After-Tax 401(k) Calculator

Calculate your maximum possible 401(k) savings and uncover your potential for a mega backdoor Roth. This calculator determines how much "after-tax" contribution space is available in your plan after accounting for your own pre-tax/Roth deferrals and your employer's match. It shows you the exact dollar amount you can contribute beyond the standard employee limit to supercharge your retirement savings.

This tool is designed for high-income earners who are already maximizing their standard 401(k) contributions and are looking for additional tax-advantaged savings strategies. If you're phased out of direct Roth IRA contributions, this calculator reveals a powerful alternative. It's also useful for anyone wanting to understand the full mechanics of the annual 401(k) contribution limits.

The results provide a clear breakdown of your 401(k) utilization, including your employee deferral, employer match, and available after-tax space. You'll see your total annual contributions, a projection of your 401(k)'s growth until retirement, and a donut chart visualizing how each contribution source adds up to the total IRS limit.

Begin by entering your financial details in the "Income & Contributions" section. Your annual salary and the percentage you contribute pre-tax or to a Roth 401(k) are the starting points for the calculation. This determines your standard employee deferral.

Next, provide your "Employer Match" details. Enter the percentage your employer matches and the salary limit for that match. For example, a common match is 100% of the first 4% of your salary. The calculator uses this to determine how much of the total annual 401(k) limit is used by your employer's contribution.

Then, in "Personal Details," enter your current age, planned retirement age, and your expected annual return on investments. These inputs power the long-term growth projection, showing how these powerful contributions can compound over time.

Finally, expand the "After-Tax Options" to customize the mega backdoor Roth scenario. You can enter a specific after-tax contribution amount you plan to make, or leave it at zero to see the maximum possible space. Crucially, indicate whether your plan allows for "In-Plan Roth Conversions." Setting this to '1' (for yes) will calculate your true mega backdoor Roth potential, which allows for tax-free growth. For more on conversions, see the Roth conversion calculator.

Annual Salary & Pre-Tax Contribution

Your annual salary is your gross pay before any deductions. The pre-tax contribution is the percentage of that salary you direct to your traditional or Roth 401(k). For 2026, the standard employee limit is $23,500 ($31,000 if you're age 50 or over). The calculator will cap your contribution at this limit, which is the first component that counts toward the total annual limit. Use the 401(k) contribution calculator to optimize this amount.

Employer Match & Match Limit

The employer match is the contribution your company makes to your 401(k). The match percentage and limit define the formula (e.g., "50% match on the first 6% of salary"). This employer money is a key part of your retirement savings and also counts toward the total annual 401(k) limit. Maximizing this match is a critical first step before considering after-tax contributions. If you have a federal plan, our TSP match calculator can provide more specific details.

Current Age, Retirement Age, & Expected Return

Your current and retirement ages determine the timeline for your investment growth. A longer timeline gives your contributions more time to compound. The expected annual return is your estimated average return on your 401(k) investments. A reasonable long-term estimate for a diversified stock portfolio is often between 6% and 8%, but you should adjust this based on your own risk tolerance and asset allocation.

After-Tax Contribution Amount

This is the amount you plan to contribute to your 401(k) with after-tax dollars, separate from your standard pre-tax or Roth 401(k) deferrals. If you enter '0', the calculator will automatically determine the maximum possible after-tax contribution you can make. This is calculated by taking the total annual limit ($70,000 in 2026) and subtracting your employee deferral and employer match.

In-Plan Roth Conversion

This is the most important input for the mega backdoor Roth strategy. Setting this to '1' signifies that your 401(k) plan allows you to immediately convert your after-tax contributions into Roth 401(k) funds. This action ensures that all future investment growth on those contributions is completely tax-free. If your plan does not allow this (set to '0'), the earnings on your after-tax contributions will grow tax-deferred and be taxed as ordinary income upon withdrawal.

This calculator's primary function is to determine the gap between the total 401(k) contribution limit and the sum of your standard contributions. This gap represents your opportunity for after-tax savings.

