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At what interest rate should debt be used as a tool?
Average return on the S&P500 over the last 10 years has been 1.6 %; so if you'd invested in that with money borrowed at 3 % you would have lost (so far). Investing with borrowed money implies you think you can beat the market: that you're a cleverer investor than whoever decided to lend you the money. Whoever decided to lend you the money decided that you are the best (return/risk ratio) investment for their money. It might make sense to invest borrowed money if you don't need to pay it back if things go wrong: if you're an investment professional whose bonus depends on the profit you make, but who won't need to repay any loss. It might also makes sense to borrow money if you're going to 'add value', e.g. sweat equity: for example if you use it to renovate a house or (if you're a business) to hire more staff. But the question was "What guidelines do you use" and the answer is, "I don't make passive investments with borrowed money." My Dad did it, i.e. didn't repay his mortgage as soon as he could have: but that was because (back in the '70s) he had a long-term (government-sponsored) mortgage for about 1.5 % (designed to help first-time buyers or something like that), at a time when banks were paying higher interest rates on (ultra-safe) deposits.