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529 vs. Extra Mortgage Payments Above 7%: The $2,512 Gap on $10,000 for a 4-Year-Old (and Why Your Own Mortgage Rate Changes the Answer)

Say you have $10,000 of spare cash on September 28, 2026. Your kid is 4. You have a mortgage. Your inbox is full of headlines saying rates are "a little lower, but still above 7%," and you're asking whether that $10,000 should go into a 529, onto your loan, or into a bank account that dangles a sign-up bonus.

Here's the honest answer, before any spreadsheets: it depends on the rate on your mortgage, not the rate in the headlines. In this post I'll run the numbers on all three options. I'll also show where each one wins, and which inputs decide it.

Every figure below is a worked example I constructed, and I've labeled the assumptions. Your numbers will differ based on your specific situation. The point is to show which variables matter.

Why the 7% headline matters, and why it might not apply to you

NerdWallet's Mortgage Rates Today, Monday, September 28 reports that rates fell today but remain solidly above 7%. Its companion piece, Your Guide to Bargain Hunting With Mortgage Rates Above 7%, suggests thinking like a grocery shopper on a budget: compare options, find savings, and stay flexible.

That advice works well for a 529 decision too. But the 7% rate applies to people taking out a loan today. If you locked a mortgage years ago at 3% or 4%, extra principal payments earn you 3% or 4%, and the comparison looks completely different. If you're buying now at 7%+, every extra dollar of principal is a guaranteed 7%-plus return, and a 529 has to beat that.

That's the first personal variable. The others are your state's tax deduction, your plan's fees, your child's age, and how much market risk you can stomach.

The worked example: $10,000, one 4-year-old, 14 years to go

Assumptions (all labeled as examples):

  • Horizon: 14 years, until the child turns 18
  • Mortgage: 7.0% fixed, with the extra payment going straight to principal
  • 529 gross return: 6.0% per year, an assumption and not a forecast
  • Low-cost plan fee: 0.15% per year, so 5.85% net
  • State income tax deduction: 5% marginal benefit on the $10,000, so a $500 refund that's reinvested in the 529
  • No federal deduction for mortgage interest (you take the standard deduction)

Mortgage paydown. $10,000 at 7% for 14 years grows to 10,000 × 1.07¹⁴ = $25,785. To be precise, that's the value of the interest you avoid, provided you keep making the same payment and the loan runs at least 14 more years.

529 in a low-fee plan. $10,000 at 5.85% net for 14 years grows to about $22,165. The $500 refund invested alongside it grows to about $1,108. That gives a total of $23,273.

The gap: $25,785 − $23,273 = $2,512 in favor of the mortgage.

So at a 7% mortgage, a 6% assumed market return, and a modest state deduction, paying down the loan wins on paper. Now I'll show how quickly that changes.

Table 1: How the mortgage rate you actually have changes the winner

Same $10,000, same 14 years, same 529 result ($23,273 with deduction, $22,165 without):

Your mortgage rateValue of extra principal after 14 years529 (6% gross, 0.15% fee, with 5% deduction)Winner
3%$15,126$23,273529 by $8,147
5%$19,799$23,273529 by $3,474
7%$25,785$23,273Mortgage by $2,512

If you're a 3% borrower who's been staring at 7% headlines and wondering whether to rush extra dollars at your loan, the math says to slow down. This is the kind of comparison Nelovanti runs for you, using your rate and your state's deduction, so you don't have to rebuild the spreadsheet every time a headline moves.

Table 2: The 529 return you need to break even

The mortgage return is guaranteed. The 529 return isn't. So the useful question is what gross return the 529 needs to catch a 7% loan:

ScenarioGross return needed to match a 7% mortgage over 14 years
0.15% fee, with 5% state deductionabout 6.78%
0.15% fee, no state deductionabout 7.15%
0.90% fee, with 5% state deductionabout 7.53%
0.90% fee, no state deductionabout 7.90%

The break-even barely moves with the deduction, but it moves a lot with the fee. A 0.75% fee gap raises the bar by roughly three-quarters of a point every single year. Check the fee before you compare anything else. I covered this in detail in 529 Plan True Cost: 0.77% Expense Ratio Gap, Missing State Deductions, and 2026 Inflation Signals.

To put that in dollars, the same $10,000 in a 0.90% plan at 6% gross ends up near $20,067, plus about $1,003 from the reinvested refund, for $21,070. That's $2,203 less than the low-fee plan, from one account and one lump sum.

Table 3: What if the market doesn't cooperate?

Mr. Money Mustache's Will the AI Bubble Destroy our Retirement? opens with a fair observation: the market keeps surprising us. That includes the times it rises to record levels, when people worry it must be about to fall. The piece is about retirement, but the lesson carries over. A record high tells you where the market has been, not where it goes next.

Here's the same 529 under different market outcomes (0.15% fee, with the $500 refund reinvested), against the 7% mortgage's fixed $25,785:

529 gross return, 14 years529 ending valueVersus 7% mortgage
4%about $17,820Mortgage wins by about $7,965
6%about $23,273Mortgage wins by about $2,512
8%about $30,248529 wins by about $4,463

The upside and downside are lopsided in dollars, and that's what risk looks like. The mortgage gives you one outcome. The 529 gives you a range, and that range needs to be weighed against how much of your kid's tuition depends on it.

