RSI Calculator (Relative Strength Index)
Paste closing prices to calculate Wilder’s Relative Strength Index (RSI) — average gain, average loss, RS and the RSI for every bar — and see whether the market is overbought, oversold or neutral.
How it works
Change = close − previous close · first average gain/loss = simple average of the first n gains/losses · then Wilder smoothing: average = (previous average × (n − 1) + current gain or loss) ÷ n · RS = average gain ÷ average loss · RSI = 100 − 100 ÷ (1 + RS)
The Relative Strength Index, introduced by J. Welles Wilder in New Concepts in Technical Trading Systems (1978), is a momentum oscillator that compares the size of recent gains with recent losses on a 0–100 scale. Wilder used 14 periods and treated readings above 70 as overbought and below 30 as oversold. This calculator uses his original smoothing — the same method as StockCharts and most charting platforms — so it needs at least period + 1 closes and becomes more accurate with 100 or more bars of history, because each value depends on all earlier ones.
Worked example
The 33 closes in the box are the classic StockCharts example. The first 14 changes give an average gain of 0.2385 and an average loss of 0.0996, so RS = 2.39 and the first RSI is 70.53 (overbought). Wilder smoothing then carries the averages forward bar by bar; after the last close of 43.1314 the averages are 0.1803 and 0.2970, RS = 0.6070 and the 14-period RSI is 37.77 — neutral, after falling from the high 60s.
Sources: StockCharts ChartSchool — Relative Strength Index (RSI) · Wikipedia — Relative strength index
Frequently asked questions
What RSI period should I use?
Wilder’s 14 is the standard. Shorter periods such as 7 or 9 react faster and hit 70/30 more often; longer periods such as 21 are smoother. Many short-term traders use 2-period RSI with 90/10 thresholds.
Why does my RSI differ slightly from my charting platform?
Because Wilder’s average depends on every earlier bar. With a short history the starting point changes the result; with 100+ bars the values converge. Some platforms also use an exponential or simple average instead of Wilder’s.
Does an RSI above 70 mean I should sell?
Not necessarily. In strong uptrends RSI can stay above 70 for weeks. Traders often look for divergences, failure swings or a cross back below 70 rather than selling the moment it reaches 70.
What is RSI divergence?
When price makes a new high but RSI makes a lower high (bearish divergence), or price makes a new low while RSI makes a higher low (bullish divergence) — a sign that momentum is fading.
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Educational estimate, not investment, trading or tax advice. Brokers, exchanges and data providers use their own conventions, fees and figures — confirm with your broker, the exchange’s documentation or a licensed adviser before acting.