Pro Rata Rule Calculator
The Pro Rata Rule Calculator tackles one of the most misunderstood corners of retirement saving: what happens, tax-wise, when you convert IRA money to a Roth IRA while holding a mix of pre-tax and after-tax dollars. Many savers imagine they can convert only their after-tax money and pay no tax. The IRS has a rule that says otherwise, and it is called the pro rata rule.
This calculator makes the rule concrete. Enter your total IRA balance, your after-tax basis, and the amount you plan to convert. It returns the taxable amount of the conversion, the tax-free amount, and the taxable percentage, so a conversion never surprises you at tax time.
Whether you are planning a backdoor Roth contribution, a partial conversion in a low-income year, or simply trying to understand a tax form, the Pro Rata Rule Calculator turns an intimidating rule into three clear numbers.
What Is the Pro Rata Rule?
The pro rata rule is the IRS method for taxing IRA distributions and conversions when an account holds both pre-tax money (deductible contributions and all earnings) and after-tax money (non-deductible contributions, called basis).
Instead of letting you choose which dollars come out, the rule treats every withdrawal as a proportional slice of the whole pie. If 25 percent of your total IRA value is basis, then 25 percent of any conversion is tax-free and the other 75 percent is taxable.
The rule follows you across accounts. For this calculation, the IRS looks at the combined value of your traditional, SEP, and SIMPLE IRAs, not just the single account you convert from.
The practical effect is simple to state and impossible to dodge: you cannot isolate the cream. Every conversion carries its proportional share of taxable money with it.
Basis: The After-Tax Money You Already Paid Tax On
Basis is money you contributed to an IRA without taking a tax deduction, most often through non-deductible traditional IRA contributions. Because you already paid tax on it once, the IRS does not tax it again when it comes out.
Basis is tracked on IRS Form 8606, filed in any year you make a non-deductible contribution or take a distribution from an account containing basis. Keeping that form’s running total accurate is essential; lost basis records mean paying tax twice on the same dollars.
Earnings, by contrast, are never basis. Interest, dividends, and growth inside the IRA are all pre-tax money, even when they grew on top of an after-tax contribution.
The calculator asks for your total basis as a single dollar figure. If you have several IRAs, add their basis amounts together, because the pro rata rule sees your IRAs as one combined pot.
The Three Numbers This Calculator Gives You
The taxable amount is the part of your conversion that will be added to your income for the year. This is the figure that drives your tax bill, and the one to test against your tax bracket before you convert.
The tax-free amount is the part of the conversion that represents a return of your basis. It moves into the Roth IRA without any tax cost today.
The taxable percentage expresses the split as a single number. It equals 100 percent minus your basis percentage, and once you know it, you can estimate the tax on any conversion size instantly by multiplying.
Together the three outputs answer the only questions that matter before signing a conversion form: how much will be taxed, how much will not, and at what rate does the split occur.
How to Use the Pro Rata Rule Calculator
Gather three figures, all in dollars, before you start.
Step 1: Find your total IRA balance. For a faithful estimate, use the combined balance of all your traditional, SEP, and SIMPLE IRAs, ideally the year-end total the IRS method uses.
Step 2: Type that total balance into the first box.
Step 3: Find your after-tax basis, the running total from your most recent Form 8606, and type it into the second box.
Step 4: Type the conversion amount you are considering into the third box.
Step 5: Press the Calculate button.
Read the taxable amount first, then check it against your tax situation. If the number is bigger than you like, press Reset and test a smaller conversion. The calculator happily runs as many what-if scenarios as you need, which is exactly how careful conversion planning works.
Worked Example 1: $120,000 Balance, $30,000 Basis, $20,000 Conversion
A saver holds IRAs worth 120,000 dollars in total, with 30,000 dollars of after-tax basis recorded on Form 8606. She wants to convert 20,000 dollars to a Roth IRA.
Step 1: Enter the total balance: 120000.
Step 2: Enter the basis: 30000.
Step 3: Enter the conversion amount: 20000.
Step 4: Press Calculate on the Pro Rata Rule Calculator.
Step 5: Find the basis percentage: 30000 / 120000 = 25 percent of the pot is after-tax money.
Step 6: The tax-free amount is reported as 5000.00 dollars, which is 25 percent of the 20,000 dollar conversion.
Step 7: The taxable amount is reported as 15000.00 dollars, the remaining 75 percent of the conversion.
