The backdoor Roth is really two separate transactions wearing one name. First you contribute after-tax money to a Traditional IRA. Then, usually a few days later, you convert that money to a Roth IRA. Most guides rush through the mechanics, but the order and the timing decide how much tax you owe.

The trap is the calendar. The conversion can happen in January, but the IRS judges it against your IRA balances on the following December 31. Money you leave behind, money that grows while you wait, and basis carried over from last year all change the split between the taxable and nontaxable parts of the conversion.

The Backdoor Roth IRA Calculator walks the two steps in order. Enter your contribution, any basis carried over from last year, your conversion amount, and your December 31 IRA balance, and it tracks your basis the way Form 8606 does, showing the taxable amount, the nontaxable amount, and the basis you carry into next year.

The backdoor Roth is two transactions, not one

Thinking of the backdoor Roth as a single move causes most of the confusion. Legally, step one is a Traditional IRA contribution and step two is a Roth conversion, and each has its own tax year, its own paperwork, and its own timing. The contribution can even be designated for the prior tax year if you make it before the April filing deadline, while the conversion always belongs to the calendar year in which it happens. Because the two steps can land in different tax years, your records must track them separately. The calculator keeps them separate too, with one section for the contribution and one for the conversion.

Step one: the nondeductible contribution

Step one is simple on the surface. You put after-tax money into a Traditional IRA and choose not to deduct it. For most savers this is the annual limit, seven thousand dollars, or eight thousand once you reach fifty. The critical detail is the word nondeductible. If your income is low enough to deduct the contribution, the backdoor structure breaks, because deductible money is pre-tax and converts as taxable income. You confirm the nondeductible status by filing Form 8606, which records your basis, the running total of after-tax money in your IRAs. Without that form, the IRS treats your contribution as pre-tax by default.

Step two: the Roth conversion

Step two moves the money from the Traditional IRA to the Roth IRA. There is no legal waiting period between the contribution and the conversion. You can convert the same day the contribution settles, and many savers do exactly that to stop investment earnings from piling up. When you convert, your brokerage reports the full converted amount to the IRS, and it is your job, through Form 8606, to show how much of it was basis and therefore nontaxable. The conversion is the event that can create a tax bill, so everything about timing exists to keep that bill as small as possible.

Why most people convert days later

Speed matters because of earnings. From the moment your contribution lands in the Traditional IRA, any growth it produces is pre-tax money. Leave seven thousand dollars invested for six months at a decent return and you might have three hundred dollars of pre-tax earnings mixed into your conversion. That three hundred dollars is taxable, and worse, it signals to the IRS that pre-tax money exists in your IRA, which feeds the pro-rata calculation. Converting within days keeps earnings near zero and keeps the math clean. The calculator assumes you convert promptly, which is why the worked examples use round numbers.

Your basis carried over from last year

Basis is sticky. If last year you made a nondeductible contribution of two thousand dollars but only converted part of it, the leftover basis carries forward. You find it on line 14 of last year Form 8606, and it becomes the opening balance of this year calculation. Many first-time backdoor savers enter zero here, which is correct for year one. But anyone who has done partial conversions, or who contributed late last year and converted this year, often has a carryover they forgot about. That forgotten basis is valuable. It is after-tax money the IRS already knows about, and it increases the nontaxable slice of this year conversion.

The December 31 balance rule

Here is the timing rule that decides everything. The IRS computes the taxable portion of your conversion using your total IRA balance on December 31 of the conversion year, not the balance on the day you converted. If you convert seven thousand dollars in February and your IRA still holds three thousand dollars of pre-tax money on December 31, that three thousand dollars dilutes your basis and makes part of the February conversion taxable. The rule looks backward from year end. This is why the calculator asks for the December 31 balance as its final input, and why rolling pre-tax money into a 401(k) in November can rescue a conversion you made in March.

Your total basis available

Total basis is the full stock of after-tax money the IRS recognizes in your IRAs. It equals your carried-over basis plus this year nondeductible contribution. With zero carryover and a seven thousand dollar contribution, your total basis is seven thousand dollars. With a two thousand dollar carryover and the same contribution, it is nine thousand. This number is the numerator of the nontaxable fraction, so every dollar of basis you can document on Form 8606 directly increases the tax-free portion of your conversion. Basis you never filed for is basis you do not have.

