If you have ever put money into a traditional IRA and later realized you could not deduct it, you are not alone. This is one of those “quiet” tax situations that can cost you real money later if you do not track it.
Here is the big idea in plain English: nondeductible traditional IRA contributions create after-tax money inside your IRA. The IRS calls that after-tax amount your basis. And the way you prove that basis and keep it updated year after year is Form 8606.
What is a nondeductible IRA contribution?
A traditional IRA contribution is deductible when you qualify to take the IRA deduction on your tax return. It is nondeductible when you contribute to a traditional IRA but you do not qualify for the deduction.
That might sound like a waste, but it is not necessarily. Nondeductible contributions can still make sense for people who want:
- Tax-deferred growth inside the IRA
- More retirement space than a workplace plan allows
- A foundation for future Roth planning, as long as you understand how the math works
The key is making sure you do not get taxed twice later. That is exactly what Form 8606 is for.
When contributions become nondeductible
Most nondeductible contributions happen for one of two reasons:
1) You are covered by a workplace plan and your income is too high
If you or your spouse is covered by a workplace plan like a 401(k), 403(b), or SIMPLE IRA, the ability to deduct a traditional IRA contribution can be reduced or eliminated at certain income levels. Those income limits are based on your modified adjusted gross income (MAGI) and filing status. MAGI is basically a modified version of your AGI that the IRS uses for IRA rules.
Depending on your income level, your contribution may be fully deductible, partially deductible, or not deductible at all.
2) You simply choose not to deduct (less common)
Some people are allowed to take the deduction but choose not to, often due to unique tax planning. If you do this, you still must track basis the same way.
Important: If you contribute to a Roth IRA directly, that is not tracked on Form 8606 as “basis.” Form 8606 is mainly about after-tax money in traditional, SEP, and SIMPLE IRAs, plus Roth conversions and certain Roth distributions.
What “basis” means and why you should care
Your IRA basis is the total amount of nondeductible contributions you have made to your traditional IRA over the years, minus any basis you have already used up through distributions or conversions.
Think of basis like a receipt folder for after-tax IRA money. When you eventually take money out of your IRA, the IRS needs to know what portion has already been taxed.
If you do not file Form 8606 when required, you risk:
- Double taxation on money you already paid tax on
- IRS questions later when you do a distribution or Roth conversion
- Extra time untangling old records when you least want to deal with it
Tip: If you have nondeductible IRA contributions, Form 8606 is not “optional paperwork.” It is the thing that keeps your after-tax dollars from getting taxed again.
Form 8606: what it does
Form 8606 is used to track and report:
- Nondeductible contributions to traditional IRAs
- Your total basis in traditional IRAs (carried forward year to year)
- Roth conversions (including taxable vs non-taxable portions)
- Certain Roth IRA distributions that are not qualified
When you must file it
You generally need to file Form 8606 if any of these happened:
- You made a nondeductible contribution to a traditional IRA for the year
- You took a distribution from a traditional, SEP, or SIMPLE IRA and you have basis
- You did a Roth conversion from any traditional, SEP, or SIMPLE IRA
- You received a nonqualified Roth IRA distribution (or another Roth distribution situation that requires Form 8606)
Form 8606 is filed with your federal tax return for that year. If you use tax software, it typically generates the form, but you still need to answer the questions correctly.
Timing note: IRA contributions are reported for the tax year they are designated for, even if you make the contribution between January 1 and the tax filing deadline and label it as a prior-year contribution. In other words, the Form 8606 ties to the contribution year, not simply the calendar date you moved the money.
Penalty note: A penalty may apply if you fail to file Form 8606 when required. The amount can change, so check the current IRS instructions if you want the exact number.
How basis carries forward
This is the part that trips people up: basis is cumulative.
Let’s say you contribute $7,000 nondeductible this year (subject to annual IRS limits). Your basis is $7,000.
If you do another $7,000 nondeductible contribution next year, and you have not taken any distributions or conversions, your basis becomes $14,000.
Form 8606 is basically keeping a running total so the IRS can see:
- What you added this year
- What your total basis was coming into the year
- How much of that basis you used (if you took money out or converted)
- What basis remains to carry forward
The pro-rata rule
If you have both pre-tax and after-tax money across your traditional, SEP, and SIMPLE IRAs, the IRS does not let you cherry-pick only the after-tax dollars when you take money out or convert to Roth.
Instead, the IRS applies the pro-rata rule.
What it means
When you do a distribution or a Roth conversion, the IRS treats it as coming proportionally from:
- Pre-tax IRA money (deductible contributions and earnings)
- After-tax IRA money (your basis from nondeductible contributions)
The key snapshot date: December 31
The pro-rata calculation looks at the value of all of your traditional, SEP, and SIMPLE IRAs as of December 31 of the year you take the distribution or do the conversion.
