Plane Geometry
Triangles, quadrilaterals, circles, area, volume and angle relationships in two and three dimensions.
10 skill topics sit under Plane Geometry on the ACT Math taxonomy. Each one has its own dedicated page with concept notes and six worked practice questions. Work through the list top to bottom, or jump straight to the topic you struggle with.
This category accounts for a meaningful slice of your overall math score, so spend the time. The fastest way to gain points is to convert one weak topic per week into a strong one — not to skim every topic at surface depth.
Need the formulas at a glance? See the Plane Geometry formula sheet. Studying for a specific score range? Use the score-band guides to focus on the topics that move the needle for your current score.
All Plane Geometry topics
Properties of Triangles
Use angle sum, exterior-angle theorem and the triangle inequality.
Open topic
Pythagorean Theorem
Use a² + b² = c² and recognise common Pythagorean triples (3-4-5, 5-12-13, 8-15-17).
Open topic
Special Quadrilaterals
Use the side, angle and diagonal properties of squares, rectangles, rhombi, parallelograms and trapezoids.
Open topic
Area and Perimeter
Compute the area and perimeter of triangles, quadrilaterals and composite shapes.
Open topic
Volume and Surface Area
Find volume and surface area of prisms, cylinders, pyramids, cones and spheres.
Open topic
Circles
Use circumference, area, central angles, inscribed angles and tangent properties.
Open topic
Similar Figures
Use side-ratio and area-ratio properties of similar polygons (k for sides, k² for area, k³ for volume).
Open topic
Three-Dimensional Figures
Visualise and compute with rectangular solids, cylinders and spheres.
Open topic
Parallel Lines and Transversals
Identify alternate-interior, corresponding and co-interior angle pairs.
Open topic
Angle Relationships
Use vertical, complementary, supplementary and linear-pair relationships.
Open topic
Recommended study order
If this category is brand new to you, start with the first three topics in the list above — they tend to be the most foundational. Then work outward to the others. If you're already comfortable, go straight to a recent past paper, identify which Plane Geometry question you missed, and drill that specific topic page.
For broader test-prep advice, see our study tips. For real test reference, browse the ACT Math past paper index.