Most people have a mix of stocks and bonds in their retirement accounts. Stocks are expected to grow faster than bonds in the long run. If you put your stock investments in a tax free account and your bonds in a pre-tax account you'll come out a bit ahead in retirement.
Lets illustrate this with an example. Pretend you've got $100,000 split evenly between Roth and 401k accounts with $50,000 in each. Now say you've got a 50 stock / 50 bonds allocation. That would give you $25,000 in stocks and bonds in each account. Now say your stocks grow 10% over 20 years and the bonds grow 5%. Assume you've got a 33% tax rate on your 401k withdrawals.
Roth 70/30 | 410k 70/30 | |
stock | $25,000 | $25,000 |
bond | $25,000 | $25,000 |
10% for 20yr | $168,187 | $168,187 |
5% for 20y | $66,332 | $66,332 |
total | $234,520 | $234,520 |
tax | $0 | $77,392 |
after tax | $234,520 | $157,128 |
TOTAL | $391,648 |
Now instead put all the stocks in the Roth and all the bonds in the 401k and you get :
Roth 70/30 | 410k 70/30 | |
stock | $50,000 | $0 |
bond | $0 | $50,000 |
10% for 20yr | $336,375 | $0 |
5% for 20y | $0 | $132,665 |
total | $336,375 | $132,665 |
tax | $0 | $43,779 |
after tax | $336,375 | $88,885 |
TOTAL | $425,260 |
In the end you've got about 8.5% more money simply due to allocating your investments per the tax advantages.
Note: I'm not saying that you should *only* have stocks in the Roth and bonds in the 401k. In the above example its set up that way just to illustrate. If you had a 70/30 split allocation then you'd still need some bonds in your Roth. Further, reallocating periodically to keep a specific % asset allocation isn't possible if all your stocks are in one and all the bonds are in the other.
Admittedly this is pretty contrived example, but I did that to illustrate the point. Its more likely you'd have money invested periodically annually over a long period rather than starting with a lump sum and its also likely your effective tax will be lower.
I didn't come up with this idea, but I can't recall where I saw it.
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