What Paperwork Tracks After-Tax 401(k) Contribution Basis?
Every dollar contributed to a 401(k) after tax carries an invisible label — proof it was already taxed once — and losing track of that label can mean paying tax on the same money twice.
The short answer
The plan recordkeeper, and separately the IRS, track after-tax 401(k) contribution basis through account statements, Form 1099-R reporting at distribution, and, on the individual’s side, Form 8606 when basis moves to an IRA. The plan is required to track which portion of a distribution is already-taxed basis versus taxable earnings, but the saver bears the practical risk if that paperwork gets lost, especially across a rollover or job change. Keeping personal copies of statements is the simplest safeguard.
Where the official tracking happens
A 401(k) plan’s recordkeeper maintains separate accounting for each contribution source — pre-tax deferrals, Roth deferrals, employer contributions, and after-tax contributions — because each is taxed differently at distribution. That recordkeeping typically shows up on periodic account statements as a breakdown by source, and it’s what allows a plan to correctly calculate, at the time of a withdrawal or rollover, how much of the distribution is a return of already-taxed principal (not taxable again) versus earnings (taxable). This distinction is entirely internal to the plan’s own bookkeeping until the money actually moves.
What happens at distribution
When money leaves the plan — through a withdrawal or a rollover to another employer’s plan — the plan issues Form 1099-R, which reports the distribution and is supposed to reflect the taxable and non-taxable (basis) portions separately. If the after-tax basis moves to an IRA rollover rather than being cashed out, the receiving IRA custodian needs that basis information to keep tracking it correctly going forward, and it’s also reported to the IRS on Form 8606 by the individual, similar to how non-deductible IRA contributions get tracked.
Why gaps happen
The handoff between a departing employer’s plan, a new plan, and an IRA is where basis tracking most often breaks down. Recordkeepers change, plans get acquired, statements get archived and become hard to retrieve years later, and a rollover’s paperwork trail doesn’t always carry basis detail forward as cleanly as it should. None of this is unusual — it’s simply a structural risk of a system where the same dollar can pass through multiple recordkeepers over a working career.
Why keeping personal records matters
Because official recordkeeping can be incomplete or hard to reconstruct after the fact, many savers keep their own file of account statements showing after-tax contribution amounts and dates, along with any 1099-R or 8606 forms received over the years. This isn’t about distrust of the plan — it’s self-insurance against a future paperwork gap. If a distribution years down the line doesn’t correctly reflect basis, having personal statements on hand from the years the contributions were made is often the only practical way to demonstrate what was already taxed. This record becomes especially useful if you’re also weighing an in-plan conversion of that after-tax money to Roth status, since that decision depends on knowing exactly how much basis and earnings sit in the account at a given moment.
- What to hold onto. Annual account statements, especially ones showing a source-by-source breakdown of the after-tax bucket, along with any Form 1099-R or Form 8606 tied to a rollover or distribution.
- When it matters most. At the time of a final distribution or a rollover between institutions, when the risk of the basis figure getting lost or estimated incorrectly is highest.
- Why it compounds. Contributions made over many years, sometimes across multiple employers, create more opportunities for a basis figure to go untracked.
A practical habit
Saving a copy of each year’s year-end account statement, or at least any statement that shows a source breakdown, takes a few minutes and creates a paper trail that’s far easier to produce than trying to reconstruct contribution history a decade later. Because plan administration and tax reporting rules can change, and because every account’s history is different, anyone unsure whether their basis has been tracked correctly should request a full contribution history from their plan administrator or a prior employer’s recordkeeper directly.