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How to Calculate After-Tax Return on Investment

Your 8% return isn't really 8% — the IRS takes a cut. Learn exactly how to calculate after-tax return for taxable accounts, 401(k)s, and Roth IRAs.

The Formula

After-Tax Return = Nominal Return − Tax on Gains − Tax on Dividends

Investment returns get taxed at different rates depending on account type and holding period. A 7% return in a taxable account might be only 5.5% after federal tax on dividends and capital gains. The same return in a Roth IRA is fully yours.

When to Use After-Tax Return

  • Comparing taxable vs tax-advantaged account returns
  • Deciding whether to hold investments in taxable or 401(k)/IRA
  • Calculating your true spendable retirement income
  • Roth conversion analysis

Worked Examples

Example 1: Stock in taxable account (held > 1 year)

Given
  • Nominal return: 8%
  • Qualified dividend yield: 2% (taxed at LTCG)
  • Long-term capital gains rate: 15%
  • Stock price appreciation: 6% (unrealized, no tax until sold)
Calculation
  1. Tax on dividends: 2% × 15% = 0.30% drag per year (assuming DRIP)
  2. If you sell at year-end: 8% × 15% = 1.20% one-time tax on full gain
  3. Average annual tax drag (assuming you sell after many years): ~0.5-1% per year
Result
After-tax return ≈ 7-7.5% per year (vs 8% nominal)
💡 Tax-efficient fund placement matters. Hold tax-inefficient investments (bonds, REITs) in 401(k)/IRA; hold tax-efficient investments (broad stock index funds) in taxable accounts.

Example 2: Tax-deferred 401(k)

Given
  • Nominal return: 8%
  • Tax bracket (in retirement): 22% federal
Calculation
  1. Tax paid on withdrawal: 8% × 22% = 1.76% drag
  2. But all growth is tax-deferred — you only pay tax on withdrawal
Result
Effective after-tax return ≈ 6.24% per year (assuming same 22% rate in retirement)
💡 The drag looks similar to taxable accounts, but 401(k) has advantages: pre-tax contributions reduce current-year taxes, and gains compound tax-deferred until withdrawal.

Common Pitfalls to Avoid

Pitfall 1: Ignoring tax drag in portfolio performance

A taxable account tracking an index fund might show 8% gross return but only 6.5% net return after taxes. Over 30 years, this 1.5% drag compounds to a 35%+ reduction in final wealth.

Fix: Track after-tax returns, not just gross returns. Use tax-loss harvesting and asset location (tax-efficient funds in taxable accounts).

Pitfall 2: Assuming tax rates stay constant

Current LTCG rate is 15% for most taxpayers. If you retire in a higher bracket, you'll pay 20% on gains. If tax laws change, all bets are off.

Fix: For retirement planning, use conservative tax rate assumptions (20-25%) regardless of current rates.

Pitfall 3: Confusing account types

Traditional IRA withdrawals are taxed as ordinary income. Roth IRA withdrawals are tax-free. 401(k) is taxed as ordinary income. HSA withdrawals for medical are tax-free.

Fix: Keep a clear chart of which accounts are taxable, tax-deferred, and tax-free at withdrawal. Plan withdrawals strategically to minimize lifetime tax.

Frequently Asked Questions

How much does tax drag cost long-term?

Roughly 1-2% per year for a typical taxable investment account. Over 30 years, a 1.5% drag on an 8% return reduces your final portfolio from $906K (pre-tax) to $603K (after-tax). That's a $300K+ difference for a $500/month contribution.

Which investments are most tax-efficient?

Broad stock index funds (VTI, VOO, FXAIX) are highly tax-efficient — low turnover, qualified dividends, low distributions. Municipal bonds are tax-efficient because their interest is federally tax-free. Tax-inefficient investments include REITs (high ordinary dividends), bonds (ordinary interest), and actively managed funds (high turnover).

Is a Roth IRA always better than a Traditional IRA?

Not always. If your tax bracket will be much lower in retirement than today, Traditional wins. If same or higher, Roth wins. For most people in their 20s-30s (in a lower bracket now than they'll be in retirement), Roth is usually better. Run numbers both ways.

Apply This to Real Numbers

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