My broker offers a margin rate of 0.66% pa. Given that the avg. growth rate of the S&P500 is 5% it looks like good opportunity to make use of the low margin rate and borrow a bit to increase the performance of the portfolio. Of course the down side is that in a crash, we lose more, therefore our margin has to be small. But how small/big should it be. If we have 100k in the S&P500, should it be ok to borrow 20k extra so we work we will have 120k invested in the S&P. such things should add up over 10-20 years to significant differences in performance. Or am I missing something?