How much do taxes reduce my compound interest?

In a taxable brokerage account at the long-term capital gains rate of 15%, your final balance is reduced by roughly 15% on gains only (not contributions). Example: $500/month for 30 yrs at 8% = $746K total ($180K contributions + $566K gains). After 15% LTCG on gains = $746K − $85K = $661K. With 20% LTCG (high earners): $633K. In a Roth IRA, zero tax. In a traditional 401(k), full income tax on withdrawal (22-24% typical) = $568K net.

What is the difference between taxable account, Roth IRA, and 401(k) for compound interest?

Taxable: pay capital gains tax when you sell (0%, 15%, or 20% based on income). Dividend tax annually (15-20% qualified, ordinary rate non-qualified). Roth IRA: tax paid on contributions; ZERO tax on withdrawals or growth after age 59½. Traditional 401(k): tax-deferred — no tax now, full ordinary income tax on withdrawal (typically 22-24%). For most middle-income earners, the priority is: 401(k) match → Roth IRA → max 401(k) → taxable.

How do I calculate after-tax compound interest?

Run the standard compound interest formula A = P(1 + r/n)^(nt), then subtract tax. For taxable: A_after = P + (A − P) × (1 − tax_rate). Example: $10K at 8% for 30 yrs = $100K. Gains = $90K. Tax at 15% LTCG = $13.5K. After-tax = $86.5K. For 401(k) tax-deferred: A_after = A × (1 − marginal_tax_rate). For Roth: A_after = A (no tax).

Does compound interest include capital gains tax?

Standard compound interest formulas DO NOT include taxes. They show pre-tax growth. To get a realistic after-tax projection, you need to apply: (1) Annual tax drag on dividends (15-20% on qualified dividends), (2) Capital gains tax when you sell (0/15/20% LTCG or ordinary rate STCG), (3) State tax (varies 0-13.3%). Long-term, annual dividend tax drag costs 0.5-1% per year of CAGR — equivalent to a 15-25% reduction in final balance over 30 yrs.

What is the 2026 capital gains tax rate?

Long-term capital gains (assets held >1 year): 0% if income under $47,025 single / $94,050 married; 15% for most middle-income; 20% above $518,900 single / $583,750 married. Short-term capital gains: taxed as ordinary income (10-37%). Net Investment Income Tax (NIIT): additional 3.8% on investment income above $200K single / $250K married. State capital gains: varies by state — 0% in 9 states (FL, TX, WA, etc.), up to 13.3% in California.

How much can I save in taxes by using a Roth IRA vs taxable account?

On $500/month for 30 yrs at 8% ($746K final balance): Roth IRA = $0 federal tax (saves ~$85K vs 15% LTCG in taxable). Traditional 401(k) = $164K federal tax at 22% retirement bracket (worse than Roth IF your retirement bracket is the same or higher than your contribution year). Rule of thumb: Roth wins for young/low-income years; Traditional wins for high-income years if your retirement bracket will be lower.

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Compound interest calculator with taxes

Standard compound interest calculators show pre-tax fantasy numbers. This one shows the after-tax reality: $500/mo at 8% over 30 years grows to $746K pre-tax → $661K in taxable, $582K in a 401(k), $746K in a Roth. See your real outcome.

After-tax reality: $500/mo at 8% over 30 years

The compound interest formula A = P(1 + r/n)^(nt) gives you the gross growth. Taxes reduce that depending on the account type. Here's what $180,000 in contributions ($500/month × 30 years) becomes at 8% APY after different tax treatments:

Account type	Pre-tax	Tax	After-tax	Drag
Taxable brokerage (LTCG 15%) 15% LTCG on $566K of gains, contributions untaxed	$746,000	$85,000	$661,000	11.4%
Taxable brokerage (LTCG 20% + NIIT 3.8%) 23.8% on gains for high earners	$746,000	$135,000	$611,000	18.1%
Traditional 401(k) at 22% retirement bracket Full ordinary income tax on entire withdrawal	$746,000	$164,000	$582,000	22.0%
Traditional 401(k) at 24% retirement bracket Higher bracket retiree (often dual-income)	$746,000	$179,000	$567,000	24.0%
Roth IRA / Roth 401(k) Tax-free growth + tax-free withdrawals after 59½	$746,000	$0	$746,000	0%
Taxable + 1% annual dividend drag 0.5-1%/yr drag from annual dividend tax compounds over 30 yrs	$746,000	$130,000	$616,000	17.4%

How to calculate after-tax compound interest

Three different formulas depending on account type:

Taxable brokerage

A_after = P + (A − P) × (1 − ltcg_rate)

Tax applies only to gains (A − P), not contributions. LTCG rate: 0% / 15% / 20% by income bracket. Add 3.8% NIIT if MAGI > $200K single / $250K married.

Traditional 401(k) / IRA

A_after = A × (1 − retirement_marginal_tax_rate)

Full ordinary income tax on the entire withdrawal (including contributions that were pre-tax). Typical retiree bracket: 22% (single $44-95K) or 24% (single $95-182K).

Roth IRA / Roth 401(k)

A_after = A

Zero tax on growth or withdrawal after 59½. Contribution limit $7,000 ($8K at 50+) for Roth IRA in 2026; income phase-out at ~$150K single / $236K married.

The hidden cost of annual dividend tax (taxable accounts)

Most calculators ignore annual tax drag on dividends. Even if you don't sell, S&P 500 funds (VOO, VTI) pay ~1.4% in dividends taxed at 15-20% each year. That's a 0.21-0.28% annual drag, which compounds: over 30 years, it reduces your CAGR from 10% nominal to ~9.7-9.8% — roughly $40K less on a $500K final balance.

Solution: hold dividend-paying funds in tax-advantaged accounts (Roth IRA, 401(k)) and growth-oriented funds in taxable accounts.

State tax matters too

9 states have zero capital gains tax: Alaska, Florida, Nevada, New Hampshire, South Dakota, Tennessee, Texas, Washington, Wyoming. If you live in California (13.3% top rate), your real after-tax outcome is much worse:

California (13.3% state): $746K pre-tax → ~$549K after fed+state — 26% drag
New York (10.9% state): $746K → ~$574K — 23% drag
Texas / Florida (0% state): $746K → ~$661K — 11% drag (same as federal-only)

Sources: IRS Publication 550 (Investment Income), Topic 409 (Capital Gains), Federal Income Tax Rates & Brackets. State tax data from individual state revenue departments. Last updated: 2026-06-01.

By Snowballr Research Team

Last reviewed June 1, 2026Fact-checked against primary sourcesEditorial standards

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