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Roth IRA vs Traditional IRA vs 401k--here's the math behind deciding which retirement account is best for you and your long-term financial success.

It’s without a doubt the most frequent question I receive from readers of this blog. People want to know whether they should put their retirement savings in a Roth or traditional retirement account.

I’ve written a lot about these retirement options. If you are new to retirement accounts, I’d suggest you start by reading some of the following articles:

The Ultimate Guide to Roth and Traditional 401(k) and IRA Retirement Accounts

401k and IRA Contribution and Deduction Limits for 2014

How to Roll Over a 401k to an IRA

401k vs IRA–Where Should You Save for Retirement

a 5-Step Plan to a Perfect Retirement Savings Plan

Today we are going to look specifically at whether a Roth or traditional (pre-tax) retirement account is best.

Podcast of this Article

It’s All About Taxes (Mostly)

With some exceptions, the key issue when deciding on a Roth or traditional retirement account is taxes. Are the taxes you’ll save today by making a pre-tax contribution to a traditional IRA or 401k higher or lower than the taxes you’ll pay when you withdraw the money in retirement?

Particularly for those who are many years away from retirement, this question is almost impossible to answer with any degree of confidence. As you consider this question, keep in mind the following:

  • Your current federal marginal tax bracket: Your marginal tax rate is typically the percentage you’ll save in taxes if you make contributions to a traditional retirement account (the contribution could move you into the next lowest tax bracket, splitting your tax savings from the contribution into two different percentages).
  • Your state and local income taxes: We can’t forget about the implications of this decision as it relates to state income tax.
  • Taxes in the place you plan to retire: This may be an unknown for many. But if you do know where you’ll be living when you begin taking withdrawals from your retirement account, this information can prove helpful in making the decision between a Roth and a traditional account.
  • Your view of future tax rates: I have no clue what our taxes will look like decades from now, but many have very strong opinions on this topic. Personally, I don’t make my decision on the type of retirement account on my view of future tax policies, but some do.
  • Your tax rates before you retire: As you’ll see in the section below on Roth conversions, there is a strategy to put away pre-tax dollars during your working years and then slowly convert the account to a Roth IRA later when your income places you in a lower tax bracket.

Roth vs Traditional–General Rule of Thumb

Let’s first look at the general rule of thumb that I follow. Then we’ll talk about the limitations of any general rule.

General Rule of Thumb

As a general rule, I look to my marginal tax rates for federal income tax as a starting point–

  • Lower two federal tax brackets (10 and 15%) -> Roth
  • Middle tax brackets (25, 28 and 33%) -> Roth and/or Traditional
  • Highest two tax brackets (35 and 39.6%) -> Traditional with eye toward Roth Conversion later

Now to the limitations. As noted above, a lot of factors go into the decision of a Roth or traditional retirement account. The focus above is on federal income tax. This is in part because state income taxes range from none to extremely high taxes. It’s important to consider your local tax situation when making this decision.

Also, there is a wide range within the middle tax brackets, from 25% to 33%. On the lower end of this range, I lean toward a Roth account. On the higher end I lean toward a traditional account. Regardless, keep in mind that it’s not all or nothing. You can choose to divide your contributions between a Roth and traditional account.

Finally, the importance of the Roth conversion option cannot be overstated. We’ll look at that below.

Traditional IRA or 401k vs. Taxable Accounts

Now it’s time to look at the math. To start, let’s first cover why a traditional retirement account is preferable to a taxable account.

Here we’ll assume a $10,000 investment in a pre-tax 401k versus the after-tax equivalent of $7,500 in a taxable account. As you can tell from the numbers, we are assuming a marginal tax rate of 25%. We’ll also assume that both accounts are invested the same way, and that over a period of time the investment grows 4x.

We’ll assume that the entire balance from both accounts is withdrawn at once in retirement and that no taxes are paid on the taxable account until the money is withdrawn. (No, these assumptions are not entirely realistic, but they are close enough for our purposes.) Finally, we’ll assume a capital gains tax rate of 20%.

401k Account

Initial investment: $10,000
Account grows to: $40,000 ($10,000 x 4)
Taxes paid on withdraw: $10,000 (25% of $40,000)
After-tax balance: $30,000

Taxable Account

Initial investment: $7,500 ($10,000 less 25% in taxes paid)
Account grows to: $30,000 ($7,500 x 4)
Taxes paid on withdraw: $4,500 (($30,000 – $7,500 (initial investment)) x 20%)
After-tax balance: $25,500

In short, tax-advantaged accounts can add a lot to a retirement nest egg. Now let’s turn to the Roth vs. traditional retirement account analysis.

Roth vs. Traditional IRA or 401k (Don’t Max Roth Contributions)

In this example, we’ll assume that we don’t have enough to max out a Roth retirement account. This is an important assumption, as you’ll see in a minute. We’ll use the same numbers we used above.

