An after-tax 401(k) calculator projects the future balance of after-tax 401(k) contributions, which are deposits made from pay after income tax. It asks for pay, contribution amounts, employer match, years to retirement, and expected return, then reports employee, match, and after-tax buckets separately.
Use it to test whether extra after-tax savings fit within plan limits. It helps maxed-out savers compare choices. Treat the result as a planning estimate, not a tax filing figure.
Table of Contents
- What do you need to enter?
- How do 2026 limits shape after-tax room?
- What return and fee assumptions should you use?
- How should you read the results?
- Who gains most, and what should you check?
What do you need to enter?
Start with your current balance, salary, and contribution rates for pre-tax, Roth, and after-tax. Add the full employer-match formula, including the match rate and pay cap. The calculator turns those figures into yearly additions.
Enter years to retirement and the expected annual return with compounding frequency. Those entries drive compound growth in the model. Keep the return steady across scenarios so comparisons stay fair.
- current balance
- salary and contribution rates
- employer-match formula
- years to retirement
- expected return and compounding frequency
How do 2026 limits shape after-tax room?
The IRS caps 2026 employee elective deferrals at $24,500, according to the IRS in the 2026 limit announcement. The tool must cap pre-tax plus Roth entries separately from match and after-tax. Check this cap first. The IRS sets the 2026 annual-additions limit at $72,000, covering deferrals, match, after-tax, and forfeitures, as shown in the IRS COLA table.
Employer match does not count toward the $24,500 deferral cap but counts toward the $72,000 total. Use the gap between them to find after-tax headroom. For example, $24,500 in deferrals plus $8,000 in match leaves $39,500 for after-tax and forfeitures. Enter each bucket separately. The split keeps the headroom math clear.
What return and fee assumptions should you use?
Fees and fund expenses lower net compounding, so enter a return net of fees, according to Investor.gov in the SEC guide to investment fees. Ask your plan for the all-in expense ratio. Small fee gaps compound into large balance gaps.
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Add an inflation view when the tool offers one. It shows future dollars in present buying power. That view keeps large ending balances in context.
How should you read the results?
Read employee deferrals, employer match, and after-tax balances as separate lines. Each bucket has different tax treatment and limits. A single total hides over-contributions. The IRS treats after-tax basis as recovered tax-free, while earnings on that subaccount face ordinary income tax at distribution.
Compare after-tax principal and earnings growth separately. That split matters for Roth conversion planning. Check projected balance against contribution headroom and timeline. Raise the after-tax rate only while headroom remains. Lower the return or add fees to stress-test the plan.
Who gains most, and what should you check?
IRS guidance notes these projections are hypothetical and plan-dependent. After-tax contributions help mainly savers who already maximize deferrals and match. Ask benefits whether the plan allows after-tax contributions and related Roth moves. IRS guidance allows one distribution to send after-tax basis to a Roth IRA while pretax earnings go to a traditional IRA or plan.
That split supports tax-efficient mega-backdoor Roth conversions. Confirm that your plan permits the split and any in-plan conversion steps. Actual taxes, eligibility, and returns vary by plan and year. Get the plan's after-tax, match, and conversion rules in writing. Model again after any rule change.
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