What is a Trendline? Definition, Formula, and Example
A trendline is a straight line drawn through two or more price pivot points on a chart to define the direction and slope of a trend and to mark dynamic support or resistance.
What is a Trendline?
A trendline is a straight line connecting two or more significant price points on a chart, extended forward to project where the trend is heading. An uptrend line is drawn beneath rising swing lows and acts as dynamic support. A downtrend line is drawn above falling swing highs and acts as dynamic resistance. The line is considered valid once price has touched it at least three times without breaking it. A decisive close through the line signals that the prevailing trend has weakened or reversed.
How a Trendline Is Drawn
A trendline is not a formula-driven indicator, but its construction follows strict rules:
1. Identify pivot points. Mark the two most significant swing lows (uptrend) or swing highs (downtrend). A pivot is a bar whose low or high is the extreme of at least two bars on either side.
2. Connect and extend. Draw a straight line through both points and extend it right. The line must not cut through any candle bodies between the anchors.
3. Require a third touch. The line earns significance only after price respects it a third time.
4. Slope. The slope of the line is simply rise over run: slope = (P2 − P1) / (T2 − T1), where P is price and T is the bar index. Steeper lines break more easily. Lines in the 30 to 45 degree range tend to persist longest.
Logarithmic scales are the correct choice for long-term charts, because a trendline on a log scale represents a constant percentage growth rate rather than a constant dollar rate.
Worked Example
Suppose NVDA prints swing lows at $110.00 in week 1 and $118.00 in week 5. The trendline slope is ($118.00 − $110.00) / 4 weeks = $2.00 per week. Projected forward, the line sits at $126.00 in week 9 and $130.00 in week 11. If price pulls back to $126.20 in week 9 and closes the week at $131.00, that is a valid third touch and a bounce. If in week 12 the stock closes at $124.50, below the line's projected value of $132.00, the uptrend line is broken and traders treat the trend as compromised until a new line can be drawn.
When Traders Use Trendlines
Trendlines serve three practical functions. First, entries: traders buy pullbacks to a rising trendline with a stop just below it, which defines risk tightly. Second, exits: a close through a respected trendline is a mechanical exit signal for trend-following positions. Third, context: trendlines frame channels, wedges, and triangles, and they anchor tools like Fibonacci retracement levels. On Tapeboard, you can draw and test trendline logic against live price action in the paper trading simulator before committing capital.
Limitations and Common Misconceptions
Trendlines are subjective. Two analysts given the same chart will draw different lines, and small anchor adjustments change the projected level by dollars. A break of a trendline does not guarantee a reversal; it often marks only a shift to a shallower trend, which is why traders redraw lines after breaks rather than assume a new downtrend. Trendlines also degrade in choppy, range-bound markets where no clean pivots exist. Finally, a trendline says nothing about volume, volatility, or catalysts, so it works best combined with confirmation tools like RSI or volume profile.