The 'relative strength indicator': A useful tool for timing trades

Knowing when to buy or sell an asset can be as important as knowing how cheap or expensive it is. Here, Tim Bennett explains the momentum indicator that tells you when to trade.

Updated October 2019

At MoneyWeek, we are big fans of value investing buying stocks, sectors or even entire markets when they are cheap, as indicated by key ratios such as price/earnings (p/e) or dividend yield. But when it comes to timing a decision to buy or sell, momentum' indicators can also be useful. One such popular measure, highlighted by Big Picture blogger Barry Ritholtz, is the relative strength indicator (RSI).

What is the RSI?

The RSI calculation is fiddly, but you don't need to do it yourself any good charting package will do it for you. However, it is worth understanding how it is put together, so that you know what you're using. The basic calculation is: 100 (100/(1+RS)). RS represents the average gain over a selected number of trading periods, divided by the average loss over those periods.

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Here's a simplified example. Let's say that over a given period the FTSE 100 gains an average of 50 points per winning session, and loses an average of 20 points per losing session. The RSI is 100 (100/(1+50/20)), which is 71.

The closer to 100 the reading is, the more overbought the index is. The closer to 0, the more oversold. Measured over a number of sessions (to smooth out the data), an RSI over 70 puts you in overbought territory and a reading below 30 is oversold, according to J Welles Wilder, who came up with the measure in the 1970s.

As with all measures, there are some pitfalls to be aware of. One is the number of trading periods used to measure average gains and losses Wilder recommended at least 14. As a rule, the more sessions you use, the greater the smoothing effect and the less likely the RSI is to hit overbought and oversold levels.

So, the more volatile the share or index, the further you should extend the calculation, to avoid constant buy' and sell' signals that could have you wipe out most of your trading gains in trading costs.

Tim graduated with a history degree from Cambridge University in 1989 and, after a year of travelling, joined the financial services firm Ernst and Young in 1990, qualifying as a chartered accountant in 1994.

He then moved into financial markets training, designing and running a variety of courses at graduate level and beyond for a range of organisations including the Securities and Investment Institute and UBS. He joined MoneyWeek in 2007.