Area of a Rectangle Formula: 5 Examples + Quick Calculation

Area of a Rectangle Formula: 5 Worked Examples + Quick Calculation Method

The area of a rectangle formula is length times width, or A = p × l. For example, if the length is 8 cm and the width is 5 cm, the area is 8 × 5 = 40 cm². This formula is used in grades 3–7 to measure floors, gardens, books, and even the TV screen at home. The unit of area is always a square unit (cm², m², km²).

💡 Quick note: if you get confused between area and perimeter — area is the inside fill of the rectangle (square units), while perimeter is the outer edge that goes around it (linear units like cm or m). See also our guide on how to calculate the perimeter of a rectangle.

What Is the Area of a Rectangle

The area of a rectangle is the size of the flat region bounded by its four sides. A rectangle has two pairs of equal parallel sides — the longer side is called the length (p) and the shorter one is called the width (l). All four corners are right angles (90°).

Picture your bedroom at home. The floor is usually shaped like a rectangle. If you want to know how many ceramic tiles you need to cover that floor, you calculate its area. That is why this concept is used by builders, by mums in the kitchen (cutting a rectangular cake tray), and by garden designers in cities across Indonesia, from Jakarta to Surabaya.

Unlike a square (where all sides are equal), a rectangle has length ≠ width. If by chance p = l, the shape becomes a square — and the area formula becomes side × side. See also the related formula in how to find the area of a triangle.

The Area of a Rectangle Formula

The main formula you need to remember:

Read out loud: Area equals length times width. Here is what each symbol means:

• A = Area of the rectangle (square units: cm², m², km²)
• p = length, the longer side (units: cm, m, km)
• l = width, the shorter side (units: cm, m, km)
• × = ordinary multiplication

If you already know the area and one of the sides, you can find the other side:

In words: length equals area divided by width, and width equals area divided by length. This is the inverse operation of multiplication.

How to Calculate the Area of a Rectangle

Follow these 4 steps every time you face a rectangle area problem. The method works for every grade, from primary school to junior high.

Step 1 — Identify the length and the width. Read the problem carefully. Find the two numbers: the larger one is usually the length, the smaller one the width. Label them p and l.

Step 2 — Match the units. If the length is in metres and the width in centimetres, convert them to the same unit first. Example: 2 m = 200 cm. Without this step, the result will always be wrong.

Step 3 — Multiply p × l. Write down the formula, plug in the numbers, and multiply. You can use long multiplication if the numbers are big.

Step 4 — Add the square unit. Don't forget: if length and width are in cm, the area is in cm² (square centimetres). If they are in metres — m². Without the unit, the answer is marked wrong at school.

Worked Examples

Here are 5 examples, from the easiest to the most challenging.

Example 1 ⭐ — Andi's bedroom

Andi's bedroom floor in Bandung is rectangular. The length is 6 m and the width is 4 m. What is the area of his bedroom floor?

Step 1. Write down what is given: p = 6 m, l = 4 m. Step 2. Write the formula: A = p × l. Step 3. Plug in the numbers: A = 6 × 4 = 24.

✅ Answer: The area of Andi's bedroom floor = 24 m².

Example 2 ⭐ — Sinta's notebook

Sinta has a rectangular notebook with a length of 25 cm and a width of 18 cm. What is the area of its cover?

Step 1. Given: p = 25 cm, l = 18 cm. Step 2. A = p × l = 25 × 18. Step 3. 25 × 18 = 25 × 20 − 25 × 2 = 500 − 50 = 450.

✅ Answer: The cover area = 450 cm².

Example 3 ⭐⭐ — A school garden with mixed units

A school garden is rectangular with a length of 12 m and a width of 250 cm. What is its area?

Step 1. Given: p = 12 m, l = 250 cm. The units differ — convert first. Step 2. Convert the width to metres: 250 cm = 2.5 m. Step 3. A = 12 × 2.5 = 30.

✅ Answer: The garden area = 30 m².

Example 4 ⭐⭐ — Finding the width from the area

A rectangular canteen table at school has an area of 120 cm² and a length of 15 cm. What is the width of the table?