Step 8: The taxable percentage is reported as 75.00, confirming the split at a glance.
So although she converts 20,000 dollars and personally thinks of 30,000 dollars of her IRA as “already taxed money,” the rule taxes 15,000 dollars of this conversion. Knowing that figure in advance lets her set aside money for the tax bill instead of being ambushed by it.
Worked Example 2: $100,000 Balance, $20,000 Basis, $10,000 Conversion
Another saver has a combined IRA balance of 100,000 dollars and basis of 20,000 dollars, and is considering a modest conversion of 10,000 dollars.
Step 1: Enter the total balance: 100000.
Step 2: Enter the basis: 20000.
Step 3: Enter the conversion amount: 10000.
Step 4: Press Calculate.
Step 5: The basis percentage is 20000 / 100000 = 20 percent.
Step 6: The tax-free amount is reported as 2000.00 dollars, which is 20 percent of the conversion.
Step 7: The taxable amount is reported as 8000.00 dollars.
Step 8: The taxable percentage is reported as 80.00.
Notice the pattern across both examples: the taxable percentage depends only on the balance and the basis, not on the size of the conversion. Doubling the conversion doubles both dollar results but leaves the percentage untouched, a fact that makes planning a series of yearly conversions much easier.
Why the Rule Catches Backdoor Roth Savers
The backdoor Roth strategy works like this: contribute after-tax money to a traditional IRA, then convert it to a Roth. If the traditional IRA holds nothing else, the conversion is nearly tax-free because almost all of it is basis.
The trap springs when the same person also holds a large pre-tax IRA, perhaps an old rollover from a former employer’s plan. The pro rata rule blends that big pre-tax balance into the calculation, and suddenly most of the “tax-free” conversion is taxable.
This is the single most common surprise the rule produces, and it is why running your real combined numbers through this calculator before executing a backdoor contribution is so important.
A common remedy is moving the pre-tax IRA money into a current employer’s 401(k) before year-end, which removes it from the pro rata calculation. Whether that suits you is a question for a tax professional, but the calculator shows you the size of the problem you are solving.
Planning Conversions Across Several Years
Because the taxable percentage is fixed by your balance and basis, the main planning lever is the conversion amount itself. Many savers convert just enough each year to fill up their current tax bracket and no more.
Low-income years, such as the gap between retirement and the start of pensions or required distributions, are golden opportunities. The same conversion that would be taxed at a high rate during peak earning years may be taxed gently in a lean year.
Each conversion also consumes a slice of your basis, so your basis figure shrinks proportionally over time and future splits change slightly. Re-running the calculator each year with updated balances keeps the plan honest.
The tool’s three outputs are designed for exactly this rhythm: test a number, judge the taxable amount against your bracket, adjust, and repeat until the plan feels right.
A Word of Care
This calculator implements the standard pro rata calculation faithfully, using the balances you provide. Your official tax result is computed on Form 8606 using the year-end balance plus certain adjustments, so treat the figures here as a close planning estimate rather than a filed return.
Retirement tax rules carry real money consequences and individual wrinkles, including state taxes and special account types. For large conversions, a short conversation with a qualified tax adviser is money well spent.
What the calculator guarantees is that the proportional logic of the rule is applied correctly to your numbers, which is the part most people get wrong when they estimate in their heads.
A Closer Look at the Formula
Stripped to its skeleton, the pro rata calculation is a single proportion. Divide your basis by your total balance and you have the fraction of every distribution that returns to you tax-free.
Multiply that fraction by any distribution or conversion amount and you get its tax-free part. Whatever remains is taxable. The calculator’s three outputs are nothing more, and nothing less, than that proportion expressed in dollars and in percent.
The elegance hides one sharp edge: the proportion is recalculated from the balances that exist when money moves. Market growth swells the pre-tax share over time, so a basis that once covered a third of your account may cover only a fifth a decade later.
That drift is why a conversion that was mostly tax-free early in retirement can become mostly taxable later, even with no new contributions. Re-running this calculation with current numbers each year is the only way to see the drift and plan around it.
It also explains why advisers obsess over the timing of contributions, rollovers, and conversions. Each move nudges the two ingredients of the proportion, and the proportion quietly decides the tax on everything that follows.
What About a Conversion with No Basis at All?
