Enter length and width to calculate the total perimeter.
Keep linear inputs in a consistent unit, verify that side lengths or coordinate points can form the stated shape, and treat sketches as explanatory unless explicitly marked to scale. Coordinate formulas can become unstable for nearly degenerate shapes, so invalid or nearly collinear inputs are rejected rather than silently returning misleading values.
The same length and width produce three useful outputs, but they map to different real tasks. Perimeter estimates boundary material, area estimates surface coverage, and diagonal length can help check squareness or fit. Use the output that corresponds to the physical job instead of substituting one measure because all three happen to appear on the same calculator.
A rectangle formula is unit-agnostic, but the inputs are not. Mixing feet and inches or centimetres and metres without conversion produces a number that has no coherent unit. Convert both side lengths to the same unit first, then label perimeter in that linear unit, area in its square unit, and the diagonal in the original linear unit.
A calculator may display many decimal places even when a tape measure, drawing, or field estimate is only accurate to a fraction of a unit. Preserve enough precision to avoid premature rounding, but report the final value at a precision justified by the source measurements. For material purchasing, apply project-specific waste, overlap, seam, or cut allowances separately from the pure geometry.
For a true rectangle the diagonal follows the Pythagorean relation √(length² + width²), and both diagonals should be equal. On a physical layout, comparing measured diagonals can expose a shape that is not square even when opposite side lengths are correct. The calculator assumes right angles; it cannot prove the measured object actually satisfies that assumption.
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