RSI — Relative Strength Index

Overview

Developed by J. Welles Wilder in 1978, the Relative Strength Index (RSI) is one of the most widely used momentum oscillators in technical analysis. It measures the speed and magnitude of recent price changes to evaluate whether an asset is overbought or oversold.

RSI oscillates between 0 and 100, with two classic levels guiding interpretation:

• Above 70 — potentially overbought (buying pressure may exhaust soon)
• Below 30 — potentially oversold (selling pressure may exhaust soon)

These levels are defaults, not absolute laws — in strong trends RSI can stay overbought / oversold for extended periods.

Formula

RSI = 100 - (100 / (1 + RS))

RS = Average Gain / Average Loss   (over N periods)

Default N = 14 periods

Wilder originally used 14 periods. Today this is the universal default across charting platforms, although shorter (7–9) or longer (21+) periods are sometimes used.

Default Settings

• Period: 14
• Overbought level: 70
• Oversold level: 30
• Centerline: 50 (sometimes used as trend filter)

How to Use It

1. Overbought / Oversold Reversals

The classic textbook use: when RSI rises above 70, look for opportunities to short / take profit; when it falls below 30, look for long entries. This works best in ranging markets, not strong trends.

2. Divergence

One of the most reliable RSI signals:

• Bullish divergence — price makes a lower low but RSI makes a higher low. Suggests weakening downward momentum.
• Bearish divergence — price makes a higher high but RSI makes a lower high. Suggests weakening upward momentum.

Divergence is a leading indicator and can signal reversals before price confirms them.

3. Centerline Crossover

RSI crossing above 50 indicates increasing bullish momentum; crossing below 50 indicates increasing bearish momentum. Useful as a trend filter alongside trend-following systems.

4. RSI Range Shifts in Trending Markets

Constance Brown popularized the idea that in bull markets RSI tends to oscillate between 40–90 (40 acts as support), and in bear markets between 10–60 (60 acts as resistance). Adjusting the overbought/oversold levels to match the market regime improves signal quality.

Strengths

• Simple, visual, well-documented — understood by virtually every trader
• Works on any timeframe and any asset class
• Divergence detection adds a leading dimension to a lagging indicator
• Effective filter in mean-reversion strategies

Weaknesses & Common Mistakes

• Stays overbought / oversold in trends — entering short on RSI above 70 in a strong uptrend is a classic way to lose money
• Divergence can persist — a divergence isn’t an entry signal by itself; price confirmation is essential
• Whipsaws on shorter periods — period 5 or 7 gives more signals but most are noise
• Single-indicator decisions — never trade RSI alone; always combine with price action or other confirmation

Best Combinations

• RSI + 200 EMA — trade RSI oversold only when price is above the 200 EMA (and vice versa for shorts). Filters out counter-trend mean reversion.
• RSI + Support / Resistance — oversold RSI at a key support level is much stronger than oversold RSI in mid-range.
• RSI + Bollinger Bands — oversold RSI combined with price touching the lower Bollinger Band offers a higher-probability mean-reversion setup.

Practical Example

EUR/USD, H4 chart, range-bound market. Price drops to a known support zone; RSI dips below 30. Within the next few candles RSI crosses back above 30 and forms a higher low while price tests support again — classic bullish divergence at support. Long entry with stop below the recent low typically offers 2–3R reward in this setup.

Bottom Line

RSI is most powerful as a context indicator — not as a trigger. Combine it with price structure and a trend filter, and it becomes one of the most reliable tools on the chart.