Confirm you actually need the backdoor. Plug MAGI into the calculator. If direct Roth room is still $7,500, contribute to the Roth IRA and stop. A backdoor on a year you did not need it is just extra 8606 work.
Hunt every traditional, SEP, and SIMPLE IRA. Old rollovers, a 2018 solo-401(k) that was never closed, a SEP from a side hustle. The IRS aggregates them. 401(k) / 403(b) / TSP do not count. Print year-end values; December 31 is the snapshot.
Ask the current 401(k) recordkeeper, in writing, whether they accept incoming IRA rollovers (pre-tax). If yes, roll the pre-tax IRA pile in before you contribute. If no, either skip the backdoor this year or convert on purpose and pay the tax from cash — do not “hope” pro-rata is small.
Wait for the IRA to show $0 pre-tax (or only the after-tax basis you already track on Form 8606). A pending rollover that settles in January still sits in this year’s aggregation.
Open or reuse a traditional IRA at the same custodian as the Roth if you can. Same-shop conversions are faster and less likely to cut a check you deposit wrong.
Contribute nondeductible up to the 2026 cap ($7,500, or $8,600 at 50+), labeled for the correct tax year. April 15 of 2027 can still be a 2026 contribution — check the box. Do not deduct it. If software defaults to deductible, override it.
Convert promptly — same week is fine. Growth between contribution and conversion is taxable ordinary income. Leaving it until December is how a $12 interest leftover becomes a story.
Convert dollars, not “the account,” if other money might land. A residual dividend on an old IRA you thought was empty is a pre-tax dollar in the pile. Sweep leftovers to the 401(k) or convert them too, on purpose.
Keep the 1099-R and the year-end IRA statements. Box 7 code 2 or 7 plus the taxable amount is what 8606 has to reconcile. If the custodian marks the whole conversion taxable because they do not know your basis, that is normal — you correct it on 8606.
File Form 8606: Part I for the nondeductible contribution and running basis. Part II for the conversion. If you did a backdoor last year, basis carries. Missing one year is how people pay tax twice.
Do not dump a leaving-job 401(k) into an IRA the same year you want a clean backdoor unless you like pro-rata. Roll to the new employer plan, or keep the old 401(k), or convert on purpose. The leave-a-job checklist is the other half of this conflict.
If the plan allows after-tax 401(k) money, the mega backdoor moves more than $7,500. Run that only if in-plan Roth conversion or in-service after-tax withdrawal is actually in the SPD — see mega backdoor.
One-page decision
Direct Roth if MAGI allows. Else empty pre-tax IRAs into a 401(k) that will take them, contribute nondeductible, convert the same week, file 8606. If you cannot empty the IRAs, do not dress a taxable conversion up as a backdoor — either pay the tax on purpose or skip the IRA and max the Roth 401(k).