RSI — how to read this indicator and when it fails

RSI is probably the most widely used indicator among individual traders — and the one most often read wrongly. The sentence "RSI 70 is a sell signal, RSI 30 is a buy signal" turns up in almost every beginner's guide, and it costs people real money. Below we explain what the indicator actually measures, when it helps, and when it walks you straight into a stop loss.

What does RSI actually measure?

RSI (Relative Strength Index) was described by J. Welles Wilder in "New Concepts in Technical Trading Systems" in 1978. Despite the name it does not measure the strength of one currency against another — it measures the strength of recent price moves on a 0–100 scale.

Simplified formula: RSI = 100 − [100 ÷ (1 + RS)], where RS = average gain over the last N periods ÷ average loss over the last N periods. Default N = 14.

What it means in practice:

• RSI around 50: the gains and losses of recent sessions roughly balance out. A neutral state.
• RSI above 70: gains have clearly dominated over the recent periods. The move up is strong.
• RSI below 30: losses have dominated. The move down is strong.
• RSI rising from 50 towards 70: the upward move is gathering pace.
• RSI rising from 70 towards 90: the upward move is still accelerating, not fading.

The "RSI 70 = sell" myth

When you hear that a reading of 70 means overbought and it is time to sell, you are dealing with a shortcut that only works in a range. In a strong trend RSI can hold between 70 and 90 for weeks on end:

The mechanism is simple: in an uptrend successive higher highs produce a run of bullish candles. RSI jams in the 70–90 band and has no way of falling until price starts giving ground. This is not a sign of overvaluation — it is a sign of strength. Trading against the trend on the strength of RSI alone is a classic trap.

First real use: RSI 50 as a trend filter

The simplest and at the same time the strongest use of the indicator:

• RSI above 50 on D1 or H4: the market is in an uptrend. Look only for opportunities to go long.
• RSI below 50 on D1 or H4: the market is in a downtrend. Look only for opportunities to go short.

This is a filter, not an entry signal. It tells you only which way the wind is blowing. You look for the entry itself somewhere else — on a reversal candle, at a support or resistance level, in the behaviour of price. RSI above 50 on the daily chart is then confirmation: if the chart says buy, you are going with the wind rather than into it.

Second real use: divergence

Divergence is a mismatch between price and RSI. You will meet two classic kinds:

Bullish divergence

Price marks a lower low, but RSI stops at a higher low than before. It means the market went lower while the move itself was weaker. Sellers are losing momentum, so a reversal upward becomes plausible.

Bearish divergence

Price marks a higher high, but RSI finishes at a lower high. The market reached further, only on a weaker move. Buyers are losing momentum, so a reversal downward becomes plausible.

Divergence on its own gives only a small edge — on H4 and D1 it ends in a false signal often enough that no strategy should rest on it. It works best as confirmation of a signal from another source. The classic arrangement looks like this:

1. Price reaches strong support on the daily chart
2. A reversal candle forms (hammer, bullish engulfing)
3. RSI shows a bullish divergence
4. All three conditions met — that is a solid case for a long position

One signal is not enough. Two that agree is already a decent case. Three that agree is about as much as a chart realistically offers.

"The Relative Strength Index does not measure the strength of one market against another, but the internal momentum of price itself on a scale from zero to one hundred." — J. Welles Wilder, New Concepts in Technical Trading Systems, 1978.

When RSI fails — and what to do

The indicator has three classic weaknesses:

1. A strong trend. RSI stays in the 70–90 or 10–30 band for weeks, and every counter-trend signal ends in a run of stop-outs.
2. Consolidation ahead of a data release. RSI oscillates around 50 and shows nothing useful. Wait for the number instead of trying to front-run it with an indicator.
3. Low intervals, M5 and M15. There the reading is random noise in practice. Work on H1 at the minimum, ideally on H4 or D1.

The practical conclusion: do not use RSI in isolation. Combine it with trend (SMA 200 or chart structure), with support and resistance levels, with reversal candles. RSI is one element of a kit, not the entire kit. If you are wondering whether it is even possible to profit from indicators alone, the answer is more nuanced than the advertisers would like.

What to do tomorrow with RSI

1. Add RSI(14) on D1 and use it only as a direction filter. Before you look for any entry, check whether the indicator is above or below 50 on the daily chart and trade only that way. Stop treating 70 and 30 as sell and buy buttons — in a strong trend they are signs of strength, not exhaustion.
2. Demand several signals that agree, never a lone one. Combine RSI divergence with a clear level of support and resistance and a reversal candle; only when all three align do you have a genuinely strong case, and when just one appears, wait. A divergence by itself gives too small an edge to carry a strategy.
3. Move up the timeframe and abandon M5 and M15 for reading RSI. On those intervals the indicator is random noise that only multiplies losing trades; work on H1 at the minimum and, where possible, on H4 or D1, where the momentum reading is stable and divergences carry meaning.
4. Set the parameters once and leave them alone for four weeks. Pick 14 periods — Wilder's standard — or, if your style calls for it, 9 for something faster, but do not hop from 14 to 9, then to 7 and back to 11: that dance destroys your feel for the readings. Log every RSI-filtered trade in your journal and review after a month whether the filter genuinely improves your process.