Backdoor Roth IRA Tax Calculator

How Backdoor Roth Works

The 'Backdoor Roth IRA' is a workaround for high-income earners who exceed the direct Roth IRA contribution income limits ($165K single / $245K MFJ for 2026, phased out from $146K/$230K). The three-step move: (1) Contribute up to $7,000 ($8,000 if 50+) to a Traditional IRA as a NON-DEDUCTIBLE contribution. (2) Convert that contribution to a Roth IRA within days. (3) File IRS Form 8606 to document the non-deductible basis.

If you have NO existing pre-tax Traditional IRA balance, this works cleanly with $0 additional tax — you're essentially making a $7K Roth contribution despite exceeding income limits. The complication is the pro-rata rule. Source: IRS Section 408(d)(2), Form 8606 instructions. Last updated: May 2026.

The Pro-Rata Rule — Why It Matters

The IRS treats ALL your Traditional IRA accounts as one combined account for conversion purposes. If you have $93,000 in existing pre-tax traditional IRA money and add $7,000 non-deductible, your total is $100K with $93K pre-tax (93%) and $7K post-tax basis (7%). When you convert any amount, that conversion is 93% taxable and 7% tax-free — proportionally.

The pro-rata rule applies to ALL Traditional IRAs (Rollover, SEP, SIMPLE) but NOT to 401(k)s, 403(b)s, or Roth IRAs. This is the key loophole exploit: roll your existing Traditional IRA into a 401(k) plan, eliminate the pre-tax balance, then do a clean Backdoor Roth.

Common Backdoor Roth Mistakes

(1) Not filing Form 8606. Without it, the IRS taxes your basis when you withdraw decades later — double taxation. (2) Waiting too long between contribution and conversion. Earnings during the wait become taxable. Convert within days. (3) Ignoring the pro-rata rule on Dec 31 balance. The IRS uses your Dec 31 total Traditional IRA balance, not the balance at conversion. (4) Doing the move late December. Better to execute Jan-March of the following year — clean accounting cycle. (5) Step transaction concerns — largely resolved by 2018 IRS clarification, but maintain separation of dates.

Mega Backdoor Roth — The Bigger Brother

The Mega Backdoor Roth uses after-tax 401(k) contributions (allowed at some employers up to a combined $70,000 total 401(k) limit in 2026) and converts those to Roth via in-plan conversion or in-service distribution. This puts $20K-$50K/year into Roth for high earners — much bigger than the $7K regular Backdoor. Requires (1) employer plan that allows after-tax contributions, (2) employer plan that allows in-service distribution or in-plan Roth conversion. Maybe 30% of US 401(k) plans support this in 2026.

Backdoor Roth Tax Calculator: 2026 Step-by-Step Worked Example

Concrete numbers make the pro-rata rule easier to see. Say a single filer earns $220,000 (over the $165K direct Roth limit) and holds $50,000 in an existing pre-tax Traditional IRA plus wants to add $7,000 non-deductible for the Backdoor Roth. Total Traditional IRA balance after contribution: $57,000. Basis fraction = $7,000 ÷ $57,000 = 12.28%. Converting the full $7,000 makes $7,000 × 87.72% = $6,140 taxable (added to ordinary income at 24% bracket = $1,474 federal tax due) and $860 tax-free. Compare to the same person with $0 pre-tax IRA balance: $7,000 × 100% basis = $0 tax. That's why rolling the $50,000 into an employer 401(k) FIRST — per IRS Publication 590-A — saves $1,474 on this single conversion. Run the calculator with and without the pre-tax balance to see your personal number.

Last updated 2026-07-01. Sources: IRS Pub 590-A, IRS Form 8606, IRS Section 408(d)(2).