How Non-Deductible Contributions Are Tracked

Some contributions to a traditional retirement account are made without a deduction. Those amounts create basis, and basis only exists in practice if it has been recorded.

Basis is the amount already taxed

Money contributed without a deduction has already been included in income once, so recognizing it again at distribution would tax it twice.

The system prevents that by treating the non-deducted amounts as basis, which is returned free of further income tax when distributions occur.

The mechanism depends entirely on someone knowing the basis figure, and the account custodian generally does not, because it does not know whether a deduction was claimed.

The reporting form is the record

Basis is reported to the tax authority on a specific form filed with the return for the year a non-deductible contribution is made.

The form carries the running total forward, which is why the historical filings matter as much as the current one.

Reconstructing decades of contributions without those filings is difficult, and the burden of proving basis rests with the account holder rather than the custodian.

Distributions come out proportionally

A distribution from a traditional account is generally not treated as coming from basis first. A proportion of every distribution is treated as basis and the remainder as taxable.

The proportion is calculated across the relevant accounts as a whole rather than account by account, which surprises people who assume they can isolate the basis.

The same principle applies to conversions, so converting an account holding basis does not convert the basis separately from everything else.

Multiple accounts are aggregated

For this calculation the traditional accounts are treated as a single pool, meaning a rollover from a workplace plan can change the ratio substantially.

A household that has carefully tracked basis in one account can find that proportion diluted by a large pre-tax balance arriving from elsewhere.

Whether that matters, and how the ordering of such moves interacts, is precisely the kind of question that belongs with a qualified tax professional.

Records outlive the people who made them

Basis carries through to a beneficiary, who inherits both the account and the obligation to account for the previously taxed portion.

A beneficiary who never receives the historical filings has no practical way to claim it, and the amounts are treated as fully taxable by default.

Keeping the filings with the estate documents, rather than in a tax folder discarded after several years, is what makes the basis survive. The rules governing all of this are technical and revised periodically.