There's also a quieter feature the mortgage doesn't have. 529 money can be withdrawn tax-free for qualified education costs, and you can pay tuition from it directly. Extra mortgage principal is locked in your house. If you need cash in year 12, you'd be looking at a home equity loan or a refinance, likely at a rate near the one this article opened with.

The bank bonus: small money, real math

NerdWallet's Should I Switch to a New Bank Just to Earn a Bonus? makes the point that bonuses usually take effort to earn. That's the right frame. Here's a hypothetical to price it out:

  • A $300 bonus that requires keeping $5,000 in the account for 60 days, plus a qualifying direct deposit
  • That $5,000 would otherwise have earned 4% in a savings account: 5,000 × 4% × 60/365 = about $33 of forgone interest
  • The bonus is taxable interest income; at a 22% federal rate, that's $66 of tax
  • Net after tax and forgone interest: $300 − $33 − $66 = about $201
  • Time spent on setup, tracking, and closing: say 3 hours, or about $67 an hour

That's decent hourly pay for a one-time task. But the amounts are small next to the choices above. If that $201 goes into the 529 at the same 5.85% net for 14 years, it becomes about $446. Compare that with the $2,512 gap in the main example. The bank bonus is a fine bonus, but it shouldn't drive the decision. If you do chase one, your future self might appreciate the proceeds landing in a 529 and not disappearing into the checking account. For a larger cash amount, I ran a similar comparison in Bank Bonus vs. Higher-Rate Savings vs. 529: The Break-Even Math on $10,000.

The two-kid problem: same $10,000, different clocks

Now suppose you have two kids, ages 4 and 8, and you're deciding how to divide the money. The child's age changes how long each dollar has to compound.

Take the 8-year-old with 10 years to go. Using the same assumptions:

  • 529 (5.85% net): $10,000 grows to about $17,657. The $500 refund adds about $883, for $18,540.
  • 7% mortgage for 10 years: 1.07¹⁰ × $10,000 = $19,672

The mortgage lead is $1,132 for the older child, compared with $2,512 for the younger one. The gap shrinks because time is what lets the 529 catch up, and the older child has less of it.

But the age of the child also changes the risk. A 4-year-old's 529 can weather a market drop and recover. For an 8-year-old with 10 years left, a 4% year hurts more, and the plan's allocation matters more. If that's your worry, my post on whether to move a 529 to conservative after a record stock run walks through that trade-off with real numbers.

For multi-child families, the question isn't which option wins overall. It's which dollars go where. Some families put the extra cash on the mortgage because a guaranteed 7% is hard to beat, and keep regular monthly 529 contributions going for the younger child, where the deduction and time work harder. Others do the reverse. Neither is wrong. The right split depends on your mortgage rate, both kids' ages, and each account's fees.

Six questions that decide your version of this comparison

Before moving any money, get answers to these:

  1. What rate is my mortgage, exactly? Below roughly 5%, the 529 generally wins the comparison above. Near or above 7%, paydown is a serious competitor.
  2. What does my state give me for contributing? If your state offers a deduction and it applies only to its own plan, the break-even shifts. See my in-state vs. out-of-state 529 checklist.
  3. What is the plan's all-in fee? Under 0.20% and over 0.80% can flip the answer.
  4. How many years until each child starts college? Fewer years means less compounding and more risk.
  5. How much liquidity do I need? Home equity is hard to get back. A 529 is easier to spend on tuition, though you can't use it for other emergencies without a penalty.
  6. What else is competing for this cash? Emergency funds and high-interest debt usually come first. I break down the fuller mortgage-versus-529 tradeoff in 529 Contributions vs. Extra Mortgage Payments: A 6-Question Framework.

What this means for you

Here's what the example does and doesn't show:

  • At a 7% mortgage, a low-fee 529 needs roughly 6.8% to 7.2% gross to match paydown over 14 years. That's an optimistic return to lean on, which is why the mortgage has a real case.
  • At a 3% or 5% mortgage, the same 529 wins comfortably.
  • A 0.75% fee gap can cost more than a state deduction earns you.
  • A bank bonus is worth doing only if the effort is cheap. It won't move the total.
  • Older kids narrow the gap on paper but raise the risk if the money is in stocks.

None of this is a case for one choice. It's a case for running the numbers with your mortgage rate, your state, your plan's fees, and your children's ages. A couple of points of return, or one fee tier, can change the winner. That's why rules of thumb break down, and why the grocery-shopper mindset in the NerdWallet piece fits. Compare, find the savings, and stay flexible.

If you'd like to see this with your own inputs, Nelovanti lets you compare 529 plans, contribution amounts, and your mortgage rate side by side for each child. You can also model different market returns and see the break-even for your situation. Plug in your numbers, look at the gap, and decide from there. The math should speak for itself.

Sources

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