Rectangle Calculator (Area, Perimeter, Diagonal)

How to use this calculator

Enter the length and width to get the area, perimeter and diagonal. Or enter any two values — for example area and length — to find the rest. Area units (m², ft², acres…) are converted automatically.

Formulas

A = l × w P = 2(l + w) d = √(l² + w²)

Worked example

A room 8 m long and 5 m wide:

• Area = 8 × 5 = 40 m² (430.6 ft²)
• Perimeter = 2 × (8 + 5) = 26 m (skirting board needed)
• Diagonal = √(64 + 25) = 9.43 m

Practical uses

• Flooring and tiles: buy area + 5–10% for cuts and waste.
• Paint: wall area = perimeter × ceiling height, minus doors and windows.
• Fencing: perimeter of a rectangular plot.

Unit reminders

1 m² = 10.764 ft² · 1 acre = 4,046.9 m² · 1 hectare = 10,000 m²

Frequently asked questions

How do I find the area of a rectangle?

Multiply length by width. An 8 m × 5 m room has an area of 40 m².

How do I find the width if I know the area?

Divide the area by the length. Press × on Width and type the area.

How do I check a rectangle is square-cornered?

Measure both diagonals. If they are equal, the corners are right angles.

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