What is the Tax Arbitrage Rule for IRAs?

The rule states: If your current tax rate is higher than your expected retirement rate, choose Traditional/SEP. If your current rate is lower, choose Roth. If rates are equal, the outcome is mathematically identical.

IRA Calculator - Free & Accurate Online Calculator

Q: What are the IRA contribution limits for 2025?

For 2025, Traditional and Roth IRAs have a limit of $7,000 ($8,000 if 50+). SEP IRAs allow up to 25% of compensation or $70,000. SIMPLE IRAs are capped at $16,500.

Q: Can I contribute to both a 401(k) and an IRA?

Yes, you can contribute to both simultaneously. However, having a workplace 401(k) may limit the tax deductibility of your Traditional IRA contributions depending on your income level.

Q: What are Required Minimum Distributions (RMDs)?

RMDs are mandatory withdrawals from pre-tax accounts (Traditional, SEP, SIMPLE) starting at age 73. Failing to withdraw results in a penalty. Roth IRAs are exempt from RMDs during the owner's lifetime.

IRA Calculator

IRA Calculator: Compare Traditional, Roth, SEP & SIMPLE Growth

Quick Results: What This Tool Solves

Metric	Why It Matters
Projected Nest Egg	Estimates your total account balance at retirement age.
Tax-Adjusted Value	Calculates what that balance is actually worth after you pay retirement taxes.
Contribution Efficiency	Compares pre-tax (Traditional) vs. post-tax (Roth) efficiency based on your specific tax bracket.
RMD Forecasting	Highlights when mandatory withdrawals (RMDs) will deplete your Traditional or SEP IRA.

Understanding the Tax Wrapper Ecosystem

An Individual Retirement Account (IRA) is not an investment itself (like a stock or bond); it is a Tax Wrapper that changes how the IRS treats your money.

The choice between Traditional, Roth, SEP, and SIMPLE IRAs fundamentally comes down to a mathematical bet on your Current Tax Rate ($T_{now}$) versus your Retirement Tax Rate ($T_{ret}$). This calculator processes these “Entities” to simulate the long-term impact of compounding under different tax codes.

Who is this for?

Employees: Deciding between a tax deduction now (Traditional) or tax-free income later (Roth).
Small Business Owners: Evaluating high-limit vehicles like SEP or SIMPLE IRAs.
High Earners: Analyzing the “Backdoor Roth” viability when income limits are exceeded.

The Logic Vault: Mathematical Precision

To compare IRA types accurately, we cannot simply look at the final account balance. We must solve for Purchasing Power ($V$).

1. Roth IRA Formula (Tax-Free Withdrawal):

Since taxes are paid upfront, the future value is simply the standard annuity formula.

$$V_{Roth} = P(1+r)^t + C \left[ \frac{(1+r)^t – 1}{r} \right]$$

2. Traditional / SEP / SIMPLE IRA Formula (Tax-Deferred):

Here, the money grows faster (due to a larger initial principal if tax savings are invested), but the government takes a cut at the end.

$$V_{Trad} = \left( P(1+r)^t + C \left[ \frac{(1+r)^t – 1}{r} \right] \right) \times (1 – T_{ret})$$

Variable Breakdown

Symbol	Name	Unit	Description
$V$	Final Value	Currency ($)	The spendable cash available after all taxes are paid.
$P$	Principal	Currency ($)	Current account balance.
$C$	Contribution	Currency ($)	Annual deposit amount.
$r$	Rate of Return	Decimal	Expected annual growth (e.g., 7% = 0.07).
$t$	Time	Years	Years until retirement.
$T_{ret}$	Retirement Tax Rate	Decimal	Your expected effective tax rate in retirement.

Step-by-Step Interactive Example

Let’s solve the classic debate: Pay taxes now or pay taxes later?

Scenario: Mark, Age 30.

