Potentially yes. A backdoor Roth generally means making a nondeductible traditional IRA contribution and then converting to a Roth IRA. It uses the annual IRA contribution allowance. Form 8606 calculates the taxable share using your traditional, SEP and SIMPLE IRAs together, including relevant year-end balances; isolating the new contribution in another IRA does not remove the pro-rata calculation.
A mega backdoor Roth generally starts with voluntary after-tax contributions to an employer plan, distinct from designated Roth salary deferrals. The plan must permit those contributions and the relevant in-plan Roth rollover or eligible distribution to a Roth IRA. Its annual-additions rules include employer and other plan contributions.
These separate channels can coexist, but each transaction must independently qualify. Converting after-tax principal generally avoids taxing it again; associated untaxed earnings converted to Roth are taxable. Check both the plan’s terms and your IRA balances before choosing amounts.
Primary sources
Instructions for Form 8606 (2025) | Internal Revenue Service
Rollovers of after-tax contributions in retirement plans | Internal Revenue Service
Retirement topics - 401(k) and profit-sharing plan contribution limits | Internal Revenue Service
Instructions for Form 5329 (2025) | Internal Revenue Service