Methodology & data sources
Last reviewed August 2026 — reflects 2026 IRS figures.
Every calculator here shows its work. This page lays out the exact formula behind each tool and the primary source for every number we use, so you can check the math and trust the inputs.
How each calculator works
The tools are deliberately transparent. Each one is a short, explicit calculation you could reproduce in a spreadsheet — no black boxes, no hidden assumptions beyond the inputs you choose.
Retirement projection (main calculator)
A month-by-month simulation from your current age to retirement. Each month: new balance = old balance × (1 + annual return ÷ 12) + monthly contribution + monthly employer match. Salary grows once a year by your chosen raise, employee deferrals are capped at the IRS limit for your age, the combined employee-plus-employer total is capped at $72,000, and estimated monthly income uses the 4% rule.
Roth vs Traditional
Compared on an equal take-home-cost basis. A Roth contribution C is after-tax; the Traditional contribution that costs the same paycheck is C ÷ (1 − taxnow). Both grow at your return; the Roth is withdrawn tax-free, the Traditional is taxed at your retirement rate. The Roth’s after-tax advantage works out to (1 − taxnow) ÷ (1 − taxret) — so the two are equal when the rates match.
Early withdrawal
Net kept = withdrawal − 10% penalty (if under 59½ and no exception) − federal tax − state tax, using the marginal rates you enter. The 10% penalty is the IRC §72(t) additional tax; the rule-of-55 and other exceptions waive the penalty but never the income tax.
Withdrawal tax
Federal tax on the withdrawal is measured by stacking it on your other income: tax(other income + withdrawal) − tax(other income), using the 2026 brackets and standard deduction for your filing status. This captures the true marginal cost as part of the withdrawal fills higher brackets. State tax is a flat estimate you set; the 10% penalty applies under 59½.
401(k) loan
Standard amortization: payment = P × i ÷ (1 − (1 + i)−n), where i is the monthly rate and n the number of payments. We show total interest (repaid to your own account) and the value the borrowed sum would have reached if left invested. Your borrowing limit follows IRC §72(p): the lesser of $50,000 or 50% of your vested balance.
Required minimum distribution
RMD = prior year-end balance ÷ the distribution period for your age from the IRS Uniform Lifetime Table. RMDs begin at age 73 under SECURE 2.0. The withdrawal percentage is simply 1 ÷ the factor.
Balance by age
Comparison figures are the average and median 401(k) balances by age band from Vanguard’s annual report. The “on track” targets use the widely cited salary-multiple benchmarks (about 1× salary by 30, 3× by 40, 6× by 50, 8× by 60, 10× by 67).
Data sources
Every financial figure in these tools comes from a primary source — the IRS, the Tax Foundation, or Vanguard’s participant data — not from memory. Figures were last verified in August 2026.
| Figure | Value / detail | Source |
|---|---|---|
| 2026 employee deferral limit | $24,500 | IRS |
| Catch-up (50+) / super catch-up (60–63) | $8,000 → $32,500 / $11,250 → $35,750 | IRS (SECURE 2.0) |
| Combined 415(c) cap | $72,000 | IRS |
| 2026 federal tax brackets & standard deduction | 7 brackets; std. deduction $16,100 single / $32,200 MFJ | IRS / Tax Foundation |
| RMD age & Uniform Lifetime Table | Age 73; factor 26.5 at 73 | IRS Pub. 590-B |
| 401(k) loan limits | Lesser of $50,000 or 50% vested; 5-year term | IRC §72(p) |
| Early-withdrawal penalty | 10% before age 59½ | IRC §72(t) |
| Average / median balance by age | By age band, 4.6M accounts | Vanguard, How America Saves 2026 |
| Retirement-income rule of thumb | 4% rule | Bengen (1994) |
Primary references: IRS — 2026 contribution limits, Tax Foundation — 2026 tax brackets, IRS — RMDs / Uniform Lifetime Table, IRS — plan loans (§72(p)), and Vanguard’s How America Saves report.
How often we update
Tax and retirement figures change every year. We refresh the calculators whenever the IRS publishes new numbers — contribution limits and tax brackets each autumn for the following year — and note the change here so you always know which year’s rules are in effect.
Changelog
- August 2026 — Launched with 2026 IRS contribution limits and federal tax brackets, the IRS Uniform Lifetime Table, IRC §72(p) loan limits, and Vanguard’s How America Saves 2026 balances. Six calculators plus the main projection.
Editorial standards
This site exists to make 401(k) math clear and honest. Content is for education only — it is not investment, tax, or legal advice, and we don’t sell products or take a cut of your decisions. Every figure is tied to a primary source, and the formulas are shown so you can verify them yourself.
Where a rule has nuance we can’t fully model — progressive state taxes, plan-specific loan terms, penalty exceptions with their own limits — we say so plainly and point you to the primary source rather than paper over it. For decisions about your own money, consider talking to a qualified tax or financial professional.
Questions, corrections, or a figure that looks off? Email hello@simple401kcalculator.com — we take corrections seriously and update promptly.
Written and maintained by the Simple 401k Calculator editorial team, checked against the primary sources listed above. Last reviewed August 2026.