Compound Interest Accounts: Where High-Income Families Keep the Most Growth

By Kevin Luu, Co-Founder and Chief Learning Officer. Reviewed on 2026-09-30.

A compound interest account is any account that adds earned interest or investment returns back to the balance, so next year’s growth is calculated on a larger number. High-yield savings accounts, Treasury bills, CDs, 401(k)s, Roth accounts, HSAs, 529 plans, custodial Roth IRAs and UTMA accounts all compound. For a high-income family, the useful question is not which account compounds, but which one lets you keep the most of that compounding after tax.

At a $900,000 household income in California, roughly half of every dollar of savings-account interest can go to federal and state income tax. That changes the ranking. This guide sorts the main compound interest accounts into three groups (cash, parents’ tax-advantaged accounts, and kids’ accounts), shows the after-tax math, and ends with the decisions a two-income W-2 family in California or Washington usually has to make.

Why the tax wrapper matters more than the interest rate at your income

Compounding works on what stays in the account. If tax is taken out of the growth every year, the account compounds on a smaller base. For most families earning $750,000 to $3 million, three taxes hit ordinary interest at the same time for 2026:

• Federal income tax at 35% or 37%. The 37% bracket starts at $768,700 of taxable income for married couples filing jointly in 2026 (IRS Rev. Proc. 2025-32).
• Net Investment Income Tax of 3.8% once modified AGI is above $250,000 for joint filers (IRS).
• California income tax of 9.3% to 13.3% on the 2025 schedules, the latest the FTB has published (13.3% applies only to taxable income over $1,000,000). California has no lower rate for investment income (FTB), and with the federal SALT deduction phased down to $10,000 at this income, state tax on interest is rarely deductible federally.

Washington families skip the state layer in 2026, since Washington has no individual income tax this year. That changes for high earners: Washington has enacted a 9.9% tax on AGI over $1,000,000 starting January 1, 2028 (WA DOR), and it already taxes long-term capital gains at 7% above an annual standard deduction ($278,000 for 2025, the latest published), and at 9.9% on taxable gains above $1,000,000 (WA DOR).

The main compound interest accounts, compared

Limits are for 2026 from IRS Notice 2025-67 (retirement plans and IRAs), Rev. Proc. 2025-19 (HSA) and Rev. Proc. 2025-32 (gift exclusion and kiddie tax).

Worked example: $150,000 of cash for a $900,000 California household

Assumptions (illustrative, for 2026): married couple, both W-2 employees, $900,000 combined salary and RSU income, each deferring $24,500 to a 401(k), standard deduction of $32,200. Federal taxable income is about $818,800, which is in the 37% bracket. California taxable income is assumed at about $845,000, inside the 10.3% bracket ($742,958 to $891,542 on the 2025 schedule, the latest published by the FTB). They hold $150,000 as an emergency and near-term fund. We assume a 4.0% yield on both options to isolate the tax effect. Real rates change often and will differ.

Same balance, same headline rate, $618 a year apart, because interest on U.S. Treasury bills, notes and bonds is exempt from state and local income tax (TreasuryDirect; California subtracts it on Schedule CA, per the FTB instructions). A Treasury money market fund can get similar treatment in California only if the fund holds at least 50% of its assets in U.S. obligations, so check the fund’s annual tax statement. The same family in Washington would keep $3,552 from either option in 2026, so for them the choice comes down to rate and convenience.

The bigger lesson is scale. $618 a year is real, but it is small next to what the tax-advantaged accounts below can do over 15 years. Cash is for safety. Long-term compounding belongs in the wrappers.

Where the long-term compounding happens for W-2 parents

401(k) deferrals and the mega backdoor Roth

For 2026 each spouse can defer $24,500, and the total of employee, employer and after-tax money can reach $72,000 per plan participant. If your plan allows after-tax contributions with in-plan Roth conversion (some large employers do, so check), the space left over is mega backdoor Roth room. Example: $72,000 minus a $24,500 deferral minus a $10,000 employer match (assumed) leaves $37,500 per spouse. At a hypothetical 6% annual return, $37,500 a year for 15 years grows to about $872,849, with no federal tax on qualified withdrawals. That is the clearest case of compounding without annual tax drag available to a salaried family.

Backdoor Roth IRA

Joint filers lose direct Roth IRA eligibility at a modified AGI of $252,000 for 2026, so this family would use a nondeductible traditional IRA contribution of $7,500 each, then convert. It is small, but it is permanent Roth space for each parent, and Roth IRAs have no lifetime required minimum distributions for the owner (IRS RMD FAQs). Pre-tax IRA balances, including old rollover IRAs, can make the conversion partly taxable, so check before you start.

