Mega Backdoor Roth 401(k) After-Tax Calculator
The Mega Backdoor Roth lets high earners contribute up to $46,500 EXTRA to Roth annually (beyond the $23,500 401(k) cap) using after-tax 401(k) contributions + in-plan Roth conversion. Calculate your total Roth contribution capacity.
Mega Backdoor Roth Mechanics
After maxing pre-tax 401(k) ($23,500 in 2026), make additional after-tax contributions up to the 415(c) total limit ($70,000 in 2026). These after-tax dollars are immediately converted to Roth — either via in-plan Roth conversion or via in-service distribution to a Roth IRA. Result: up to $46,500 in extra Roth contributions per year for high earners.
Plan Requirements
Your 401(k) plan must allow: (1) After-tax contributions beyond elective deferrals (different from Roth elective deferrals). (2) Either in-plan Roth conversions OR in-service withdrawals while still employed. Only about 50% of US 401(k) plans support both. Check your Summary Plan Description (SPD) or ask plan administrator directly.
Pro-Rata Tax Trap
If you also have pre-tax 401(k) balance, conversions trigger pro-rata taxation on the conversion (similar to backdoor Roth IRA). Best strategy: convert after-tax contributions immediately (same day or week) to minimize earnings on after-tax balance that would be taxable on conversion.
Source: IRS Notice 2014-54, IRS 415(c) limits 2026, Plan Sponsor Council of America (PSCA) 401(k) Plan Design Trends. Last updated: May 2026.