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529 overlap calculator: when two kids are in college at once

The total cost of educating several children is rarely the problem. The problem is the years two of them are enrolled together, when the bill doubles and your income does not. Enter birth years to find those years and what they cost.

Birth years
Costs and savings
Total cost across all children
$294,345
with 1 overlap year, peaking at $73,389 in a single year
Savings cover 35% of the total, leaving $189,952 from cash flow or loans
YearEnrolledCostFrom 529Gap
20291$32,621$32,621—
20301$33,926$33,926—
20311$35,283$35,283—
20322$73,389$2,563$70,825
20331$38,162$0$38,162
20341$39,689$0$39,689
20351$41,276$0$41,276
Highlighted rows are years with more than one child enrolled. Savings are projected to 2029 ($98,181) and drawn down from there.

The rule that creates the problem

A 529 distribution counts as qualified only for the account's named beneficiary at the moment you withdraw. That is federal law, not a quirk of any particular state plan. While an account names your oldest child, it cannot pay your middle child's tuition in the same year, and switching the beneficiary mid-year simply moves the problem rather than solving it.

For children spaced far apart it never comes up: one finishes, you change the beneficiary, the next starts. For children born within a few years of each other, there are years where two are enrolled at once, and a single account can only ever cover one of them. The other tuition comes from cash flow, savings, or loans.

This is the argument for an account per child, and it is the part most college calculators leave out, because they model a total rather than a schedule. We wrote up the full version, including the strategy that looked clever and was not, in 529 plans when multiple kids overlap in college.

Why the peak year matters more than the total

Families usually plan against the total and are surprised by the peak. Two children enrolled together can produce a single year costing more than most households earn after tax, and it tends to land in what should be peak saving years, just as retirement contributions matter most. That collision is the real planning problem, and it is why the schedule above is more useful than a single number.

What this calculator assumes

Four years of enrolment starting at 18, a cost that grows at the rate you set, and a combined pot of savings drawn down in year order. It does not model financial aid, scholarships, state tax deductions, or the beneficiary constraint itself: the savings column is deliberately shown as one pool so you can see the shape of the problem before deciding how to split accounts.

For the version that respects per-child accounts, links each plan to a beneficiary, and runs the whole thing against your retirement projection at the same time, the planbend app models it properly. If you only need a single-child estimate, the 529 college savings calculator is simpler.

Common questions

Can one 529 account pay for two children at the same time?
No. A 529 distribution is a qualified expense only for the account's named beneficiary at the time of withdrawal. You can change the beneficiary, but while the account names one child it cannot pay another child's tuition in that same year. For families with overlapping enrolment this is the constraint that decides how many accounts you need.
What are overlap years and why do they matter?
An overlap year is any year two or more of your children are enrolled at once. The household bill roughly doubles in those years while income usually does not, which is what turns a manageable total into a cash-flow problem. Children spaced three years apart typically produce one or two overlap years; children spaced one or two years apart produce more.
Should I open one 529 per child or one shared account?
One per child avoids the beneficiary problem entirely during overlap years, which is the main argument for it. A single account is simpler and can be moved between children as each finishes, which works well when they are spaced far enough apart that enrolment never overlaps. State tax deductions sometimes differ by account rather than by taxpayer, so check how your own state counts them before deciding.
What college cost inflation rate should I use?
Published college costs have risen faster than general inflation for a long time, and 4% to 5% is a common planning assumption. The default here is 4%. Using general inflation will understate the bill for a child who is still young, because the gap compounds over the whole period.
Is leftover 529 money wasted?
No. You can change the beneficiary to another qualifying family member, hold it for a future grandchild, or, subject to rules and limits introduced by SECURE 2.0, roll a portion to a Roth IRA for the beneficiary. Non-qualified withdrawals are possible but the earnings portion is taxed and penalised, so they are a last resort rather than a plan.
planbend is a planning tool, not financial, tax, or legal advice. 529 rules change and vary by state; confirm your own plan's terms before acting.