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Suppose I have a high-interest savings account that earns 2% interest. Suppose also that the stock market will grow 5% per year. It seems you'd have to choose to divide your money between the two and settle for an at-most 5% gain on your investments, but why not have both? Consider the following scheme:

Invest all your money in stocks, except for the minimum amount required to hold the savings account without penalties (keep this amount in the savings account at all times). Two days before the "interest-earning day" of each month, sell all stocks and deposit the money temporarily in the savings account. You will then earn 2.5% (divided by 12) interest on all that money. Immediately after the interest is earned, reinvest everything in the stock market. Repeat this process each month. This way you earn interest as if the money was sitting in savings, but simultaneously have it invested in the stock market for most of the year. Does this scheme work?

The main downside to this that I can think of is that since it takes a few days to buy and sell stocks, you're losing a few days of having your money invested in stocks each month, hence you don't get the full 5% each year from stocks. But supposing your money was not invested in stocks for 6 days each month, then your money is invested in the stock market for about 80% of the year, so you earn about 4% from the stock market instead of 5%, plus the 2% interest, for a total of 6% instead of the 5% from stocks alone. Still seems like a sensible deal, no?

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    Better yet - one day before stocks pay dividends, withdraw all money from savings account, and invest all in stocks. Next day, sell it. Hint: that would not work either. – void_ptr Oct 17 '19 at 22:30
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    Sorry but there are no free lunches. But if you're lucky, you'll get a free toaster :->) – Bob Baerker Oct 18 '19 at 0:28
  • A $7000 margin deposit will hold a Mini S&P500 futures contract. The size of the contract is about $149300. The $142300 not holding the futures contract can be in a savings account. When there is a margin call then more funds must be wired to the futures account. – S Spring Oct 19 '19 at 1:05
  • I see this got voted to -2. I'm voting it up, since this is a reasonable question, and an interesting one, even though the answer is "no." – Tanner Swett Oct 19 '19 at 22:40
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No. Virtually all savings accounts pay interest based on what your average balance was over the course of the month, not based on what it was at one particular moment in time.

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  • Ah, what a shame. – WillG Oct 18 '19 at 0:11
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    " pay interest based on what your average balance was over the course of the month" Do you have a citation for that? Because I think some banks with daily compounding calculate the daily interest and accrue it to your account monthly. – RonJohn Oct 19 '19 at 0:57
  • Those two are almost mathematically equivalent. And pretty close in practice. I just calculated the interest for an account earning 2% APR with $1000 in it, but a single day of $50,000 balance, and the difference in accrual methods was about four cents per month. – Shawaron Oct 21 '19 at 14:24
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Besides the fact that bank accounts don't earn interest that way you are ignoring many other aspects.

  • Fees. If that buying and selling costs you money you have to figure that in your math.
  • Taxes. If you sell for a profit you can trigger taxes being due.
  • Missing out. There was a question recently about missing a few days a year could cost you a lot of money. Your days out of the market might miss the best days or the worst days. Your assumption that 80% invested only costs you 20% of the increase might or might not be true.
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    Your first two points are good, but your third is not. That's like saying "Well, the lottery ticket that you passed on buying might have been the winner". – Acccumulation Oct 19 '19 at 0:36

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