Work out angle between two planes instantly with clear inputs, formula shown and shareable results.
The angle between two planes equals the angle between their normal vectors: cos θ = |n₁·n₂| / (|n₁||n₂|). The absolute value keeps the answer in the range 0° to 90°, the usual convention for a dihedral angle.
Angle between planes
cos θ = |n₁ · n₂| / (|n₁| |n₂|)
The dot product is 4, both magnitudes are 3, so cos θ = 4/9 and θ ≈ 63.6122°.
Flipping a normal's direction does not change the plane, so without the absolute value you could get the supplement of the intended angle.