ACT Plane Geometry Formula Sheet
Triangle, quadrilateral, circle, and 3-D solid formulas for the ACT Plane Geometry reporting category.
Below are the 18 formulas tested in this category, with the situation each one applies to. Drill until you can write the whole sheet from memory in under two minutes — that's the standard top scorers reach by test day.
Each formula links back to the topic page where it's worked into example problems. If a formula doesn't yet feel automatic, click into the topic page and grind that topic's six practice questions.
Formulas
| Concept | Formula | When to use it |
|---|---|---|
| Area of triangle | A = ½ b h | Base × height ÷ 2. |
| Area of equilateral triangle | A = (√3 / 4) s² | Side s. |
| Heron's formula | A = √(s(s−a)(s−b)(s−c)) | s = (a+b+c)/2. |
| Pythagorean theorem | a² + b² = c² | Right triangle. |
| 45-45-90 triangle | 1 : 1 : √2 | Both legs equal. |
| 30-60-90 triangle | 1 : √3 : 2 | Short : long : hyp. |
| Area of parallelogram | A = b h | Base × perpendicular height. |
| Area of trapezoid | A = ½ (b₁ + b₂) h | Average of bases × height. |
| Circumference of circle | C = 2 π r = π d | Just remember 2πr. |
| Area of circle | A = π r² | Square the radius. |
| Arc length | s = r θ | θ in radians. |
| Sector area | A = ½ r² θ | θ in radians. |
| Inscribed angle | ½ × central angle | On the same arc. |
| Sum of interior angles | (n − 2) · 180° | n-sided polygon. |
| Volume of cylinder | V = π r² h | Round prism. |
| Volume of sphere | V = ⁴⁄₃ π r³ | Solid ball. |
| Volume of cone | V = ⅓ π r² h | ⅓ of cylinder. |
| Surface area of sphere | A = 4 π r² | All sides combined. |
How to memorise this sheet in a week
Day 1. Read every row out loud. Connect each formula to a worked example by clicking into the topic page on which it appears. Goal: understand why each formula is true, not just memorise it.
Day 2. Cover the right-hand columns and quiz yourself on each concept. Write the formula and the use case from memory. Mark the ones you missed.
Day 3. Re-quiz only the ones you missed yesterday. Keep cycling until your error count hits zero.
Day 4. Re-do the full quiz. You should know every row.
Day 5. Apply each formula in a problem context — pick a topic page from the list below and solve all six questions.
Day 6–7. Sit a real past paper. Note any formulas you blanked on under time pressure and add them back to your daily quiz.
Topics that use these formulas
- Properties of Triangles6 Qs
- Pythagorean Theorem6 Qs
- Special Quadrilaterals6 Qs
- Area and Perimeter6 Qs
- Volume and Surface Area6 Qs
- Circles6 Qs
- Similar Figures6 Qs
- Three-Dimensional Figures6 Qs
- Parallel Lines and Transversals6 Qs
- Angle Relationships6 Qs
Common mistakes when applying these formulas
Three errors account for the majority of dropped points on formula-driven questions:
- Substituting before simplifying. Always simplify symbolically as far as you can before plugging numbers in. It cuts arithmetic errors in half.
- Mixing units. If a formula expects metres and you have centimetres, convert first, not at the end. The number you compute is meaningless until the units agree.
- Forgetting the ½ in area or volume formulas. Triangle, trapezoid, sector, cone, pyramid — all have a ½ or ⅓ that's easy to drop. Write the formula on the page before plugging in.