Method #1 – Diagonal from Length & Width
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Formula – How to calculate the area of a rectangle from the length & width
Diagonal = √(Length2 + Width2)
Example
A rectangle has a length of 10 and a width of 30.
Diagonal = √(102 + 302) = √(100 + 900) = √1000 = 31.62
Method #2 – Diagonal from Area
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Formula – How to calculate the diagonal of a rectangle using the area
Step 1: Find the Width
Width = Area / Length
Step 2: Find the diagonal
Diagonal = √(Length2 + Width2)
Example
A rectangle has a length of 10 and an area of 200.
Width = 200 / 100 = 20
Diagonal = √(102 + 202) = √(100 + 400) = √500 = 22.36
Method #3 – Diagonal from Perimeter
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Formula – How to calculate the diagonal of a rectangle using the perimeter
Step 1: Find the Width
Width = (Perimeter – Length x 2) ÷ 2
Step 2: Find the diagonal
Diagonal = √(Length2 + Width2)
Example
A rectangle has a length of 10 and a perimeter of 90.
Width = (90 – 10 x 2) ÷ 2 = (90 – 20) ÷ 2 = 70 ÷ 2 = 35
Diagonal = √(102 + 352) = √(100 + 1225) = √1325 = 36.40