Free · 2D geometry calculators 📏

Rectangle Perimeter & Missing Side

Find a rectangle’s missing side from its perimeter and the other side, with independent length-unit selectors.

  • Formula & worked example
  • Private in your browser
  • No signup
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Calculator inputs

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How to use this calculator

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

Private by default

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

Formula and method

A rectangle with a 30 cm perimeter and one 9 cm side has a 6 cm other side: 30 ÷ 2 − 9 = 6.

P = 2(L + W); W = P/2 − L; L = P/2 − W

Worked example

Enter these known values and leave the other values blank.

Full perimeter
30 cm
Length
9 cm
Width
6 cm

Assumptions and limitations

  • Both side lengths must be positive. A known side must be smaller than half the perimeter.
  • A perimeter alone does not determine a unique rectangle. Enter a perimeter and one side, or both sides.
  • The shape has four right angles, with opposite sides equal. Select the correct units for each field; length and width need not use the same display unit.

Common questions

Can perimeter alone give both side lengths?

No. A 30 cm perimeter could describe 9 × 6 cm or 10 × 5 cm, among many other rectangles.

Why does my missing side fail validation?

If the known side is at least half the perimeter, the remaining side would be zero or negative. That does not form a rectangle with positive sides.

References

The calculation equations, inverse formulas, units, and input rules were imported from this source. Bookify provides the interface and equation solver.

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