RSI

Relative Strength Index — momentum oscillator, 0–100.

The Relative Strength Index (RSI), developed by J. Welles Wilder in 1978, is a bounded momentum oscillator. It compares the magnitude of recent up-closes to the magnitude of recent down-closes over a look-back window (Wilder's default is 14) and rescales the result onto a 0–100 axis.

Conventional reads: above 70 = overbought, below 30 = oversold. In strongly trending markets those thresholds shift (Constance Brown's work argues 40–80 for uptrends and 20–60 for downtrends). RSI is also used to spot divergences — price makes a new high but RSI does not, suggesting weakening momentum.

Wilder used a smoothing scheme equivalent to an EMA with α = 1/N, not the standard EMA α = 2/(N+1). Different platforms use slightly different smoothing, so RSI values can disagree across charting tools.

RSI = 100 - 100 / (1 + RS),  where  RS = avg gain / avg loss  over the look-back window

Read the full Relative strength index (RSI) definition in the glossary →