The HSA, with a California caveat

Federally, an HSA is the only account that can be deductible going in, tax-free while invested and tax-free coming out for medical costs. California does not conform. It does not allow the deduction, treats employer HSA contributions as taxable wages, and taxes HSA interest and earnings every year as they are earned (FTB Schedule CA instructions). For a California family it still saves federal tax, but invested HSA money needs state-tax record keeping. For a Washington family it works as designed.

Compound interest accounts for your kids

For children under 12, time is the biggest input. A hypothetical illustration at 6% a year: $95,000 placed in a 529 at birth (the full five-year gift tax election for one parent in 2026) grows to about $271,162 by age 18. The same $95,000 started at age 6 grows to about $191,159. Six years of delay costs about $80,000 of growth before any new money goes in. A married couple splitting gifts can elect up to $190,000 per child, which requires filing Form 709 (Form 709 instructions).

• 529 plan: earnings grow tax-deferred and qualified education withdrawals are tax-free federally and in California. California gives no deduction for contributions in 2026. See our comparison of superfunding versus regular 529 contributions.
• Custodial Roth IRA: only possible when the child has taxable compensation. Allowance and gifts do not count (IRS Pub 590-A). The limit is the lesser of $7,500 or what the child earned in 2026.
• UTMA account: flexible, but taxed. For 2026, for a dependent child under 18, the first $1,350 of unearned income is tax-free, the next $1,350 is taxed at the child’s rate, and anything over $2,700 is taxed at the parents’ rate. In California the child takes control at 18 by default, or as late as 21 or 25 if the transfer documents say so (Cal. Probate Code §3920.5). In Washington, a gift to a UTMA ends at 21 by default and can be extended to 25 when the custodian is first named (RCW 11.114.200). Our guide to UTMA gift rules covers the details, and 529 plans versus custodial accounts walks through the trade-off.

The decisions a family at this income actually makes

1. How much cash, and where. Pick a number (often three to six months of spending). Many California families keep the near-term part in a savings account and the rest in Treasury bills.
2. Whether both spouses are filling their 401(k) and mega backdoor Roth space. Check each plan document for after-tax contributions and automatic in-plan conversion.
3. Whether to front-load the 529s. Superfunding early uses time; spreading gifts keeps the annual exclusion free for other gifts.
4. How much, if anything, goes into a UTMA. Money given to a UTMA belongs to the child, permanently.

Common mistakes

• Chasing the highest headline savings rate while leaving 401(k) and mega backdoor Roth space unused (up to $72,000 of total additions per spouse, per plan, for 2026).
• Holding large balances in a fully taxable savings account in California when Treasury bills would keep more after tax.
• Assuming the HSA is tax-free in California.
• Opening a custodial Roth IRA for a child with no earned income.

What to write down for your kids

Your children will see the balances one day. What they usually will not see is why the cash sat in Treasury bills, why you filled the Roth before buying a bigger house, or why the 529 was funded at birth. A one-page note on each of those choices is worth more to them than the extra growth. As Kevin Luu puts it: “Conversations fade. Documentation compounds.” That record is what the FamilyKnowledge® System is built around, and it is part of Age Five Wealth Education.

Frequently asked questions

Is a high-yield savings account a good place for long-term savings?

For money you need within a year, yes. For long-term goals at a $750,000-plus income, the interest is taxed at roughly 48% to 54% combined in California (2026 federal rates plus 2025 California rates, the latest published) and about 39% to 41% in Washington in 2026, so a savings account keeps roughly 46% to 61% of its headline rate. Long-term money usually compounds better in a 401(k), Roth or 529, invested for the time horizon.

Which accounts compound tax-free?

Roth 401(k) and Roth IRA balances, including mega backdoor and custodial Roth money, grow free of federal tax if withdrawals are qualified. 529 earnings are tax-free federally and in California when used for qualified education costs. An HSA is tax-free federally for medical costs, but California taxes HSA earnings each year.

Do Treasury bills really avoid California tax?

Yes. Interest on U.S. Treasury bills, notes and bonds is subject to federal income tax but exempt from state and local income tax, and California subtracts it on Schedule CA. Funds qualify for California only if at least 50% of their assets are U.S. obligations, so check the fund’s year-end statement for the exempt share.

Next step

List every account your family holds, its balance, and how its growth is taxed in your state, then compare it with the table above. If you want a second set of eyes, our tax management and retirement and employee benefits teams can review it as part of a Tax & Wealth Assessment. This is general education, and the right move depends on your family’s full picture.

Written by

Kevin Luu

Co-Founder and Chief Learning Officer, Bright Advisers

Kevin has advised high-income W-2 tech and biotech families since 2015, and leads the education-first Age Five Family Office from Brea, California.

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