RSI Explained

The Relative Strength Index (RSI) is a momentum oscillator that compares the size of recent gains to the size of recent losses and expresses the result on a scale from 0 to 100. Developed by J. Welles Wilder and published in 1978, it is designed to show whether price has been rising or falling with unusual speed, which traders interpret as overbought or oversold conditions. RSI is one of the most widely used indicators in forex, and also one of the most misused, so understanding its construction is essential before relying on it.

What RSI measures

RSI measures the ratio of average upward price changes to average downward price changes over a lookback period. When recent candles have closed higher far more often and by larger amounts than they have closed lower, RSI is high. When declines dominate, RSI is low. A reading of 50 means gains and losses have been roughly balanced.

Crucially, RSI measures the speed and consistency of recent movement, not the level of price. A currency pair can be at an all-time high with an RSI of 55 if the climb has been gradual, or at a modest level with an RSI of 85 after a sharp short-term spike.

How RSI is calculated

The calculation takes four steps. Using the standard 14-period setting:

1. Separate gains and losses. For each of the last 14 candles, record the close-to-close change. Positive changes are gains; negative changes are losses (recorded as positive numbers). A period with no change counts as zero in both.
2. Average them. Compute the average gain and the average loss over the 14 periods. Wilder used a smoothed average: after the first calculation, each new average is (previous average × 13 + current value) ÷ 14, which behaves like an exponential average.
3. Relative strength (RS). RS = average gain ÷ average loss.
4. Index. RSI = 100 − (100 ÷ (1 + RS)).

If average gains are three times average losses, RS = 3 and RSI = 100 − (100 ÷ 4) = 75. If gains and losses are equal, RS = 1 and RSI = 50. If there are no losses at all in the window, RSI reaches 100.

The smoothing means that a single large candle influences RSI for many periods after it occurs, which is why the indicator does not snap back instantly when momentum pauses.

Common settings

Wilder's original recommendation of 14 periods remains the default on almost every platform, including the JDGlobalFX terminal. Shortening the period increases the frequency of extreme readings; lengthening it reduces them.

Common interpretations

Overbought and oversold

The textbook reading treats RSI above 70 as overbought and below 30 as oversold, suggesting that price has moved too far too fast and may pull back. In ranging markets this works reasonably well: buying near support when RSI dips below 30 and selling near resistance when it rises above 70 aligns momentum with location.

The 50 midline as a trend filter

When RSI holds above 50 during pullbacks, momentum remains net bullish; when it stays below 50 during rallies, momentum is net bearish. Some traders use the midline as a trend filter and ignore overbought and oversold readings entirely, treating a dip toward 40–50 in an uptrend as a potential buying zone rather than waiting for 30.

Divergence

Divergence compares the direction of price swings with the direction of RSI swings.

• Bearish divergence: price makes a higher high, RSI makes a lower high. Momentum is fading even though price advanced.
• Bullish divergence: price makes a lower low, RSI makes a higher low. Selling pressure is weakening.

Divergence is a warning rather than a trigger. It can persist through several further swings before price finally turns, so most traders wait for a price-action confirmation, such as a break of the most recent swing point, before acting.

Failure swings

A bullish failure swing occurs when RSI drops below 30, rallies, pulls back without falling below 30 again, then breaks above its prior rally high. The bearish version mirrors this above 70. Wilder considered these more reliable than simple threshold crossings because they show momentum rejecting the extreme.

A worked example

Suppose USD/CAD is trading in a well-defined range on the 4-hour chart between 1.3500 (support) and 1.3650 (resistance). Price falls to 1.3510 and the 14-period RSI reads 27. A bullish pin bar forms with its low at 1.3495 and a close at 1.3530. A trader enters long at 1.3535 with a stop at 1.3480, a 55-pip risk below the range low, and targets 1.3640, just under resistance, for a 105-pip reward, roughly 1.9:1. The RSI reading added weight to the location; it did not create the trade by itself.

Limitations and false signals

RSI's weaknesses are well documented and worth internalising.

It stays extreme in trends. In a strong uptrend, RSI can remain above 70 for dozens of candles while price rises steadily. Selling because RSI is "overbought" in these conditions means selling into strength repeatedly. The same applies to oversold readings in downtrends. This is the single most expensive misuse of the indicator.

Divergence can persist. Multiple divergences can form as a trend extends. Acting on the first one often means being early by a wide margin.

It is derived from price. RSI contains no information that is not already in the candles. It presents that information differently, which is useful, but it cannot anticipate a news release or an order-flow shift.

Sensitivity to period. A 9-period RSI produces far more extreme readings than a 21-period RSI on the same chart, so signals depend on a parameter choice that has no objectively correct value.

Ambiguity in ranges near the thresholds. Readings of 68 or 32 are neither clearly extreme nor clearly neutral, and traders who treat the thresholds as precise boundaries will be inconsistent.

The practical response is to use RSI as one input among several: define the trend and levels first using structure, treat RSI extremes as more meaningful when they occur against the trend direction at a level, and rely on candlestick confirmation for the actual trigger. MACD Explained covers a complementary momentum tool that behaves differently in trends.

Key takeaways

• RSI measures the ratio of average gains to average losses over a lookback period, scaled from 0 to 100; it reflects momentum, not price level.
• The default is 14 periods with thresholds at 70 (overbought) and 30 (oversold); shorter periods produce more frequent extremes.
• Useful readings include threshold extremes in ranging markets, the 50 midline as a trend filter, divergence between price and RSI, and failure swings.
• RSI can stay overbought or oversold for long stretches in strong trends, and divergence can persist through several swings before price reacts.
• Treat RSI as supporting evidence at a level with a confirming candle, not as a standalone buy or sell signal.