To calculate surface area, identify your 3D shape, choose the matching formula, measure the dimensions, substitute the values, and solve. The answer is always in square units.
Step 1: Identify the Shape
Look at your object. Is it a cube, rectangular prism, cylinder, sphere, cone, or pyramid? Each shape has its own formula. If unsure, check our formulas chart.
Step 2: Write Down the Formula
| Shape | Formula |
|---|---|
| Cube | SA = 6s² |
| Rectangular prism | SA = 2(lw + lh + wh) |
| Cylinder | SA = 2πr² + 2πrh |
| Sphere | SA = 4πr² |
| Cone | SA = πr² + πrl |
Step 3: Measure and Substitute
Measure the required dimensions (radius, height, side lengths). Plug them into the formula. Make sure all units match before calculating.
Step 4: Calculate
Example — Cylinder:
r = 3 cm, h = 10 cm
SA = 2π(3)² + 2π(3)(10)
= 2π(9) + 2π(30)
= 18π + 60π = 78π
= 245.04 cm²
r = 3 cm, h = 10 cm
SA = 2π(3)² + 2π(3)(10)
= 2π(9) + 2π(30)
= 18π + 60π = 78π
= 245.04 cm²
Step 5: Check Your Answer
- Is the answer in square units? (cm², m², ft²)
- Does the number seem reasonable for the object's size?
- Did you use radius (not diameter) where required?
Tip: The #1 mistake is confusing radius and diameter. See 10 common mistakes.
Try our calculators: Sphere, Cylinder, Cone, Cube, Rectangular Prism. Learn more with surface area using nets and what is surface area.