RSI Indicator: How It Works and How It Is Misread

The Relative Strength Index measures how one-sided recent price movement has been, on a scale from 0 to 100. That is all it does, and almost every mistake made with it comes from expecting it to do something else.

It was published by J. Welles Wilder in 1978, and his own use of it was considerably more careful than the way it is usually taught.

The calculation

RSI = 100 − (100 ÷ (1 + RS)), where RS = average gain ÷ average loss over the period.

With the default 14 periods, the calculation looks at the last 14 bars, averages the gains on up days and the losses on down days, and expresses the ratio on a scale of 0 to 100.

What falls out of that arithmetic: RSI 100 would require 14 consecutive up periods with no down periods at all. RSI 50 means gains and losses have been roughly balanced. RSI 70 means recent gains have substantially outweighed recent losses.

That last one says nothing about value, nothing about whether the move has gone too far, and nothing about what happens next. It is a description of what already happened.

Why “overbought” loses money

The most common RSI strategy is to sell above 70 and buy below 30. In a ranging market this works acceptably. In a trending market it is close to a reliable way to lose.

Strong uptrends hold RSI above 70 for weeks at a time. That is the indicator correctly reporting that gains have consistently outweighed losses, which is the definition of an uptrend. Selling each overbought reading means shorting strength repeatedly, and every one of those trades is against the dominant flow.

Market condition	RSI above 70 means	Correct read
Strong uptrend	Buyers are firmly in control	Confirmation, not a warning
Established range	Price is near the top of the range	Potential fade, with a level
Late in an extended move	Momentum is stretched	Tighten stops, stop adding

The determining variable is whether there is a trend, which you establish from price structure before ever looking at the oscillator.

Divergence: the signal worth learning

Divergence is when price and RSI disagree, and it is far more informative than a threshold reading.

In a bearish divergence, price makes a higher high while RSI makes a lower high. The new high was achieved with less momentum than the previous one: fewer participants, less conviction. In a bullish divergence, price makes a lower low while RSI makes a higher low, so the new low came with less selling force behind it.

Failure swings

Wilder’s own preferred signal, and one almost nobody uses.

In a bearish failure swing, RSI rises above 70, pulls back, rallies again but fails to exceed its previous peak, then breaks below the pullback low. The point is that the signal is generated entirely within the indicator, and it requires RSI to break its own structure rather than merely reach a number.

The bullish version mirrors it below 30. Both are stricter than a threshold cross, which is exactly why they generate fewer and better signals.

Using RSI sensibly

1. Establish the trend first, from price. RSI is interpreted differently in a trend than in a range.
2. In a range, an RSI extreme at a level you already marked is a reasonable fade. The level is the reason; RSI is corroboration.
3. In a trend, use RSI for divergence and for pullback timing. RSI dipping to 40 in an uptrend is often a better entry than RSI at 30 in a downtrend.
4. Never trade the number alone. RSI 72 with no level and no structure is arithmetic.

Next: MACD for a trend-following counterpart, and moving averages for the simplest trend filter of all. Size every resulting trade with the position size calculator.