclays and Amex: a high-yield savings account isn't the wrong tool for every dollar. It's the right tool for money you'll need within 2-3 years (private K-12 tuition, a gap-year cost, an upcoming semester's expenses), for emergency-fund dollars you don't want tied to market risk, or for a family that values zero withdrawal restrictions over tax-advantaged growth. A 529's tax benefits and higher expected return only pay off over a long enough horizon to overcome the volatility and the 10% penalty risk on non-qualified withdrawals.
If your oldest is a junior in high school, the calculus above inverts — short timelines favor the safety of FDIC insurance over 7% return assumptions that might not have time to play out.
Run Your Own Numbers
The $111,661 figure above is one family's example, built on a $100,000 starting cost, 5% tuition inflation, a 7% 529 return, and a 24% tax bracket. Change any one of those — your kids' actual ages, your state's tuition trajectory, your real marginal rate, your risk tolerance — and the gap moves with it. That's the entire point: the math should tell you which tool fits your timeline, not a headline APY or a rule of thumb.
You can plug in your own family's numbers — kids' ages, current savings, target school type, and tax bracket — at Nelovanti and see exactly where your monthly contribution target lands, and how much a savings-account-only strategy would actually cost you over your specific timeline.
Sources
- Barclays Savings Interest Rate: How It Compares — NerdWallet
- American Express Savings Rate: How it Compares — NerdWallet
- Major Economic Indicators Latest Numbers — Bureau of Labor Statistics
- Looking Back at the Economic Aftershocks of 9/11 — NerdWallet
- Mortgage Rates Today, Wednesday, September 9: A Little Higher — NerdWallet