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The Second Eroder: Tax on the Compounding Itself

Inflation is one leak; tax is the other, and it does more damage than most people expect because it attacks the compounding rather than the balance. Shelter the growth first and the return second — that ordering is what the entire tax-advantaged-accounts chapter is built on (chapter “Tax Advantaged Accounts”), and here is the arithmetic underneath it.

Compare one dollar earning $r$ for $n$ years under two regimes. If the gain is taxed annually at rate $τ$ — interest, non-qualified dividends, a fund distributing realized gains — only $r (1 - τ)$ survives each year to compound:

F V_{annual} = {(1 + r (1 - τ))}^{n}

If instead the gain compounds untouched and is taxed once on withdrawal:

F V_{deferred} = {(1 + r)}^{n} (1 - τ) + τ

Both pay the same rate. Deferral wins anyway, and the gap widens with every year and every point of volatility-free growth, because the annual version keeps removing the seed capital that would have compounded.

Run it at $r = 8 %$ , $τ = 35 %$ , $n = 30$ . Annual taxation compounds at $8 % \times 0.65 = 5.2 %$ , turning $1 into $1.05 2^{30} = $ 4.58$ . Deferral turns $1 into $10.06 \times 0.65 + 0.35 = $ 6.89$ — half again as much terminal wealth, from identical gross returns and an identical tax rate. Expressed as an effective compound rate, the taxable account delivered 5.2% and the deferred account 6.6%. The 1.4-point difference is not a fee anyone disclosed to you.

Three consequences worth carrying into the rest of the book. Tax drag is a rate reduction, not a one-time haircut, so it compounds against you exactly as inflation does — combine both and the real after-tax return is

r_{net} = \frac{1 + r (1 - τ)}{1 + π} - 1

which at 8% nominal, 35% tax, and 2.5% inflation is a sobering 2.6%. Second, the drag depends on turnover, not just on the rate: a broad index fund you never sell defers almost all of its gain by default and behaves far more like the second formula than the first, which is most of why index funds beat active funds after tax by more than they beat them before tax. Third, deferral is not forgiveness — $τ$ still arrives, and the full comparison of pre-tax versus Roth versus taxable, including the case where your future rate differs from today’s, is worked in section “Tax-Advantaged Accounts: Choosing Between Pre-Tax and Post-Tax Options”.

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