What is the Directional Movement Index (DMI)? Definition, Formula, and Example
The Directional Movement Index (DMI) is a trend indicator that compares positive and negative directional movement, +DI and -DI, to show which side of the market controls price.
What is the Directional Movement Index (DMI)?
The Directional Movement Index (DMI) is a trend-following indicator developed by J. Welles Wilder that measures which direction price is moving and which side, buyers or sellers, controls the tape. It consists of two lines: the positive directional indicator (+DI), which measures upward movement, and the negative directional indicator (-DI), which measures downward movement. When +DI sits above -DI, bulls control the trend; when -DI sits above +DI, bears control it. Crosses between the two lines generate trend-change signals. The DMI is the engine inside the average directional index (ADX), which Wilder built by smoothing the difference between the two DI lines to measure trend strength.
How the DMI is Calculated
The calculation runs in four steps, typically over 14 periods:
1. Directional movement. +DM = current high minus prior high, if positive and larger than the down move, else 0. -DM = prior low minus current low, if positive and larger than the up move, else 0.
2. True Range (TR). The largest of: high minus low, high minus prior close, or prior close minus low, the same TR used in average true range.
3. Smooth. Wilder-smooth +DM, -DM, and TR over 14 periods.
4. Compute the indicators.
+DI = 100 x (smoothed +DM / smoothed TR)
-DI = 100 x (smoothed -DM / smoothed TR)
Both lines oscillate between 0 and 100. Readings above 25 on the dominant line indicate a firm trend; readings below 20 on both lines indicate a directionless market.
Worked Example: NVDA DI Cross
Suppose NVDA has chopped sideways for a month with +DI at 17 and -DI at 19, both weak. The stock then breaks out of its range. Over the next 14 sessions, up moves dominate: the smoothed +DM rises to 3.6 while smoothed TR sits at 4.0, and smoothed -DM falls to 0.8.
+DI = 100 x (3.6 / 4.0) = 90
-DI = 100 x (0.8 / 4.0) = 20
The +DI crosses above the -DI and holds a wide spread, confirming buyers control the move. A trend trader takes the cross as an entry signal and stays long while +DI remains above -DI. Weeks later, -DI climbs through +DI as NVDA rolls over, and the same cross in reverse triggers the exit. Pairing the cross with a rising ADX above 25 filters out the whipsaw crosses that occur in flat markets.
When Traders Use the DMI
Traders use DI crosses as mechanical trend entries and exits, especially in futures and trending large-caps where false signals cost less than in choppy small-caps. The standard filter combines DMI with the ADX: only trade crosses when ADX exceeds 20 to 25, which confirms a real trend exists behind the signal. The DMI also serves as a regime gate for other strategies, disabling mean-reversion systems when one DI line dominates, and enabling mean reversion trades only when both lines sit below 20. Swing traders combine the +DI/-DI spread with relative strength index readings to avoid buying overbought breakouts. You can screen for stocks in strong directional trends on the live scanner.
Limitations and Common Misconceptions
The DMI lags because it is built on smoothed data; by the time the lines cross, part of the move is gone. In sideways markets the lines cross repeatedly with no follow-through, which is why the ADX filter is mandatory rather than optional. A common misconception is that +DI above -DI means price is rising; it only means up moves exceed down moves over the lookback, and price can still be below where the cross occurred if the path was volatile. The 14-period default is arbitrary, and shortening it increases sensitivity at the cost of more whipsaws. Like all trend tools, the DMI measures direction and says nothing about how far a move will travel or where volatility-based stops belong, which is a job for ATR.