The methodology follows these steps:

1. Determine Total Limit: It identifies the total IRS limit for all 401(k) contributions (employee, employer, and after-tax) for the current year. For 2026, this limit is $70,000, or $77,500 for those age 50 and over who are eligible for catch-up contributions.
2. Calculate Employee Deferral: It calculates your standard employee contribution based on your salary and contribution percentage, capping it at the 2026 employee deferral limit of $23,500 (or $31,000 with the $7,500 catch-up).
3. Calculate Employer Match: It computes the amount your employer will contribute based on their matching formula.
4. Find After-Tax Space: It subtracts your employee deferral and the employer match from the total annual limit. The result is your maximum available after-tax contribution space.
5. Project Growth: Using your age, retirement age, and expected return, the calculator runs a year-by-year projection. Each year, it adds the total annual contributions (employee + employer + after-tax) to the balance and applies the investment return to show how the account could grow over time.

The calculator does not account for future increases in salary or contribution limits, providing a projection based on the current year's data entered.

The calculations are performed in a sequence to determine your after-tax potential and project future growth.

Contribution Formulas

First, the calculator determines each component of your annual contributions.

// Employee Deferral (capped at IRS limit)
max_deferral = 23500 (or 31000 if age 50+)
employee_deferral = min(annual_salary * pre_tax_contribution_percent, max_deferral)

// Employer Match
employer_match_amount = calculated based on your salary, contribution, and employer's match formula

// Total Annual Limit
total_limit = 70000 (or 77500 if age 50+)

// After-Tax Contribution Space
after_tax_space = max(0, total_limit - employee_deferral - employer_match_amount)

// Total Annual Contributions
total_annual_contributions = employee_deferral + employer_match_amount + after_tax_space

Growth Projection Formula

The projection is a year-by-year loop from your current age to your retirement age.

// For each year until retirement:
investment_growth = current_balance * expected_annual_return
ending_balance = current_balance + total_annual_contributions + investment_growth

This loop repeats for the number of years you have until retirement, with current_balance updating each year to become the next year's starting balance.

The "mega backdoor Roth" is an advanced retirement savings strategy that allows high-income earners to contribute significantly more to a Roth account than standard limits permit. It's made possible by specific features in some 401(k) plans. If your plan allows it, you can contribute up to the total IRS 401(k) limit ($70,000 in 2026) in a combination of pre-tax, employer match, and after-tax dollars.

The strategy works in three steps:

1. Max Out Standard Contributions: First, you contribute the maximum amount to your regular pre-tax or Roth 401(k) ($23,500 in 2026, plus a $7,500 catch-up if you're 50 or older).
2. Make After-Tax Contributions: After maxing out your standard deferrals, you contribute additional money to your 401(k) on an after-tax basis, up to the overall $70,000 limit.
3. Convert to Roth: You immediately convert those after-tax contributions into Roth dollars, either within your 401(k) (an "in-plan conversion") or by rolling them over to a Roth IRA. This step is crucial. It ensures all future earnings on that money are 100% tax-free.

This strategy is different from the standard backdoor Roth IRA, which involves non-deductible IRA contributions. The "mega" version allows for much larger contribution amounts, making it a powerful tool for those who can afford it.

Understanding the different 401(k) limits is key to using this calculator. There are two main limits set by the IRS that you need to know for 2026.

1. Employee Elective Deferral Limit: $23,500 This is the maximum amount you, as an employee, can contribute from your salary to your 401(k) on a pre-tax or Roth basis.

• Catch-Up Contribution (Age 50+): If you are age 50 or older, you can contribute an additional $7,500, bringing your total employee limit to $31,000.
• SECURE 2.0 "Super" Catch-Up (Ages 60-63): Under the SECURE 2.0 Act, individuals aged 60, 61, 62, and 63 can make a higher catch-up contribution of $11,250 (instead of $7,500), bringing their total employee limit to $34,750.

2. Overall Contribution Limit: $70,000 This is the total limit for all contributions made to your 401(k) in a single year. It includes:

• Your employee elective deferrals (pre-tax or Roth).

• Your employer's matching contributions.

• Any employer profit-sharing contributions.

• Your voluntary after-tax contributions.

• Overall Limit with Catch-Up (Age 50+): The $7,500 catch-up contribution increases the total limit, so for those 50 and over, the overall limit is $77,500.

• Overall Limit with Super Catch-Up (Ages 60-63): The $11,250 super catch-up increases the overall limit to $81,250 for those aged 60-63.

