Backdoor Roth Calculator
A backdoor Roth means a nondeductible contribution to a traditional IRA, then a conversion of that money to a Roth IRA. If you hold other traditional IRA money, you cannot convert only the nondeductible part. Form 8606 spreads your basis over all your traditional IRAs, which people call the pro-rata rule. The form and its instructions use neither name, but this is the sum they run. Enter your figures and the calculator works through the lines to the taxable part of the conversion and the basis you carry forward. This is not tax advice.
Line 1 of Form 8606.
Line 2. Generally line 14 of your last Form 8606, or 0 if this is your first. The instructions for line 2 list exceptions.
Line 4. Paid from January 1 to April 15 of the next year.
Line 6. Includes traditional SEP and SIMPLE IRAs and outstanding rollovers, after the conversion.
Line 7. Not conversions, rollovers, qualified charitable distributions, HSA funding distributions, certain returned contributions or recharacterizations. See the form. Empty counts as 0.
Line 8.
The form asks for at least 3.
The numbers in the fields at the start are an example. This is not tax advice. The calculator runs the lines of the 2025 Form 8606 on the figures you type, for one person and one year. It works out no tax, checks no income or contribution limit and does not say whether a conversion suits you. Your return and a tax professional decide the real figure.
How to use the Backdoor Roth Calculator
- Enter your nondeductible contributions for the year and your basis from earlier years. That is generally line 14 of your last Form 8606, or 0 if this is your first. The instructions for line 2 list exceptions.
- If part of line 1 was paid from January 1 to April 15 of the next year, enter it as well. That part stays out of the share for this year.
- Enter the value of all your traditional IRAs on December 31, after the conversion. Traditional SEP and SIMPLE IRAs count, and so do outstanding rollovers. A statement from each custodian shows the value.
- Enter any other distributions in the year and the net amount you converted, then press Calculate.
- Read the taxable part of the conversion on line 18, then copy the line figures over to your own form and check them there.
The Form 8606 lines
Line 10 = (line 1 + line 2 − line 4) ÷ (line 6 + line 7 + line 8), 1.000 at most Line 11 = line 8 × line 10 Line 18 = line 8 − line 11 Line 14 = line 1 + line 2 − line 11 − line 7 × line 10
Line 10 is the share of your IRA money that has already been taxed, and the same share of each dollar you convert comes out free of tax. The form asks for that share "as a decimal rounded to at least 3 places", with 1.000 as the most it can be. The calculator starts at 3 places and takes up to 10, rounding half up, and rounds each amount to the cent. Line 14 is the basis left for next year. The lines come from the 2025 Form 8606. Check the form for the year you file, because the IRS can change it. If you made no nondeductible contribution that year, took no other distribution and converted everything, Part I does not apply. The instructions then put line 2 on line 17, so line 18 is the amount converted minus your basis, with no share to round.
Worked example
These are made-up figures, and they are the ones the fields start with. You contribute $7,000 for 2025 and deduct none of it, with no earlier basis. You convert $7,000 and still hold $93,000 in another traditional IRA on December 31. Line 9 is $7,000 + $93,000 = $100,000, so line 10 is 0.070. Line 11 is $7,000 × 0.070 = $490.00, the nontaxable part. Line 18 is $6,510.00, the taxable part, and line 14 leaves $6,510.00 of basis for next year. With no other IRA money on December 31, line 10 is 1.000 and line 18 is $0.00. Convert $7,020 after $20 of earnings and line 10 is 0.997, which gives $6,998.94 nontaxable and $21.06 taxable. At 6 places it is 0.997151, and the taxable part is the $20.00 of earnings. Now take $7,000 of basis from earlier years and no contribution this year, and convert all $7,020. Part I does not apply, so line 17 is $7,000.00 and line 18 is $20.00.
Frequently asked questions
What is the pro-rata rule?
The name people give to lines 5 to 11 of Form 8606. Your basis, the money already taxed, is divided by all your traditional IRA money, and that share of what you convert is nontaxable. You cannot pick the nondeductible dollars out. In the example, $7,000 of basis against $100,000 in all makes 7% of the conversion nontaxable.
Which accounts count on line 6?
All your traditional IRAs, and the form says that includes traditional SEP IRAs and traditional SIMPLE IRAs, plus outstanding rollovers, at their value on December 31. Roth IRAs do not go there, and neither does a 401(k) at work. The instructions for Form 8606 explain outstanding rollovers and a few adjustments this calculator leaves to you.
Why does the December 31 value matter?
Line 6 asks for the value on that date, not on the day you converted. So money in a traditional IRA on December 31 goes into the share, whenever it arrived. The converted amount has its own place on line 8.
Why is a little taxable even with no other IRA money?
Earnings before the conversion have not been taxed, so they are taxable when converted. Rounding adds to it, since line 10 at 3 places can round down. In the example, $20 of earnings shows $21.06 taxable at 3 places and $20.00 at 6. The form allows more places. If the basis is all from earlier years and you convert everything, Part I does not apply and no share is rounded, so the same $20 shows $20.00.
What about contributions made in the next year?
Line 1 includes contributions for the year made from January 1 to April 15 of the next year. Line 4 takes them out again, so they stay out of this year's share. Line 14 still carries them forward as basis.
Do my spouse and I run this together?
No. Form 8606 is per person. If you both need one, the instructions say to file a separate form for each of you, so run the calculator once for each spouse with that spouse's own IRAs.
Is a conversion subject to the 10% early distribution tax?
IRS Publication 590-A says the 10% additional tax on early distributions does not apply if the money is properly and timely rolled over to the Roth IRA, while part or all of it may be included in gross income. It also says a conversion made in 2018 or later cannot be recharacterized, so it cannot be undone that way. The calculator works out no tax on the taxable part.
Is this tax advice?
No. This is not tax advice, and it does not say whether a conversion suits you. It runs the form's lines on the figures you type. It does not check whether you may make the contribution, what your income is or what tax you owe, and it stops short of lines 15b and 15c, which cover disaster distributions and repayments. Your return and a tax professional decide the real figure.