Geometry
Rectangle Area & Perimeter Calculator
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How to use it
Using the rectangle area & perimeter calculator
- 01
Enter length and width
Straight-line measurements of adjacent sides.
- 02
Optionally label the units
Feet, meters, inches; purely cosmetic, applied to every output.
- 03
Use all three numbers
Area for coverage materials, perimeter for edging, diagonal to verify corners are square.
Good to know
The builder’s diagonal check
Framers measure both diagonals of a laid-out wall or deck; matching diagonals prove the rectangle is truly rectangular, not a subtle parallelogram. This single check prevents crooked decks and doors that will not hang plumb.
Perimeter versus area economics
Fencing and trim price by perimeter, flooring and paint by area. Optimizing one often trades against the other; a square minimizes perimeter for given area, which is why warehouses trend toward square footprints.
How it's calculated
The math behind this calculator
A = l × w P = 2(l + w) d = √(l² + w²)Area multiplies the sides; perimeter counts the walk around them; the diagonal applies the Pythagorean theorem to the right triangle each half of the rectangle forms; the longest straight path inside the frame.
An optional unit label decorates the output for readability: areas get “square” appended automatically while perimeter and diagonal carry the plain unit.
Assumptions & limitations
- Length and width are perpendicular measurements.
- One shared unit across all inputs.
- Values must be positive.
Worked example
A 12 × 8 rectangle covers 96 square units, takes 40 units of trim to go around, and measures about 14.42 corner to corner.
FAQ
Frequently asked questions
- Does a square work here?
- Yes; enter equal sides. Area becomes s², perimeter 4s and diagonal s√2.
- Which side is “length”?
- Whichever you call it; the formulas are symmetric in l and w, so results are identical either way.
- Can I leave the units blank?
- Absolutely; outputs then appear as bare numbers ready for whatever unit system you are using.
- How is the diagonal computed?
- √(l² + w²) via the Pythagorean theorem; the same math as measuring across with a tape.
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