The after-tax 401(k) calculator works by finding the difference between the Overall Limit and the sum of your Employee Deferral and Employer Match. This remaining room is your after-tax contribution opportunity.

The mega backdoor Roth strategy isn't for everyone. It's best suited for a specific type of saver, often referred to as a "super saver." You might be a good candidate if you meet these criteria:

• You're a High-Income Earner: Your income is high enough that you are already maxing out your standard retirement accounts.
• You've Maxed Out Other Accounts: You are already contributing the maximum to your 401(k) ($23,500, or $31,000 if 50+) and your IRA or a backdoor Roth IRA.
• You Have Additional Savings Capacity: After maxing out those accounts and covering your living expenses, you still have money you want to invest for retirement.
• Your 401(k) Plan Has the Right Features: This is non-negotiable. Your plan must allow both (1) voluntary after-tax contributions and (2) in-plan Roth conversions or in-service withdrawals to a Roth IRA.

If you don't meet these criteria, it's often better to focus on foundational retirement planning steps, such as building an emergency fund, paying off high-interest debt, or simply ensuring you contribute enough to get your full employer match. Use our general retirement calculator to assess your overall plan.

The calculator provides several key metrics to help you understand your after-tax 401(k) potential.

• 401(k) Utilization Score: This gauge shows what percentage of the total annual 401(k) limit you are using. A score near 100 means you are fully maximizing your plan's potential.
• After-Tax Space Available: This is the core result. It's the maximum amount of money you can contribute on an after-tax basis for the year.
• Total Annual Contributions: This card shows the sum of your employee deferral, your employer's match, and your after-tax contributions. This is the total amount going into your 401(k) each year.
• Projected Balance: This is an estimate of what your 401(k) could be worth at your stated retirement age, assuming you continue making these total contributions each year.
• Mega Backdoor Roth Potential: This shows the amount you can convert to Roth annually for tax-free growth, assuming your plan allows it. If it shows $0, it means your plan features may not support the strategy or there's no available after-tax space.
• 401(k) Growth Projection Chart: This visualizes how your balance grows over time, illustrating the power of compounding on these large annual contributions.
• Annual Contribution Breakdown Chart: This donut chart breaks down your total annual contribution into its three parts: your deferral, your employer's match, and your after-tax portion.

If you want to maximize your use of the after-tax 401(k) strategy, focus on these areas:

1. Confirm Plan Features: The most important step is to contact your 401(k) plan administrator or HR department. Ask them two specific questions: "Does our plan accept voluntary after-tax contributions?" and "Does the plan allow for in-plan Roth conversions of those after-tax funds?" If the answer to both is yes, you're in a great position.
2. Maximize Your Standard Deferral: Before making any after-tax contributions, ensure you are contributing at least enough to get your full employer match, and ideally up to the $23,500 employee limit.
3. Automate After-Tax Contributions: If your payroll system allows, set up your after-tax contributions to be automatic, just like your regular 401(k) deferrals. This ensures you consistently fill the available space throughout the year.
4. Convert Immediately: To get the full tax-free growth benefit of the mega backdoor Roth, convert your after-tax contributions to Roth as soon as your plan allows. If you let them sit in the after-tax sub-account, any investment gains will be taxable upon withdrawal.

Avoid these common pitfalls when exploring the after-tax 401(k) and mega backdoor Roth strategy:

• Confusing "After-Tax" with "Roth": They are not the same. Roth contributions grow tax-free. Standard after-tax contributions grow tax-deferred, meaning the earnings are taxed on withdrawal. The conversion step is what turns after-tax into the more powerful Roth.
• Assuming Your Plan Allows It: Never assume your plan has the necessary features. Many 401(k) plans do not allow after-tax contributions or in-plan conversions. Always verify first.
• Forgetting to Convert: Making after-tax contributions without converting them to Roth negates much of the benefit. The earnings will be taxable, which is less favorable than a standard taxable brokerage account where gains are taxed at lower capital gains rates.
• Exceeding the Overall Limit: Be careful not to contribute more than the total annual limit ($70,000 in 2026) across all sources. Your plan administrator should prevent this, but it's good to be aware of the ceiling.
• Ignoring Other Financial Goals: Don't divert funds from essential goals like emergency savings or paying down high-interest debt just to execute this strategy. It's a tool for those with significant disposable income.