Step 1. Given: A = 120 cm², p = 15 cm. Find: l. Step 2. Use the inverse formula: l = A ÷ p. Step 3. l = 120 ÷ 15 = 8.

✅ Answer: The width of the table = 8 cm.

Example 5 ⭐⭐⭐ — A futsal court

Pak Budi owns a rectangular futsal court with a length of 25 m and a width of 15 m. The floor will be covered with 50 cm × 50 cm tiles. How many tiles are needed?

Step 1. Given: court p = 25 m, l = 15 m. Tile = 50 cm × 50 cm. Step 2. Calculate the court area: A = 25 × 15 = 375 m². Step 3. Calculate the area of one tile: 50 cm × 50 cm = 2,500 cm² = 0.25 m². Step 4. Number of tiles = 375 ÷ 0.25 = 1,500.

✅ Answer: 1,500 tiles are needed.

Common Mistakes When Calculating the Area of a Rectangle

The five mistakes teachers see most often in class:

1. Adding instead of multiplying. Many students write A = p + l. That is wrong — that is half the perimeter. The right way: multiply (×).
2. Forgetting the square unit (cm², m²). Writing the answer as "24 m" instead of "24 m². Half the marks disappear because the squared unit was missed.
3. Mismatched units. Multiplying 6 m × 80 cm directly produces nonsense. Always convert to the same unit first.
4. Mixing up length and width on a diagram. This actually does not matter, because p × l = l × p. The real mistake is using the same side twice (e.g., length × length).
5. Confusing it with perimeter. Perimeter uses P = 2 × (p + l), not p × l. Read the question: "perimeter" → fence/edge, "area" → surface/floor.

Area of a Rectangle Table

Here is a handy reference table for the rectangle sizes that show up most often in problems:

Area vs Perimeter — What's the Difference

Many primary and junior-high students mix up these two concepts. Here is a side-by-side comparison so you won't get them confused again.

An easy way to remember: if you want to paint the floor → area. If you want to put up a fence → perimeter.

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Practice Problems

Try them yourself before checking the answer key.

1. ⭐ A rectangular postcard has a length of 15 cm and a width of 10 cm. What is its area?
2. ⭐⭐ The garage floor in Pak Joko's house is 5 m long and 350 cm wide. What is the area of the garage floor in m²?
3. ⭐⭐ A rectangular rice field has an area of 240 m² and a length of 20 m. What is the width of the field?
4. ⭐⭐⭐ A classroom wall is rectangular, 8 m long and 3 m wide. The wall will be painted. If one can of paint covers 6 m², how many cans of paint are needed?

Answer Key

1. A = 15 × 10 = 150 cm². Multiply directly — both units already match.
2. A = 5 × 3.5 = 17.5 m². Convert the width first: 350 cm = 3.5 m, then multiply.
3. l = 240 ÷ 20 = 12 m. Use the inverse formula l = A ÷ p because we are looking for the width.
4. 24 ÷ 6 = 4 cans. First compute the wall area: 8 × 3 = 24 m². Then divide by the capacity of one can: 24 ÷ 6 = 4.

Summary

1. The area of a rectangle formula is A = p × l, read as length times width.
2. The unit of area is always a square unit: cm², m², or km², matching the unit of the sides.
3. To find an unknown side, use the inverse formula: p = A ÷ l or l = A ÷ p.
4. Before multiplying, match the units of length and width — never mix cm with m.
5. Area measures the inside surface, while perimeter measures the outer edge — they are not the same.

Learn Maths in a More Engaging Way at Algonova

The area of a rectangle is one of the foundations of geometry that students keep using all the way through high school. At Algonova, primary and junior-high kids learn maths through coding and real-world projects — not memorisation. More than 1,000,000 students in 90+ countries already study with us, with a maximum of 8 students per class so the teacher can focus on every child.

Explore more:

• How to calculate the perimeter of a rectangle — the companion formula
• How to find the area of a triangle
• How to find the surface area of a cylinder
• How to solve quadratic equations
• Coding for primary school kids — learn maths through coding

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