Traditional 401k

Initial investment: $10,000
Account grows to: $40,000
Taxes paid on withdraw: $10,000
After-tax balance: $30,000

Roth 401k

Initial investment: $7,500
Account grows to: $30,000
Taxes paid on withdraw: $0
After-tax balance: $30,000

In this example given our assumptions, there is no difference between the two accounts. If I were faced with this situation, I’d select the Roth option. With a Roth, there is no Required Minimum Distribution. This means I can allow the money to sit in my account indefinitely after I reach retirement age, a significant advantage.

If, contrary to this example, we assumed that our tax rate was lower during our working years than in retirement, the Roth would be the better choice. If we assumed that our tax rate was higher during our working years than in retirement, the traditional retirement account would be the better option.

Roth vs. Traditional IRA or 401k (You DO Max Roth Contributions)

Now if we change the assumption so that we can maximize the Roth, the numbers change dramatically. The analysis also gets a bit more complicated. Here’s why.

To compare apples to apples, we must calculate how much income we must earn to generate $17,500 (the current 401k contribution limit) in after-tax income for the Roth. Why? So that we can use the same figure for our traditional 401k comparison.

The 401k contribution limit is the same regardless of whether it is a traditional or Roth account. As a result, we will need to compare a Roth with a combination of traditional (up to the $17,500 contribution limit) and a taxable account (for the excess). Hopefully the math will help make this more clear.

Assuming a marginal tax rate of 25%, how much do we need to earn pre-tax to generate $17,500 in after-tax dollars? The math is simple: $17,500 / (1 – .25) or $17,500 / .75 = $23,333. With this number in hand, let’s compare the two options starting with the Roth

Note: 401k contribution limits can change every year. This example uses the limits set a few years ago just for the sake of comparison. For current contribution limits, check out this article.

Roth 401k

Initial investment: $17,500
Account grows to: $70,000 ($17,500 x 4)
Taxes paid on withdrawal: $0
After-tax balance: $70,000

The Roth analysis is simple. Now let’s look at the traditional 401k equivalent

Traditional 401k

Initial investment (401k): $17,500
Account grows to: $70,000
Taxes paid on withdraw: $17,500
After-tax balance: $52,500

Initial investment (taxable): $4,374.75 ($23,333 – $17,500 – 25% in taxes)
Account grows to: $17,498 ($4,374.75 x 4)
Taxes paid on withdraw: $2,624.65 (($17,498 – $4,374.75) x 20%)
After-tax balance: $14,873.35

Total: $52,500 + $14,873.35 = $67,373.35

So why did the Roth come out on top just because we maximized our contributions? The reason is that with a Roth, 100% of our retirement nest egg belongs to us. With a traditional account, some portion belongs to Uncle Sam. True, we may not have to pay the taxes for decades. But eventually, we will have to pay the taxes. While that does give us some tax savings initially ($5,833 in our example), we must pay taxes on that amount and pay taxes on any gains we earn from investing that amount.

The result is what I call the Roth Advantage. If you max out your contributions and believe your taxes will remain unchanged now and in retirement, the Roth wins.

Roth Conversion

One available option is the conversion of a traditional IRA into a Roth IRA. You can also convert a traditional 401k to a Roth 401k in some plans. And even if you can’t, when you leave your job you can roll over the 401k to an IRA and then convert it to a Roth. This option has significant implications, particularly for this in the higher tax brackets during their working years.

For example, let’s assume you are in the 39.6% marginal tax bracket during your working years. As a result, you are contributing to traditional retirement accounts given the significant tax savings.

Let’s further assume that before you begin taking Social Security and Required Minimum Distributions from your pre-tax retirement accounts, you retire early. The result is that your taxable income would drop significantly, placing you in much lower tax brackets. It’s during these early retirement years that strategic conversions to a Roth could save you a bundle in taxes. You’ll have to pay taxes on the conversion, of course. But the rate will be significantly less than the 39.6% saved during your working years.

An important decision will be how much to convert each year. You don’t want to convert so much in one year that it places you back into the higher tax brackets. Working with a tax professional can help you map out an ideal plan.

Backdoor Roth

Finally, I want to mention the Backdoor Roth strategy. For those whose income prevents them from either deducting a traditional IRA contribution or making contributions to a Roth IRA, the Backdoor strategy is ideal.

The strategy is simple to execute. You make after-tax contributions to a traditional IRA and then convert it to a Roth. While your contributions don’t enjoy any tax benefits, the investment grows tax free. You can get more details on this strategy in my article on How to Use the Backdoor Roth IRA Strategy.

Author Bio

Total Articles: 1080
Rob founded the Dough Roller in 2007. A litigation attorney in the securities industry, he lives in Northern Virginia with his wife, their two teenagers, and the family mascot, a shih tzu named Sophie.

Article comments

brian says:

Well done. Simple, straight forward and easy to understand.

One great advantage to the Roth conversion strategy not mentioned is that it gives you options.

Lets say you convert in January 2014. The taxes are not due until April 2015, and; if you file and extension you have until October 15, 2015 to decide whether to keep the conversion or put the money back. The tax is still due and payable by April 15th. However, that gives you a total of 22 months for the Roth conversion to grow and offset the tax bill.