This catches people by surprise because it is not account-by-account. It is one big combined bucket for these IRA types.
One important nuance: It is not “just your December 31 balance” in isolation. Form 8606 uses a fraction that effectively compares your total basis to your total IRA value for the year, generally measured as year-end value plus the amount you distributed or converted during the year (and certain amounts in transit, like rollovers that are still outstanding). The December 31 value is the anchor number, but the math also reflects what came out during the year.
A simple example
Suppose on December 31 you have:
- $90,000 of pre-tax money across your IRAs
- $10,000 of basis (after-tax) tracked on Form 8606
- Total IRA value: $100,000
If you convert $10,000 to a Roth IRA, the IRS says the conversion is:
- 10% non-taxable (basis) = $1,000
- 90% taxable (pre-tax) = $9,000
So even though you “converted $10,000,” most of that conversion is still taxable if most of your IRA money is pre-tax.
Form 8606 and partial conversions
This article is not a step-by-step backdoor Roth walkthrough, but it is important to understand how basis tracking and partial conversions connect.
What happens in a partial conversion year
If you convert only part of your IRA to Roth, Form 8606 determines how much of that conversion is taxable using:
- Your total basis going into the year
- Your nondeductible contributions for the year (if any)
- Your total year-end IRA balances (December 31)
- The amount converted or distributed
After the conversion, you will typically still have some basis left over (unless you converted or withdrew enough to use it up). That remaining basis carries forward to next year on Form 8606.
Why “I only converted the nondeductible part” is usually wrong
People often assume they can convert just the nondeductible contribution and pay little or no tax. The pro-rata rule is what prevents that in many real-life situations.
If you are planning Roth conversions, basis awareness is not a nice-to-have. It directly impacts your tax bill.
A planning note many people ask about
If pro-rata treatment is going to make a conversion more taxable than you expected, a common approach (when eligible) is to roll pre-tax IRA money into an employer plan like a 401(k) so it is no longer sitting in the IRA “bucket” for pro-rata purposes. Whether that is available or a good idea depends on the plan rules and your situation.
Common mistakes
Forgetting to file Form 8606 in nondeductible years
If you make a nondeductible contribution and do not file Form 8606, the IRS has no easy record that you already paid tax on that contribution.
Thinking basis is tracked by your brokerage
Brokerages track contributions and send Form 5498, but they do not track your IRS basis the way Form 8606 does, especially across multiple accounts and years.
Ignoring old SEP or SIMPLE IRAs
The pro-rata rule includes traditional, SEP, and SIMPLE IRA balances. People sometimes focus only on one traditional IRA and forget an old SEP IRA still sitting at a prior brokerage.
Mixing up Roth IRA “basis” with traditional IRA basis
Roth IRAs have ordering rules for contributions and conversions, but that is a different concept than the after-tax basis tracked on Form 8606 for traditional IRAs.
How to keep records clean
Here is a simple system that keeps you out of trouble without turning your life into a filing cabinet:
- Save PDFs of every filed Form 8606 and your full tax return
- Keep a note with your current total basis (the carryforward number) in your tax folder
- Track all IRA accounts you own, including SEP and SIMPLE IRAs from old self-employment income
- Before doing any Roth conversion, confirm your year-end IRA balances will affect pro-rata
If you are missing old Forms 8606, you may still be able to reconstruct basis from prior returns and contribution records. In messier cases, it can be worth working with a tax pro to get it cleaned up correctly.
Quick FAQs
Do I file Form 8606 every year?
You file it in any year you make a nondeductible contribution, do a Roth conversion from an IRA, take an IRA distribution while you have basis, or have a Roth IRA distribution situation that triggers the form.
Does basis ever expire?
No. It carries forward until it is fully used through distributions or conversions. The catch is you have to document it properly.
Is Form 8606 only for backdoor Roth contributions?
No. It is used for nondeductible IRA basis tracking in general. Backdoor Roth discussions often mention it, but the form is bigger than that one strategy.
Does the pro-rata rule include my 401(k)?
No. The pro-rata rule for this purpose looks at traditional, SEP, and SIMPLE IRAs. Employer plans like 401(k)s are not included in that specific IRA pro-rata calculation.
The bottom line
Nondeductible IRA contributions are not automatically a problem. The problem is losing track of your basis.
If you have after-tax money in a traditional IRA, Form 8606 is how you:
- prove you already paid tax on part of your IRA
- carry your basis forward correctly
- avoid nasty surprises when you take distributions or do Roth conversions
If you are considering conversions or you have multiple IRA accounts, take five minutes to confirm your current basis and your year-end IRA balances. That small check can save you a very expensive “wait, why is this taxable?” moment next April.