What is the 30° rule?

The 30° rule states that for a rolling-CB shot, over a wide range of cut angles, between a 1/4-ball hit (49 degree cut) and 3/4-ball hit (14 degree cut), the CB will deflect or carom off by very close to 30° (the “natural angle“) from its original direction after hitting the OB. If you want to be more precise, the angle is a little more (about 34°) closer to a 1/2-ball hit and a little less (about 27°) closer to a 1/4-ball or 3/4-ball hit. The rule is described and illustrated in the following videos:

The Dr. Dave peace-sign technique is very useful in applying the 30° rule. The 30° rule angle templates can be useful to help you calibrate your V-sign. If you want to know how the CB path varies with speed and how to account for this, see speed effects. And if you want to see how numbers change a little with typical pool equipment conditions, see “Rolling Cue Ball Deflection Angle Approximations” (BD, November, 2011).

Here a video demonstrations showing examples of how the 30° rule can be used in different shot situations:

and here are some others:

Here’s a convenient 1-page summary resource page summarizing all of the important points of the 30° rule, and here’s a useful handout covering advanced topics.

For more info, see “The 30° rule: Part I – the basics” (BD, April, 2004) and where the CB goes for different cases.

See also: using inside spin to narrow the follow angle.

Is the 30° rule exact?

30° is an approximate carom angle that works fairly well over a wide range of cuts between a 1/4-ball and 3/4-ball hit. However, the carom angle over this range based on the equations and plots here:

TP B.13 – Rolling cue ball carom angle approximations

An improved approximation is to use a slightly larger angle (34°) closer to a 1/2-ball hit and a slightly small angle (about 27°) closer to a 1/4-ball or 3/4-ball hit. For more information, see the peace-sign technique and the following video:

Here’s a graph from Bob Jewett (with the approximation lines added by Pat Johnson) that show how the carom angle changes with ball-hit fraction:

rolling CB carom angle plot

Here’s a good demonstration from Pubo Huang showing how the deflected CB angle varies with cut angle:

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