If the market drops and your conversion investment along with it, you can put the money back and not have to pay taxes on an investment thats lost value. You could also make two conversions one fixed and one equity. Using separate accounts one for the fixed and another for the equity. Which ever account does best is kept and the other is put back.

It should also be stated that any decision to convert to a Roth requires the ability to pay the tax; otherwise the IRS will fine and charge interest thus defeating the desire to reduce taxes.

I would strongly recommend working with a financial advisor or tax accountant to make sure everything gets done correctly.

Perhaps Rob will provide an article expounding on the ins and outs of this strategy. There are several other components that I’m not mentioning here.

Rob Berger says:

Brian, excellent point on the recharacterization. I should prepare a podcast on that topic on how to take advantage of it. Thanks for mentioning it here!

Jamax says:

The Traditional IRA’s RMD is based on the Fair Market Value at year end. The original contribution of the Traditional IRA’s plus any gains in it are taxed. However, when you take a distribution from your Roth IRA there is no tax on the amount contributed as well as on any of its gains. The tax cost difference between the two types of IRAs could be very significant in favor of the Roth IRA.

Therefore doesn’t the Roth investment, which grows tax free, always come out ahead assuming both investment had the same rate of return?

Rob Berger says:

No, the Roth doesn’t always come out ahead. The key is to understand your tax rates when you make the contribution versus your best estimate of the tax rates you’ll when you make withdrawals. If, for example, you are in the 35% tax bracket when you make the contributions and only 15% in retirement, a traditional IRA comes out well ahead of a Roth.

Daniel says:

Very thorough analysis. This year I am in the 25% tax bracket and plan to contribute enough into my Traditional 401K to get into the 15% tax bracket. I will max out a Roth IRA. I enjoy life, but live quite frugally, in the spirit of MMM. I am on track to save roughly 37% of my full salary this year (45% if you factor in my home loan principal payments).

My reasoning for the Traditional 401K is that because of my frugal living I should be able to stay within the 15% tax bracket (or like bracket) at retirement. The Roth provides some additional flexibility should I need more money in a given year.

Obviously what tax rates do in the future is a question no one has an answer too, but does my reasoning seem logical?

Rob Berger says:

Daniel, it sounds like you are doing great and have made a reasonable decision based on the information you have.

Matt says:

Thanks for the explanation. Most articles on investment accounts don’t stress the benefit of not paying capital gains tax on these accounts. Then they reach the conclusion that taxable accounts are better under certain circumstances.

I thoroughly enjoy the podcast and blog. One thing that I keep thinking about in these Roth vs tIRA conversations is the net contribution amount. You did a nice job laying that out. The reason that the Roth comes out ahead is the 15% capital gains tax. If you do the calculation without the capital gains tax, it’s a wash. They come out exactly the same. So if you’re in the 15% marginal tax bracket and pay no capital gains, there’s no difference. I agree, that then gives the Roth the advantage as there are no RMD’s.

Brian says:

Why would anyone put the $4,374 into a taxable account. In the 25% tax bracket you will be eligible for a Roth IRA which will grow tax free. This will also limit your taxable income when you withdraw from your traditional 401k. Take that into account and the difference is $2.
The initial contributions to the Roth 401k or the tax saving for the traditional 401k are being calculated from the marginal rate. The traditional 401k withdrawals will most likely be taxed at the effective tax rate which is much lower. Also if a person lives in NYC and retires to TX where there is no state tax. These are things to consider too.

Justin says:

I think your comparison between the traditional and Roth 401k’s is unfair. You are assuming the same contribution but you have to make another $5833 in pretax to pay for the same post-tax Roth 401k contribution. Therefore, I think you need to compare the Roth 401k against a traditional 401k plus an additional $4375 in after tax investments. That will give a more accurate result on investing the same amount of income. I think this will give you an extra $14,875 in after tax income when you withdraw it. (Sorry if these numbers are slightly off. I just did a quick calculation in my head.)

Justin says:

Sorry. Just saw that you did that when I scrolled back up to compare the numbers. You can delete these comments. Good work!

BillC says:

The 401k vs Roth math. I see one area the math here doesn’t cover; the effective 2017 tax rate that my 401k money is taxed. When I retire my salary goes to zero, so when I take out my 401k $40,000, the first ~$10k is taxed at 10% ($1k), next $18k at 12%($2.2k), next $12k at 25% ($3k). My tax is $6,200 on the $40,000 withdraw. I would think most people would have less total income once retired. I’m not seeing the benefit of the Roth even with young kids who don’t make enough money to save for a retirement. Once they start making enough money to pay their bills, they’ll be in the 25% taxable range, pushing them toward the 401k pre-tax…. IMHO.

My math (401k vs Roth)
401k – $33,800
Roth – $30,000

SEN says:

I am an advisor who gets asked this as well. I agree with the most of what Rob says here. I will add that I have put together a spreadsheet to do an analysis of the roth vs traditional 401k conundrum. The results are nearly same for all but the lowest tax brackets. When comparing investing in a roth and paying the taxes now vs investing in a traditional 401k and paying taxes on RMDs, the Roth option does not become the more valuable strategy until one is in their late 80’s.