How to Use RSI in Forex (Relative Strength Index)
One of the most misused indicators. Here's how to actually use it.
Researched and written by Laurent What RSI Measures
The Relative Strength Index, created by J. Welles Wilder in 1978, measures the speed and magnitude of recent price movements. It compares average gains to average losses over a period (typically 14 bars) and normalizes the result to a 0-100 scale.
RSI = 100 − (100 / (1 + RS))
Where RS = Average Gain over N periods / Average Loss over N periods
If price went up every day for 14 days: RS → infinity → RSI approaches 100
If price went down every day for 14 days: RS = 0 → RSI = 0
The 70/30 Levels: The Trap
RSI above 70 is called "overbought." RSI below 30 is called "oversold." These terms have misled more retail traders than almost any other concept in technical analysis.
The logic seems sound: "If something is overbought, it should reverse." But "overbought" doesn't mean "about to fall." It means momentum has been strong. In a genuinely strong uptrend, RSI can stay above 70 for weeks, even months, as price continues climbing.
RSI above 70 in a ranging market → genuine overbought, watch for reversal
RSI above 70 in a strong uptrend → trend continuation is likely, DON'T short just because of RSI
RSI Divergence: The Real Power
This is where RSI becomes genuinely useful. Divergence occurs when price and RSI disagree:
- Bearish divergence: Price makes a new high, but RSI makes a LOWER high. Momentum is weakening even as price rises. This often precedes a reversal.
- Bullish divergence: Price makes a new low, but RSI makes a HIGHER low. Sellers are losing strength. Often precedes a reversal upward.
Divergence alone is not a trade signal: you still need price confirmation (a break of a trend line, a reversal candlestick pattern at resistance). But it's a powerful early warning that the current move is losing conviction.
RSI as a Trend Filter (The 50 Level)
One of the best uses of RSI is treating the 50 level as a trend boundary:
- RSI consistently above 50: Bullish momentum. Look for buy setups.
- RSI consistently below 50: Bearish momentum. Look for sell setups.
- RSI crossing 50: Momentum shift occurring, pay attention.
Combined with the 200 SMA (price above = bullish trend) and RSI above 50 (bullish momentum), you have a simple but powerful two-layer filter before even looking for an entry signal.
Standard Settings
The default RSI period is 14, which Wilder himself recommended. Some traders use:
- RSI(9): More sensitive, more signals, more noise. Used by short-term traders.
- RSI(21): Smoother, fewer but higher-quality signals. Used by swing traders.
Start with 14. Only change the period after you understand why you're changing it.
Key Takeaways
- • RSI measures momentum on a 0-100 scale. Above 70 = overbought, below 30 = oversold.
- • CRITICAL: "Overbought" does NOT mean "sell." RSI can stay above 70 for weeks in strong trends.
- • RSI divergence (price vs RSI moving in opposite directions) is more valuable than overbought/oversold levels.
- • RSI as a trend filter: consistently above 50 = bullish momentum, below 50 = bearish.