How to use this calculator
- Identify the measurements labelled Perimeter P, Known side a. Check whether the figure satisfies the assumptions described below.
- Enter positive measurements in a common unit. Use the degree symbol convention for angles. Ratios such as eccentricity have no unit.
- Select Calculate. Inspect the result labels: lengths, areas and volumes have different dimensions. Copy the output or print the page if you need a record.
The formula
b = P/2−a
Perimeter P = 26; Known side a = 8
Worked example
For Perimeter P = 26 cm, Known side a = 8 cm, applying the displayed relationship gives length ≈ 8 cm. The remaining related measurements appear in the result panel. The example uses ideal dimensions and rounds only for display.
Model and limitations
The diagonals are equal and bisect one another. A square is a rectangle whose adjacent sides happen to be equal. Unless it is a square, the diagonals are not perpendicular.
Do not use a sloping distance as a side. A diagonal must exceed the known adjacent side. Inputs are limited to 10⁹ for lengths, 10¹⁸ for areas and 10²⁷ for volumes. Extremely thin or nearly degenerate figures may lose numerical accuracy; avoid treating displayed rounding as a measurement tolerance.