Rectangle Calculator
Works out a rectangle's area and perimeter from its length and width.
Recent
Formula
How the calculation works
A rectangle is fixed by two numbers, its length and its width. Every other measurement follows, so the calculator needs only those two to produce the area and the perimeter.
The area counts how many unit squares fit inside, which is why it multiplies the two sides. The perimeter walks around the edge and so adds them instead, then doubles the sum because opposite sides are equal.
Both figures scale differently. Double only the length and the area doubles while the perimeter rises by less than double; double both sides and the area grows fourfold but the perimeter only doubles, which is why a larger shape encloses far more per unit of edge.
Common mistakes
- Adding length and width and calling that the area. The sum is half the perimeter, not a surface.
- Multiplying the two sides to get the perimeter. That product is the area; the perimeter needs the sum doubled.
- Reporting the perimeter in square units. Only the area carries a squared unit; the perimeter is a plain length.
When to use it
- Use it when planning a floor, a sheet of material or a garden bed, where the area drives paint or covering and the perimeter drives edging or fencing.
- For an L-shaped or circular space, split it into rectangles or use a circle tool; a single rectangle would miss the extra edges.
Worked example
A length of 8 and a width of 5 give an area of 40 and a perimeter of 26.
Common questions
Do area and perimeter share a unit?
The area is in the unit of length squared, while the perimeter is in that unit, so centimetres give a square-centimetre area.
What is the perimeter for?
The perimeter is the total length around the edge, needed for fencing, framing or borders; the area is for paint or flooring.
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