RSI Calculator

The Relative Strength Index is a bounded momentum oscillator developed by J. Welles Wilder. It compares the magnitude of recent gains with the magnitude of recent losses and converts that relationship into a reading from zero to one hundred. This calculator accepts chronological closing prices and applies Wilder's original smoothing procedure. The result is not a forecast. It is a compact description of how strongly prices have recently moved upward or downward relative to the selected lookback period.

For every pair of consecutive closes, the calculator records a gain when the change is positive and a loss when the change is negative. The first average gain and first average loss are simple means over the selected period. Every later observation uses Wilder smoothing, which retains most of the previous average and adds one part of the newest gain or loss. This method reacts more gradually than a rolling simple average and matches the convention used by many charting platforms.

The ratio of average gain to average loss is called relative strength. RSI equals one hundred minus one hundred divided by one plus relative strength. When no losses occurred, RSI is one hundred. When no gains occurred, it is zero. If every price is unchanged, this implementation returns fifty because upward and downward momentum are balanced. Values at or above seventy are labeled overbought, values at or below thirty are labeled oversold, and readings between those thresholds are neutral.

The gauge places the latest reading within the full oscillator range, while the history table shows recent calculated values. A high reading can persist during a strong uptrend, and a low reading can persist during a strong downtrend. Therefore a zone is context, not an automatic order to trade. Compare RSI with trend structure, volume, support and resistance, and your planned holding period. Changing the lookback changes sensitivity: shorter periods react faster, while longer periods produce smoother readings.

Consider fifteen daily closing prices with mixed advances and declines. The calculator finds each daily change, separates positive changes from absolute negative changes, and averages the first fourteen gains and losses. If the average gain is 0.24 and the average loss is 0.10, relative strength is 2.4. RSI is then one hundred minus one hundred divided by 3.4, which is about 70.59. That reading enters the conventional overbought zone.

Suppose the next close declines by 0.20. With a fourteen period lookback, the prior average gain is multiplied by thirteen and the new gain of zero is added before dividing by fourteen. The prior average loss is treated the same way, but the new loss of 0.20 is included. The updated RSI falls gradually rather than discarding the older observations at once. This persistence explains why Wilder RSI differs slightly from calculators that recompute simple averages on every row.

RSI can help compare momentum across points in the same security, examine whether a pullback is losing force, or confirm that a breakout has strong participation. Swing traders often watch threshold crossings, centerline behavior near fifty, and divergences between price and the oscillator. Long term investors may use a weekly or monthly series to place short term volatility within a broader trend rather than reacting to every daily move.

Use the same data frequency and lookback when comparing readings. A fourteen day RSI and a fourteen week RSI describe very different horizons. The indicator is most informative when combined with a clearly stated decision rule and risk limit. It should not replace valuation, diversification, liquidity analysis, or position sizing. Test any proposed signal on data that was not used to design it and include realistic trading costs.

The most common mistake is treating seventy as an instruction to sell and thirty as an instruction to buy. Strong trends can remain beyond these levels for many observations. Another mistake is entering prices in reverse order, because smoothing depends on chronology. Missing sessions, mixing intraday and daily closes, or combining prices adjusted differently for splits and dividends can also create readings that do not match a trusted chart.

Do not compare results without checking the smoothing convention. Some tools use a simple moving average or a standard exponential average instead of Wilder smoothing. Small rounding differences are normal, especially when a provider carries additional decimal places. Avoid choosing a period only because it produced attractive historical trades. That practice invites overfitting. Document the data source, frequency, adjustment method, lookback, thresholds, and confirmation rules before evaluating performance.

RSI is derived entirely from past closes and cannot know future information. Fixed thresholds behave differently across assets, volatility regimes, and trends. The calculation assumes correctly ordered, comparable observations and does not repair missing data or corporate actions. Persisted inputs remain only as reliable as browser storage. Historical indicator performance can be overstated by selection bias, overfitting, ignored costs, and lookahead errors. This calculator is educational and does not provide personalized financial advice or guaranteed signals.

RSI focuses on the balance of gains and losses. A moving average focuses on trend level, while Average True Range focuses on volatility without direction. Combining different categories can add context, but adding many correlated momentum indicators may only repeat the same information. Choose a small set whose role in the decision process is explicit.

The MACD calculator can help examine trend momentum without a bounded scale. The ATR calculator can translate current volatility into a possible risk based position size. Bollinger Bands compare price with a volatility envelope. None of these tools removes uncertainty, and agreement among indicators does not guarantee a profitable outcome.