If your IRAs contain no after-tax money, the proportion at the heart of this article collapses to its simplest form: the basis is zero, the tax-free fraction is zero, and every dollar you convert is taxable income.
That is the situation most traditional savers are in, because most people deducted their contributions on the way in. For them, conversion planning is purely a question of tax brackets: how much extra income can this year absorb before the rate jumps?
The calculator handles this case without complaint. Enter a basis of zero and it will report the full conversion as taxable and the taxable percentage as 100.00, which is the correct and complete answer.
Tips for Getting the Most from This Calculator
- Combine all your IRAs: the rule looks at your traditional, SEP, and SIMPLE IRAs together, so the total balance you enter should too.
- Get basis from Form 8606: use the running total on your most recent filing rather than guessing from old contribution memories.
- Test several conversion sizes: the percentage stays fixed, so scale the conversion up and down to find the taxable amount your bracket can absorb.
- Recheck every year: balances and basis change after each conversion and each market year, so refresh the inputs before every new conversion.
- Plan around low-income years: run the numbers for a gap year after retirement to see how much gentler the same conversion becomes.
- Remember earnings are never basis: only non-deductible contributions count as basis; growth inside the account is always pre-tax money.
- Keep your records: saving your Form 8606 copies protects your basis, and losing them can mean paying tax twice on the same dollars.
Frequently Asked Questions
1. What is the pro rata rule in simple terms?
It is the IRS rule that taxes IRA withdrawals and conversions proportionally when an account holds both pre-tax and after-tax money.
Every dollar that comes out is treated as the same blend as the whole pot.
2. What does this Pro Rata Rule Calculator compute?
It splits a planned conversion into its taxable amount and tax-free amount, and shows the taxable percentage.
You supply the total balance, the after-tax basis, and the conversion size.
3. What counts as basis?
Basis is the total of your non-deductible traditional IRA contributions, money you already paid tax on.
Earnings inside the account never count as basis.
4. Which accounts are included in the total balance?
Traditional, SEP, and SIMPLE IRAs are combined for this rule.
Roth IRAs and workplace plans such as 401(k)s are not part of the balance used here.
5. Can I convert only my after-tax money to avoid tax?
No. That is precisely what the pro rata rule prevents.
Every conversion carries the same proportion of taxable money as the overall account.
6. How is the taxable percentage found?
First find the basis percentage: basis divided by total balance.
The taxable percentage is 100 percent minus that figure, and the calculator displays it directly.
7. Does the conversion size change the percentage?
No. The percentage depends only on the balance and the basis.
A bigger conversion scales both dollar results up while the split stays identical.
8. Where do I find my basis?
On your most recent IRS Form 8606, which tracks non-deductible contributions.
If you never filed it, reconstructing your contribution history with a tax professional is wise before converting.
9. What happens to my basis after a conversion?
The tax-free part of the conversion uses up that much of your basis.
Your remaining basis shrinks by the same amount, which is why the calculation is refreshed each year.
10. Why does the backdoor Roth strategy sometimes fail?
It stumbles when a large pre-tax IRA exists alongside the new after-tax contribution.
The rule blends the balances, making most of the conversion taxable, as the examples in this article illustrate.
11. Is a workplace 401(k) part of the calculation?
No. Money inside a 401(k) is outside the IRA pro rata calculation.
That is why rolling pre-tax IRA money into a 401(k) can change the result so dramatically.
12. When is the balance measured for the official tax form?
Form 8606 uses the year-end balance of your IRAs, with adjustments for amounts moved during the year.
Using your expected year-end balance in this calculator gives the closest planning estimate.
13. Does this calculator file anything with the IRS?
No. It is a planning tool only.
Your conversion is reported on your tax return, with Form 8606 doing the official calculation.
14. Can I run the calculator for a spouse separately?
Yes, and you should. The rule applies per person, not per couple.
Run each spouse’s own balances and basis through the tool individually.
15. Why does the Reset button reload the page?
Reloading clears the three inputs and the results at once.
It is the fastest way to set up the next what-if conversion scenario.
CONCLUSION
The Pro Rata Rule Calculator replaces the most common Roth conversion surprise with simple foresight: enter your total balance, your basis, and a conversion amount, and you will know the taxable amount, the tax-free amount, and the taxable percentage before you commit a single dollar. Plan your conversions with these three numbers in hand, refresh them every year, and the pro rata rule becomes a planning tool instead of a tax-time ambush.