The Value of Tax Deferral

We often hear the question “why should I contribute to a qualified retirement plan if tax rates might go up”?  Good question; here’s why:  you’ll probably end up with more money after tax.  That’s true even if tax rates go up in the future.

How much more you’ll end up with depends on your investment return, the deferral period, and your marginal tax rates at three different times:

• when you contribute the money,
• while it’s invested, and
• when you withdraw it.

The key is that in a taxable account, you lose some of the power of compound interest every time you’re taxed on your contributions and investment earnings.

Let’s start with a simple example, using a 33-1/3% marginal tax rate all the way through.  That’s a bit lower than the 35% top Federal rate, but it makes the math easy.  Suppose you have $3,000 to contribute, and investment earnings average 6%.

Tax deferred account / qualified plan

Contributing your $3,000 to a 401(k) or other qualified plan, you have the whole amount to invest and investment earnings are tax free – but you have to pay tax when you withdraw it.  Leaving it in for, say, 20 years you would have $6,414 after paying your tax:  $3,000 x (1.06 ^ 20) x (1-.3333).

Taxable account

Contributing to a taxable account, you have $2,000 to invest after tax ($3,000 x (1-.3333)) and investment earnings are taxable so your effective investment return is 4% (6% x (1-.3333)).  But then you’re done paying taxes.  After 20 years you would have $4,382:  $2,000 x (1.04 ^ 20).

What if’s:  rising tax rates, capital gains, return, deferral period, Roth

In this simple example, the qualified plan clearly beats the taxable account.  But what if tax rates are higher at withdrawal?  For the $4,382 in the taxable account to beat the qualified plan, the tax rate would have to suddenly jump to 54.5% at withdrawal:  $3,000 x (1.06 ^ 20) x (1-.545) = $4,378.   Any tax increase that happens more gradually would be worse for the taxable account, with no effect on the qualified plan.

What about capital gains?  If the current 15% long term capital gains rate is sustainable and all your investments qualify, your effective return is 5.1% (6% x (1-.15)).  You still start with $2,000 to invest after tax, so after 20 years you would have $5,408:  $2,000 x (1.051 ^ 20).  That’s not bad, but it’s still less than the $6,414 you would have had from a qualified plan.

What about different investment returns and deferral periods?  We’ve used 6% return for 20 years in this simple example, but how does it change for other returns and time periods?  The short answer is that higher investment returns and longer deferral periods favor the qualified plan.  Lower returns and shorter time favor the taxable account.

What about a Roth IRA or 401(k)?  As it turns out, Roth and regular 401(k) results are identical if your marginal tax rates are equal at contribution and withdrawal.  Roth is better if your marginal rate at withdrawal is higher than at contribution time; otherwise a regular 401(k) is better.  And they both blow the taxable account out of the water.