About

Miscalculating rectangular area leads to material waste in construction, incorrect land valuation, or failed engineering specs. This calculator computes area (A = l × w), perimeter, and diagonal for any rectangle using exact arithmetic. It supports 9 unit systems from mm² to ha. Results cross-convert automatically so you can read area in ft² while entering dimensions in m.

The tool assumes a perfect Euclidean rectangle with four right angles. It does not account for survey-grade geodesic curvature. For plots larger than roughly 10 km per side, Earth's curvature introduces measurable error. Pro tip: when ordering flooring or paint, add 5 - 10% waste factor on top of the calculated area.

Formulas

The area of a rectangle is the product of its two adjacent side lengths.

A = l × w

The perimeter is the total boundary length.

P = 2 ⋅ (l + w)

The diagonal connects opposite vertices and follows from the Pythagorean theorem.

d = √l² + w²

Where l = length of the rectangle, w = width of the rectangle, A = area, P = perimeter, d = diagonal. All values share the same linear unit; the area result carries the squared unit.

Reference Data

Unit	Symbol	Relative to 1 m²	Common Use
Square Millimeter	mm²	1,000,000	PCB design, micro-engineering
Square Centimeter	cm²	10,000	Paper sizes, small surfaces
Square Meter	m²	1	Rooms, construction, SI standard
Square Kilometer	km²	0.000001	Cities, countries, geography
Square Inch	in²	1,550.0031	Screens, stamps, US manufacturing
Square Foot	ft²	10.76391	US real estate, flooring
Square Yard	yd²	1.19599	Carpet, fabric, landscaping
Acre	ac	0.000247105	US/UK agriculture, land sale
Hectare	ha	0.0001	EU agriculture, forestry, zoning
Are	a	0.01	Garden plots, small land parcels
Square Mile	mi²	3.861×10⁻⁷	US geography, county-level data
Tatami (Nagoya)	畳	0.5688	Japanese room sizing
Ping	坪	0.3025	Taiwan / Japan real estate
Board Foot	bd ft	423.776 (volume-based)	Lumber trade (1″×12″×12″)

Frequently Asked Questions

Linear dimensions are converted to meters using exact SI conversion factors before multiplication. For example, 1 ft = 0.3048 m exactly (by international agreement since 1959). Area is then computed in m² and converted back. IEEE 754 double-precision floating point guarantees about 15 significant digits, so rounding error is negligible for any practical rectangle.

This calculator returns gross geometric area (l × w). Usable floor area (UFA) subtracts walls, columns, shafts, and stairwells. In residential construction, UFA is typically 85 - 92% of gross area depending on wall thickness and layout. Always verify which standard applies: BOMA (US), RICS (UK), or DIN 277 (EU).

JavaScript handles numbers up to approximately 1.8 × 10³⁰⁸. The calculator accepts any positive value within that range. For micro-scale work (MEMS, semiconductor), use mm input to avoid floating-point issues with very small decimals. For planetary-scale areas, use km.

Perimeter is needed for fencing, baseboards, trim, and edge banding. The diagonal (d = √l² + w²) is critical for verifying squareness during layout: measure both diagonals of the rectangle and confirm they match within tolerance.

Add a waste factor: 5% for paint or stain, 10% for tile or vinyl flooring (straight lay), and 15% for diagonal or herringbone tile patterns. Multiply the calculator's area output by 1.05, 1.10, or 1.15 respectively before ordering materials.

Two rectangles with identical area can have vastly different perimeters. A 1 m × 100 m rectangle has the same 100 m² area as a 10 m × 10 m square, but its perimeter is 202 m versus 40 m. This matters for cost: more perimeter means more fencing, more edge trim, and greater heat loss through walls.