Rectangle Area Calculator FAQ
What is the formula for the area of a rectangle?
The formula is A = length × width. Example: 8 m × 5 m = 40 m². Example: 12 cm × 7 cm = 84 cm². For a square, both sides are equal: a 10 m × 10 m square = 100 m². The unit of area is always the square of the length unit — cm gives cm², m gives m², etc.
How do I calculate the area of a rectangle from length and width?
Multiply length by width. Example: room 4.5 m × 3.2 m = 14.4 m². Example: garden 15 m × 8 m = 120 m². Example: A4 paper is 29.7 cm × 21 cm = 623.7 cm². For flooring, always add 5–10% for waste and cutting. A 20 m² room needs roughly 22 m² of flooring material.
Can I find the area of a rectangle from perimeter and one side?
Yes. If you know one side s and the perimeter P, use A = s × (P/2 − s). Example: perimeter 26 m and one side 8 m → other side = 13 − 8 = 5 m → A = 8 × 5 = 40 m². Example: perimeter 30 cm, side 9 cm → other side = 15 − 9 = 6 cm → A = 54 cm². Useful when you have a measured perimeter but know only one dimension.
Can I calculate the area of a rectangle from diagonal and one side?
Yes. If you know one side s and the diagonal d, the other side = √(d² − s²), giving A = s × √(d² − s²). Example: diagonal 10 cm, one side 6 cm → other side = √(100 − 36) = √64 = 8 cm → A = 48 cm². Example: diagonal 13 m, side 5 m → other side = √(169 − 25) = 12 m → A = 60 m².
Can I find rectangle side lengths from the area?
Yes. Enter the area and one known value (other side, perimeter, or diagonal), and the calculator solves the missing dimension. Example: area 60 m², one side 10 m → other side = 60/10 = 6 m. In perimeter and diagonal modes, the calculator uses the reverse formula and shows the shorter result as Side and the longer result as Length.
What if I know the area and perimeter of a rectangle?
The calculator solves both side lengths using s = (P ± √(P² − 16A)) / 4. Example: area 40 m², perimeter 26 m → s = (26 ± √(676 − 640)) / 4 = (26 ± 6) / 4 → sides are 5 m and 8 m. Example: area 36 cm², perimeter 24 cm → (24 ± √(576 − 576))/4 = 6 cm → it's a 6×6 square.
What if I know the area and the diagonal of a rectangle?
The calculator solves both side lengths from the area and diagonal. For area A and diagonal d, the sides satisfy s² + l² = d² and s × l = A, giving a quadratic. Example: area 48 cm², diagonal 10 cm → sides are 6 cm and 8 cm (since 6 × 8 = 48 and √(36+64) = 10). The shorter result is shown as Side and the longer as Length.
What are practical uses for rectangle area calculation?
Flooring and tiling: room 5 m × 4 m = 20 m² — multiply by price/m² to get material cost. Painting walls: wall 4 m × 2.5 m = 10 m², subtract door/window areas. Gardening: lawn 12 m × 8 m = 96 m² of turf or seed needed. Fabric and wallpaper: 2.4 m × 3.6 m = 8.64 m² per roll. Land area: a 100 m × 50 m plot = 5,000 m² = 0.5 hectares. Always measure in the same unit before multiplying.
