Surface Area of a Triangular Prism: Formula, Steps & Examples

The concept of Surface Area of a Triangular Prism Formula is a vital geometry topic for students in middle and high school. Understanding how to calculate the surface area helps in solving practical problems in school exams, competitive tests, and even real-life situations like designing, packaging, and construction.

Understanding the Surface Area of a Triangular Prism

A triangular prism is a three-dimensional (3D) shape with two identical triangular bases joined by three rectangular faces. You can imagine it as a tent or a Toblerone chocolate bar. The surface area of a triangular prism is the total area of all its external faces including the triangles and rectangles. This concept plays a key role in measurement topics covered in classes 7, 8, 9, and 10, and helps students visualize connections between two-dimensional and three-dimensional figures.

Formula for the Surface Area of a Triangular Prism

To find the total surface area of a triangular prism, we need to add the areas of all three rectangular faces and both triangular bases. Here is the main formula:

• Surface Area = (Perimeter of the triangular base × Length of the prism) + (2 × Area of the triangular base)

Written with variables:

• Surface Area = (S₁ + S₂ + S₃) × L + 2 × (Area of Triangle)

Where:

• S₁, S₂, S₃ = Sides of the triangular base
• L = Length (height) of the prism
• Area of Triangle = ½ × base × height (or use Heron's formula if all sides are known)

This formula can be applied to any triangular prism, including prisms with equilateral, isosceles, or scalene triangles as bases.

Step-by-Step Example: Calculating Surface Area

Let's work through a real example:

1. Identify the sides of the triangle: Suppose the triangular base has sides of 5 cm, 12 cm, and 13 cm.
2. Find the perimeter of the base:
   Perimeter = 5 + 12 + 13 = 30 cm
3. Find the area of the triangle (using ½ × base × height):
   Base = 5 cm, Height = 12 cm
   Area = ½ × 5 × 12 = 30 cm²
4. Find the length of the prism: Suppose the prism has a length of 11 cm.
5. Plug into the formula:
   Surface Area = (30 × 11) + 2 × 30 = 330 + 60 = 390 cm²

Lateral Surface Area of a Triangular Prism

The lateral surface area (LSA) means the area of only the rectangle faces, not including the triangular bases. The formula is simple:

• Lateral Surface Area = (Perimeter of base) × (Length of the prism)
• LSA = (S₁ + S₂ + S₃) × L

For the example above: LSA = 30 × 11 = 330 cm²

Surface Area of a Right Triangular Prism

A right triangular prism has bases that are right triangles. The calculation is the same, but it's often easier because you know one angle is 90°. For right triangles, area = ½ × base × height.

• Surface Area = (Perimeter of base × Length) + (2 × ½ × base × height)
• Surface Area = (S₁ + S₂ + S₃) × L + (base × height)

This works for any right triangular prism. If you have three side lengths and it's a right triangle, use the sides directly for perimeter.

Worked Examples

Example 1

Find the surface area of a right triangular prism with base = 3 cm, height = 4 cm (triangle), hypotenuse = 5 cm, and prism length = 10 cm.

1. Perimeter = 3 + 4 + 5 = 12 cm
2. Area of triangle = ½ × 3 × 4 = 6 cm²
3. Surface Area = (12 × 10) + (2 × 6) = 120 + 12 = 132 cm²

Example 2

If the area of each triangular base is 30 cm², perimeter is 11 cm, and prism length is 25 cm, what is the total surface area?

1. Combined area of triangles = 2 × 30 = 60 cm²
2. Area of rectangles = 11 × 25 = 275 cm²
3. Total Surface Area = 60 + 275 = 335 cm²

Practice Problems

• A triangular prism has base sides 6 cm, 8 cm, and 10 cm. Its length is 7 cm. Find the surface area. (Use Heron's formula if needed)
• If all sides of the triangular base are 5 cm and prism length is 12 cm, what is the surface area?
• Base area = 15 cm², perimeter = 12 cm, prism length = 8 cm. Find surface area.
• The LSA of a prism is 90 cm² and the length is 5 cm. What is the perimeter of the base?
• If the height of the triangle is not given, but all sides are 7 cm, prism length is 9 cm, find the surface area.

Common Mistakes to Avoid

• Mixing up the base of the triangle and the length of the prism. Always identify which measurement is which!
• Forgetting to include both triangular bases in total surface area calculations.
• Incorrectly finding the area of the triangle – use the correct formula (½ × base × height or Heron's formula).
• Using wrong units or forgetting to label the answer in cm², m², etc.
• Confusing lateral surface area with total surface area – remember LSA excludes the bases.

Real-World Applications

The surface area of a triangular prism is used when calculating the amount of material needed for tents, aquariums, roof structures, and packaging. For example, to paint a Toblerone-shaped chocolate box or design a triangular wedge in construction, engineers and designers use this formula. At Vedantu, we teach surface area concepts with practical examples to help students connect mathematics to the world around them.

For more geometry concepts, check out our pages on Surface Area of Prism and Heron's Formula for irregular triangles. You can also practice more using this surface area worksheet (PDF).

In this topic, we learned how to use the Surface Area of a Triangular Prism Formula to solve geometry problems. Mastering this formula helps in exams and practical life. For more in-depth lessons and personalized learning, explore Vedantu’s courses and resources to excel in maths and beyond.