How the conversion splits into taxable and nontaxable

The split follows one fraction. Take your total basis, divide it by the sum of your conversion amount and your December 31 balance, and multiply by the conversion amount. That product is the nontaxable portion. Everything else in the conversion is taxable. When your December 31 balance is zero, the fraction is basis divided by basis, which is one, so the whole conversion is nontaxable. When money remains on December 31, the fraction drops below one and part of the conversion becomes taxable. The calculator shows both pieces so you can see exactly what the timing cost you.

Your basis carried to next year

Basis does not vanish when the year ends. Whatever basis you did not use up in the conversion carries forward to next year Form 8606. The calculator computes it as total basis minus the nontaxable portion of the conversion. In a clean backdoor year, where you converted everything and your December 31 balance was zero, the carryforward is zero. In a messy year, where a December balance stranded some basis, the carryforward preserves it for next year. This is the number you will enter as prior year basis twelve months from now, so write it down or keep the Form 8606 copy.

Worked Example: The textbook clean backdoor

Lena contributes seven thousand dollars as a nondeductible contribution, her first backdoor year, so her carried-over basis is zero. She converts seven thousand dollars in the same week and empties her Traditional IRA, leaving a December 31 balance of zero. Her total basis is seven thousand dollars. The denominator is seven thousand plus zero, which is seven thousand. Her nontaxable conversion is seven thousand times seven thousand divided by seven thousand, which is the full seven thousand dollars. Her taxable conversion is zero dollars, and her basis carried to next year is zero dollars. Two steps, a few days apart, zero tax. This is the result the two-step discipline is designed to produce.

Worked Example: The leftover December balance

Marcus contributes seven thousand dollars with zero carryover and converts seven thousand dollars in March. But he leaves an old three thousand dollar pre-tax balance sitting in the IRA all year, so his December 31 balance is three thousand dollars. His total basis is seven thousand dollars. The denominator is seven thousand plus three thousand, which is ten thousand. His nontaxable conversion is seven thousand times seven thousand divided by ten thousand, which is 4,900 dollars. His taxable conversion is seven thousand minus 4,900, which is 2,100 dollars. His basis carried to next year is seven thousand minus 4,900, which is 2,100 dollars. The March timing did not save him. The December balance did the damage.

Worked Example: The multi-year basis stack

Aisha has a two thousand dollar basis carryover from last year, when she converted only part of a contribution. This year she contributes seven thousand dollars, giving her a total basis of nine thousand dollars. She converts eight thousand dollars and leaves four thousand dollars in the IRA on December 31. The denominator is eight thousand plus four thousand, which is twelve thousand. Her nontaxable conversion is eight thousand times nine thousand divided by twelve thousand, which is 6,000 dollars. Her taxable conversion is eight thousand minus 6,000, which is 2,000 dollars. Her basis carried to next year is nine thousand minus 6,000, which is 3,000 dollars. The old basis did its job, shielding six thousand of the eight thousand dollar conversion.

Worked Example: Converting more than your basis

Tom contributes seven thousand dollars with zero carryover, then waits four months while the money grows. He converts twelve thousand dollars, the original seven thousand plus five thousand of pre-tax earnings and an old balance, and his December 31 balance is zero. His total basis is seven thousand dollars. The denominator is twelve thousand plus zero, which is twelve thousand. His nontaxable conversion is twelve thousand times seven thousand divided by twelve thousand, which is 7,000 dollars. His taxable conversion is twelve thousand minus 7,000, which is 5,000 dollars. His basis carried forward is zero. Basis caps the tax-free portion. You can never convert more tax-free dollars than the basis you actually have.

The Form 8606 formula

The formula is:

Nontaxable conversion = Conversion amount x (Total basis / (Conversion amount + December 31 balance))

Then taxable conversion equals the conversion amount minus the nontaxable conversion, and basis carried to next year equals total basis minus the nontaxable conversion. Total basis itself equals prior year basis plus this year nondeductible contribution. These three lines are the entire arithmetic of Form 8606, and the calculator runs all three from your four inputs.