Annual Contribution: $6,000
Time Horizon: 30 Years
Growth Rate: 7%
Current Tax Bracket: 24%
Expected Retirement Bracket: 15%

Step 1: Calculate Traditional IRA (Pay Tax Later)

Mark gets a tax deduction now, but pays 15% later.

Gross Balance: $\approx \$566,764$ (Standard Annuity Growth)
Taxes Due (15%): $\$566,764 \times 0.15 = \$85,014$
Net Spendable Cash: $481,750

Step 2: Calculate Roth IRA (Pay Tax Now)

Mark pays 24% tax on his income before contributing. If he contributes the same $6,000 face value:

Gross Balance: $\approx \$566,764$
Taxes Due (0%): $0
Net Spendable Cash: $566,764

Note: This comparison assumes Mark contributes the same dollar amount to both. In reality, the Traditional IRA cost him “less” in net pay today. To be fair, he should invest the tax savings.

Information Gain: The “Reinvestment” Fallacy

Most calculators show the Roth winning (as seen above) because they ignore the Opportunity Cost of the Tax Deduction.

The Hidden Variable: Invested Tax Savings ($I_{tax}$).

With a Traditional IRA, Mark saved $24\%$ of $6,000 = **$1,440/year** in taxes today.

If Mark spends that $1,440 on lifestyle, the Roth wins.
If Mark invests that $1,440 in a brokerage account at 7%:
- The brokerage account grows to $136,000.
- After Capital Gains Tax (15%), that’s $115,600.
- Total Traditional Strategy Value: $481,750 (IRA) + $115,600 (Tax Savings) = **$597,350**.

Expert Takeaway: When you account for the reinvested tax deduction, the Traditional IRA actually wins for Mark because his tax rate dropped from 24% to 15%.

Strategic Insight by Shahzad Raja

In my 14 years of financial SEO strategy, I’ve seen the ‘Roth Obsession’ blind investors to simple math.

The ‘Tax Arbitrage’ Rule:

If $T_{now} > T_{ret}$: Choose Traditional/SEP. (Save taxes while they are high, pay when they are low).

If $T_{now} < T_{ret}$: Choose Roth. (Lock in the low rate now).

If $T_{now} = T_{ret}$: It is mathematically identical.

Don’t just guess. Use this calculator to define your brackets. If you are in your peak earning years (highest bracket), a Traditional or SEP IRA is often the mathematically superior tool.”

Frequently Asked Questions

What are the IRA contribution limits for 2025/2026?

For 2025, the limit for Traditional and Roth IRAs is $7,000 (plus a $1,000 catch-up for those 50+).

For SEP IRAs, the limit is much higher: up to 25% of compensation or $70,000 (whichever is less).

For SIMPLE IRAs, the limit is $16,500 (plus $3,500 catch-up).

Does a SEP IRA allow Roth contributions?

Historically, no—SEP IRAs were strictly pre-tax. However, the SECURE 2.0 Act now allows for Roth SEP contributions, though many custodians (banks/brokerages) are still updating their systems to support this. Check with your specific provider.

What are Required Minimum Distributions (RMDs)?

Traditional, SEP, and SIMPLE IRAs force you to start withdrawing money at age 73 (increasing to 75 in 2033). If you don’t withdraw, the penalty is severe (up to 25% of the amount not withdrawn). Roth IRAs do not have RMDs during the owner’s lifetime.

Can I contribute to both a 401(k) and an IRA?

Yes. You can max out your 401(k) ($23,500 for 2025) AND max out your IRA ($7,000). However, if you have a 401(k) at work, the tax deductibility of your Traditional IRA contribution may be limited if your income exceeds certain thresholds.

Related Tools

[Required Minimum Distribution (RMD) Calculator]: Forecast your mandatory withdrawals and tax impact.

[Roth IRA Calculator]: Focus specifically on tax-free growth projections.

[401(k) Calculator]: Compare your employer plan against self-directed IRAs.