Free · Plane geometry calculators 🔺

Rectangle Area, Boundary Length and Diagonal

Calculate rectangle area, boundary length and diagonal with bidirectional inputs, compatible units and clear geometric assumptions.

Calculator inputs

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Enter the known values in the units shown. Results update as you type.
Where results are editable, change one to solve backwards. Lock a value to hold it fixed.
Use the worked example to check the method. Reset restores the starting fields.

Inputs and results stay in this browser tab. Bookify does not upload or store the values you enter.

A 4-by-3 rectangle has area 12, perimeter 14 and diagonal 5 in compatible units.

A = ab; P = 2(a+b); diagonal² = a²+b²

Enter these known values and leave the other values blank.

Adjacent sides are perpendicular. Area and perimeter alone determine an unordered side pair; the retained root branch is stated in the title.
Length and area selectors are converted before calculation. A squared-length unit is not the same as a length unit.
The supplied measurements must be mutually consistent. Clear an unknown value rather than supplying conflicting dimensions.
Arithmetic uses 64 significant digits and full-precision π where applicable; displayed values are rounded separately.

The unordered dimensions are determined when P² ≥ 16A. Swapping the two sides describes the same rectangle. This page retains the captured branch.

The international acre is exactly 43560 square international feet; Bookify replaces the rounded captured area factor.