Mapping the results to Form 8606

The calculator outputs line up with the form. Your nondeductible contribution is the basis you report for this year. Your carried-over basis is line 14 from last year return. The conversion amount is what your brokerage reports on the 1099-R. The December 31 balance is the year-end total the form asks for. The nontaxable result is the portion of the conversion that escapes tax, and the carryforward is your new line 14 for next year. If you keep the calculator results with your tax files, filling out Form 8606 becomes a copying exercise instead of a reconstruction project.

Why the order still matters when December 31 decides

A common objection is that timing should not matter if only the year-end balance counts. It matters for two reasons. First, converting early stops the clock on earnings, so no new pre-tax dollars appear between the contribution and the conversion. Second, converting early gives you most of the year to fix problems, such as rolling a forgotten pre-tax balance into a 401(k) before December 31. Someone who converts in December has no time left to clean up the year-end balance. The two-step order, contribute then convert quickly, is really a strategy for maximizing your options before the December snapshot.

Timing mistakes that cost real money

The classic mistake is the slow conversion. Contributing in January and converting in November lets eleven months of earnings become pre-tax money. The second mistake is the January contribution for the prior tax year, which is legal but creates a year where the contribution and the conversion live on different returns, confusing the basis tracking. The third is the December 401(k) rollover into an IRA, which lands pre-tax money in the IRA just in time for the December 31 snapshot. The fourth is forgetting to file Form 8606 entirely, which erases your basis from the IRS records. Each mistake is a timing problem, and each is fixed by moving faster or planning earlier.

Frequently Asked Questions

1. How long should I wait between contributing and converting?

There is no required waiting period. Most savers convert within a few days, as soon as the contribution settles, to keep pre-tax earnings near zero.

2. Can I convert on the same day I contribute?

Yes, once the funds have settled and are available. Same-day conversions are common and keep the two steps tightly linked.

3. What if I contribute in January but designate it for the prior tax year?

The contribution counts for the prior year, but the conversion always counts for the calendar year in which it happens. Your basis tracking must span both returns.

4. Where does my basis appear on Form 8606?

Your nondeductible contribution is entered as this year basis, and your carryover from last year comes from line 14 of the prior return. The form combines them into total basis.

5. What is line 14 of Form 8606?

It is your total basis at the end of the year, the amount that carries forward. Next year you enter it as your prior year basis.

6. What happens if I forgot to file Form 8606 in a past year?

File it now. The IRS can assess a penalty for a missing form, but filing late and documenting your basis is far better than losing the basis entirely.

7. Does my December 31 balance include my Roth IRA?

No. Only Traditional, SEP, and SIMPLE IRA balances count. Roth IRA balances are excluded from the year-end total.

8. What if I roll a 401(k) into an IRA in December?

That pre-tax money lands in your IRA just in time for the December 31 snapshot, which dilutes your basis and increases the taxable part of any conversion that year.

9. Can I fix a bad December 31 balance after the year ends?

No. The snapshot is final once the year closes. The fix has to happen before December 31, for example with a reverse rollover into a 401(k).

10. Do I have to empty my Traditional IRA before converting?

Not legally, but any balance left on December 31 reduces the nontaxable portion. Emptying the IRA is how you get the clean full-basis result.

11. What happens to basis I do not use this year?

It carries forward to next year. Unused basis is never lost as long as you keep filing Form 8606 and recording the carryforward.

12. Can basis disappear if I stop tracking it?

Effectively yes. If you skip Form 8606 for a few years, reconstructing old basis becomes difficult, and the IRS will treat undocumented money as pre-tax.

13. Does the contribution have to come before the conversion?

In practice yes, because you cannot convert money you have not contributed. The contribution must also be nondeductible to create basis.

14. Can I do a backdoor Roth with no earned income?

You need taxable compensation to contribute to an IRA, though a working spouse income can fund a spousal IRA for a nonworking spouse.

15. Should the contribution and conversion happen in the same calendar year?

It keeps the paperwork simplest, since both steps land on one return. Splitting them across years is allowed but makes the basis tracking span two filings.