--- title: "Trend Strength Index: Formula, Meaning" description: "Commonly implemented as the correlation between closing prices and a time index over a lookback window, so a value near +1 means price has risen steadily..." canonical: https://www.getswoopr.com/indicators/trend/trend-strength-index/ source: "Swoopr Investment: https://www.getswoopr.com" --- ## Direct Answer > Commonly implemented as the correlation between closing prices and a time index over a lookback window, so a value near +1 means price has risen steadily and near -1 fallen steadily. Implementations vary. Values close to +1 or -1 indicate a straight-line trend up or down, while values near zero indicate no linear relationship between price and time. # Trend Strength Index: Formula, Meaning, Signals, Examples and How to Use It ## How Is Trend Strength Index Calculated? Commonly implemented as the correlation between closing prices and a time index over a lookback window, so a value near +1 means price has risen steadily and near -1 fallen steadily. Implementations vary. **Formula:** `TSI = Pearson correlation between Close and bar number over the last n bars` **Inputs:** - Closing prices - Lookback n - A running bar count used as the time index **Reading it.** Values close to +1 or -1 indicate a straight-line trend up or down, while values near zero indicate no linear relationship between price and time. It measures how consistently price has trended, not how far it moved. **Limits.** It captures only linear trend and says nothing about the size of the move. Short windows give unstable values, and different sources use the same name for different formulas. ## What is Trend Strength Index? Commonly implemented as the correlation between closing prices and a time index over a lookback window, so a value near +1 means price has risen steadily and near -1 fallen steadily. Implementations vary. ## How to use Trend Strength Index Values close to +1 or -1 indicate a straight-line trend up or down, while values near zero indicate no linear relationship between price and time. It measures how consistently price has trended, not how far it moved. A reading alone is never a buy or sell signal; it needs context from price structure, volume, volatility and the broader market regime. ## Strengths - Correlating price with bar number gives a bounded -1 to +1 reading, so direction and consistency are summarized by one number. - The correlation is scale-free, so a steady rise reads the same whether the instrument trades near 10 or near 1,000. - Reproducible when the formula and data convention are fixed. ## Weaknesses and limitations - A series that rises quickly and then flattens still correlates positively with time, so the reading stays high while momentum has faded. - Equal weight across the window means a reversal in the last few bars moves the value only slowly if the earlier path was steady. - Correlated indicators can create false confidence by repeating the same underlying information. ## Editorial and risk note This page is educational content, not individualized investment advice. It should not imply guaranteed prediction, accuracy or outperformance. ## Frequently Asked Questions Is Trend Strength Index a buy or sell signal? No. Values close to +1 or -1 indicate a straight-line trend up or down, while values near zero indicate no linear relationship between price and time. It captures only linear trend and says nothing about the size of the move. A trade built on it still needs a trigger, an invalidation rule, a position size and realistic execution assumptions. What is the best setting for Trend Strength Index? Trend Strength Index takes these inputs: closing prices; lookback n; a running bar count used as the time index. There is no universal best setting for the tunable ones, so test them across instruments, regimes and out-of-sample periods and favor stable parameter regions over one historical winner. Can Trend Strength Index be used by itself? It captures only linear trend and says nothing about the size of the move. Short windows give unstable values, and different sources use the same name for different formulas. For that reason it is read next to measures of direction, participation, valuation, liquidity and risk, not alone. Does Trend Strength Index work on every timeframe? The calculation (TSI = Pearson correlation between Close and bar number over the last n bars) can be run on any bar size, but behavior is not portable: a reading on a five-minute chart describes a very different market window from the same reading on a daily chart. Validate the exact timeframe before relying on it. Why does Trend Strength Index give false signals? A series that rises quickly and then flattens still correlates positively with time, so the reading stays high while momentum has faded. Equal weight across the window means a reversal in the last few bars moves the value only slowly if the earlier path was steady. Lag, noise and regime changes add to this, because market participants react to new information after the reading is calculated. How should Trend Strength Index be backtested? Rebuild Trend Strength Index from closing prices using point-in-time data, write the rules before testing, include delisted securities where relevant, model realistic fills and costs, reserve a validation sample and compare with a simpler baseline. S Swoopr Editorial Team The Swoopr Editorial Team produces independent investment education, research, and tools for understanding markets, evaluating opportunities, and managing risk. All content is educational only and does not constitute personalized investment advice. See our [editorial policy](https://www.getswoopr.com/legal/editorial-policy/) and [corrections policy](https://www.getswoopr.com/legal/corrections-policy/).