Rectangle Area Calculator
Free rectangle area calculator. Enter length and width, or work backwards from perimeter or diagonal to find area, perimeter, diagonal, aspect ratio and optional cost. Includes formulas and FAQs.
Full original guide (expanded)
Rectangle Area Calculator
Compute rectangle area, perimeter, diagonal, aspect ratio, and an optional material cost. Start from length–width, perimeter–side, or diagonal–side.
L & W in this unit
P = 2(L + W)
other side is derived
must be < diagonal
Rectangle formulas used
Area: \( A = L \times W \)
Perimeter: \( P = 2(L + W) \)
Diagonal: \( d = \sqrt{L^2 + W^2} \)
From perimeter + one side: \( W = \frac{P}{2} - L \)
From diagonal + one side: \( W = \sqrt{d^2 - L^2} \)
FAQs
Can I use different units for length and width?
No — convert to a single unit first (e.g. all in inches) so the area can be computed correctly.
What if length equals width?
Then your rectangle is a square. The calculator will notice and tell you.
What does cost per unit² mean?
If you know, for example, that flooring costs 20 €/m², enter 20 and we’ll multiply area × 20.
Formula (LaTeX) + variables + units
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Area: \( A = L \times W \) Perimeter: \( P = 2(L + W) \) Diagonal: \( d = \sqrt{L^2 + W^2} \) From perimeter + one side: \( W = \frac{P}{2} - L \) From diagonal + one side: \( W = \sqrt{d^2 - L^2} \)
- No variables provided in audit spec.
- NIST — Weights and measures — nist.gov · Accessed 2026-01-19
https://www.nist.gov/pml/weights-and-measures - FTC — Consumer advice — consumer.ftc.gov · Accessed 2026-01-19
https://consumer.ftc.gov/
Last code update: 2026-01-19
- Initial audit spec draft generated from HTML extraction (review required).
- Verify formulas match the calculator engine and convert any text-only formulas to LaTeX.
- Confirm sources are authoritative and relevant to the calculator methodology.