Main perimeter
Enter your data and press Calculate
Rectangle perimeter calculator
Compute rectangle perimeter from sides, diagonal or area, with automatic unit conversions.
Rectangle perimeter calculator
Calculate the perimeter with length and width, length and diagonal, or length and area.
Get main perimeter, side lengths, diagonal, and area with instant conversions to mm, cm, m, in, and ft.
Inputs
e.g. 4.8
Longer side of the rectangle.
e.g. 2.9
Second value depending on the chosen method.
0–10
Rounding applied to every result.
Results
Length (a)
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Width (b)
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Diagonal (d)
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Area (A)
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Aspect a:b
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Perimeter conversions
mm
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cm
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m
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in
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ft
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How to calculate a rectangle’s perimeter
The basic rule is to add the length and the width and then multiply that sum by two. If you know the diagonal, you can find the width with Pythagoras: square the diagonal, subtract the square of the length, and take the square root. When you start from an area value, divide the area by the length to recover the width.
The calculator normalizes any unit you provide (mm, cm, m, in, or ft), applies the right operation for your method, and shows perimeter, sides, diagonal, and area in the unit you choose. Everything is rounded with the selected decimals and paired with automatic conversions.
Worked example
For a 4.8 m by 2.9 m rectangle, add both sides (4.8 + 2.9 = 7.7) and double the result to get a 15.4 m perimeter. The diagonal comes from squaring each side, adding those results, and taking the square root, which is about 5.6 m. The area is the length multiplied by the width, giving 13.92 m². The tool repeats this process for any valid pair of inputs.
What inputs can I use?
Pick length + width, length + diagonal, or length + area. With those values the calculator fills in the missing side, gets the perimeter, and handles the unit conversions.
What is the rectangle perimeter formula?
Add length and width and multiply that result by two. With a diagonal, square the diagonal and the length, subtract those squares, and take the square root to get the missing side. With an area, divide the area by the length to find the width before adding and doubling.
Which units are supported?
You can work in millimeters, centimeters, meters, inches, or feet. The main perimeter follows your selected unit and you also get conversions to mm, cm, m, in, and ft.
