Funny Numbers – The Rule of 72

The Rule of 72

After looking at the magic of compound growth in the last post, the Rule of 72 is another easy way to figure out what compounding does. Back in the 1400s, an Italian friar, Luca Pacioli, came up with this neat little rule.

Here’s how it works …

Divide 72 by the rate of return percentage on your investment to find out how many years it will take for your money to double. If you have an annual rate of return of 10%, divide 72 by 10 & your money will double in 7.2 years. A return of 3% means it’ll take 24 years for the double. One percent means it’ll take 72 years & so on. It’s not as accurate as a proper compound growth calculator, & it’s not accurate at extremes. But it’s close enough for quick mental math while you’re chatting about your amazing portfolio performance with your buddies over a latte. Of course, like many things in the investing world, there is opportunity for it to mislead us.

You could use the historical average rate of return of the American stock market to calculate what you might earn going forward, for example. That historical 10% rate of return may, or may not, continue into the future. But hoping that an index fund will double our money, on average, every 7.2 years is not a bad assumption to justify going into a low-cost index-tracking fund. Especially for anyone with a very long time to go before retirement. There are other funds out there with big yields. Many of them exceed that 10% market return rate. Wow! A 10 or 15% yield and the potential for capital appreciation, are you kidding me? The funds aren’t but, sometimes, we kid ourselves. A 15% distribution may come with a declining share value over time, for example. You can’t automatically assume that the 15% yield number will double your money every 4.8 years (72 divided by 15), forever. If it did, we’d all be in that one!

If the share value has a downward trend, the 15% yield delivers an ever-downward amount of distribution too. When comparing two funds, regardless of the distribution percentage, it is important to understand the total return potential over time. The yield percentage is not the total return. There may be a time & place that will work for some high yield exposure in a portfolio. But it’s good to understand what you’re getting into. Whenever you are tempted by a high yielding fund, compare the historical returns against a market fund or your favourite ETF. Don’t mistake yield for return. A 15% yield doesn’t always, indeed seldom does, give you back all your money in 4.8 years & then go on to give market beating performance thereafter. While historical performance of any fund is no guarantee of future performance, comparing the total return percentage is a more useful metric for comparing two such different strategies.
And that total return percentage is the number you need to plug into the Rule of 72.

Use the online tools at sites like Portfolio Visualizer, StockCharts & the fund comparison tools at the BMO & Vanguard Canada sites to get a bigger & better picture of the historical performance of different funds. And use the Rule of 72 for total return estimates.
Though you can use it for dividend growth rates too. But that’s a story for another day.

PS … If you do know of a fund that delivers 15% total return consistently, please message me. And don’t tell anyone else ’til I rejig my portfolio!

Important – this is not investing, tax or legal advice, it is for entertainment & educational purposes only. Do your own due diligence & seek professional advice before investing your money.

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