Rectangle Area Calculator

What is the Rectangle Area Calculator?

Calculate the area, perimeter and diagonal of a rectangle. Area, perimeter and diagonal of a rectangle cover most room, garden and material calculations around the home.

How to use the Rectangle Area Calculator

1. Enter your length, your width.
2. Results update automatically as you type; you can also press Calculate.
3. Read the output: area, perimeter, diagonal.
4. Use the 📊 chart to see the result visually, then copy or share it.

Formula and method

Worked example

With length = 12, width = 8, the Rectangle Area Calculator returns area: 96; perimeter: 40; diagonal: 14.4222051019.

Where the Rectangle Area Calculator helps in real life

You are pricing laminate for a 13 × 16 ft home office. The area of 208 square feet is the figure flooring stores quote against.

You want to know whether a 75-inch TV fits between two windows. The diagonal of the wall space tells you whether the screen size, measured diagonally, will fit.

Common mistakes to avoid

• Confusing area (square units) with perimeter (straight units).
• Measuring one side in feet and the other in inches.
• Forgetting to add waste when buying materials.
• Using a TV’s diagonal size as its width.

💡 Practical tip: Area is length times width, but perimeter is twice length plus twice width — they answer different questions.

Tips for better results

• Measure each side twice at different points.
• Add 10% waste for flooring and tile.
• Use the perimeter for trim and baseboard.

Related tools