What will my Roth 401(k) be worth?

How it works

Year k is this year (k = 0), next year (k = 1), and so on, until the year before your retirement age. Your age in year k is your age + k, and your salary is salary × (1 + raise)^k. An empty raise or balance counts as 0.

The IRS limits are the 2026 amounts, used for every year:

Each year:

1. Your limit = $24,500, plus $11,250 if your age that year is 60 to 63, or plus $8,000 if it is 50 or more (and not 60 to 63).
2. Roth contribution = the lesser of (your % × salary) and your limit.
3. Traditional contribution with the same take-home pay = the least of (Roth contribution ÷ (1 − tax rate now)), your limit, and your salary that year. The salary cap is the IRS rule that contributions can be no more than 100% of pay (the annual additions limit, IRC 415(c)); the $72,000 dollar part of that rule is never reached here, because the deferral limit is lower. When the salary cap cuts the traditional contribution, traditional capped at your pay shows in how many years (for example “1 year”); otherwise it is not shown.

Each year’s contribution is paid in 12 equal amounts at the end of each month. With the yearly return R and the monthly rate g = (1 + R)^(1/12) − 1, one year’s payments are worth (contribution ÷ 12) × s₁₂ at the year’s end, where s₁₂ = R ÷ g (12 at 0%). So each year: balance = balance × (1 + R) + (contribution ÷ 12) × s₁₂.

The results:

• Your Roth 401(k) at retirement starts from today’s balance. Your contributions add up every Roth contribution; investment growth is the balance minus today’s balance and your contributions.
• You put in this year and your IRS limit this year are the year k = 0 amounts.
• For the comparison, both accounts start at $0. Roth, new contributions only is the Roth balance from the contributions alone. Traditional balance before tax is the traditional balance, and after tax is that × (1 − tax rate in retirement).
• Roth comes out ahead by = Roth (new only) − traditional after tax, when that is more than −$0.005 (a gap under half a cent is even and shows as $0); otherwise traditional comes out ahead by traditional after tax − Roth (new only).

If the retirement age is not more than your age, there is no answer.

Rules: your age is 18 to 79 and the retirement age 19 to 80; the salary is more than $0 and at most $100 million; the raise and return are 0% to 20% and 0% to 30%; the percent you put in is 0% to 100%; tax rates are 0% to 60%. Money shows to the cent, with halves rounded up.

Assumptions

• The 2026 IRS limits stay the same in later years. Employer contributions are not included.
• The return is the same every year; fees are not included.
• All withdrawals are qualified, so Roth withdrawals are tax-free and traditional withdrawals are taxed at your retirement rate.
• This is an estimate for planning, not financial advice.

When the data is out of date

The contribution limits are the IRS limits for 2026. After December 31, 2026, the calculator keeps using the 2026 limits until the page is updated, and the result says “Uses 2026 IRS 401(k) contribution limits”. The IRS usually raises the limits each year.

Worked examples by hand

Age 55, $300,000 salary, 20% in. 20% is $60,000, but the limit is 24,500 + 8,000 = $32,500, so you put in $32,500.

Age 61, $200,000 salary, 25% in. The limit is 24,500 + 11,250 = $35,750.

Age 30, $60,000 salary, 10% in, 25% tax now and later, one year at 0%. Roth: $6,000. The same take-home pay buys 6,000 ÷ 0.75 = $8,000 pre-tax; after tax at 25% that is 8,000 × 0.75 = $6,000. The two are even: the Roth is ahead by $0.

Age 40, $250,000 salary, 10% in, 32% tax now and later, one year at 0%. 10% is $25,000, capped at $24,500. The traditional would need 24,500 ÷ 0.68 = $36,029.41, also capped at $24,500; after tax, 24,500 × 0.68 = $16,660. The Roth is ahead by $7,840.

Age 30, $10,000 salary, 100% in, 50% tax now and later, one year at 0%. Roth: $10,000. The same take-home pay would buy 10,000 ÷ 0.5 = $20,000 pre-tax, but that is more than 100% of pay, so the traditional gets $10,000; after tax, $5,000. The Roth is ahead by $5,000, and the traditional is capped at your pay in 1 year.

Age 30 to 40, $10,000 today, $50,000 salary, 12% in ($500 a month), 7% a year. s₁₂ = 0.07 ÷ (1.07^(1/12) − 1) = 12.3803…; each year adds 500 × 12.3803… = $6,190.15 at its end. After 10 years: 10,000 × 1.07¹⁰ + 6,190.15 × (1.07¹⁰ − 1) ÷ 0.07 = $105,197.38, of which you put in $60,000.