Rectangle Area Calculator

A = l × w

That’s the whole formula. Length times width. Rectangles are the workhorse 2D shape — most rooms, most boards, most pieces of paper, most parcels of land.

Where rectangles show up in measurements:

• A 12 ft × 15 ft bedroom has 180 sq ft of floor. A gallon of paint covers about 350 sq ft, so one gallon does the floor (or one wall and you’ll be cutting it close).
• A standard 4 ft × 8 ft sheet of plywood is 32 sq ft of usable area. A 200 sq ft project needs at least 7 sheets — buy 8 for waste and bad cuts.
• The US letter page is 8.5 in × 11 in = 93.5 sq in. A4 paper is 210 mm × 297 mm = 62,370 sq mm (about 96.7 sq in) — slightly larger.

Working with rectangles in practice:

Order materials by area, not perimeter. A 10 ft × 10 ft room and a 5 ft × 20 ft room both have 40 ft of perimeter, but the 5 × 20 has the same 100 sq ft floor. Perimeter doesn’t tell you flooring needs.

For paint: divide the room area by paint coverage rate. Two coats means double the math.

For flooring: add 5 to 15 percent waste depending on the material. Tile that runs diagonally wastes more; laminate planks waste less. Hardwood with random plank lengths usually needs 10 percent extra.

Worked example — covering a wall:

Wall 14 ft wide × 9 ft tall = 126 sq ft. One door takes 21 sq ft (3 ft × 7 ft) and one window takes 12 sq ft (3 ft × 4 ft). Paintable area = 126 − 21 − 12 = 93 sq ft. Two coats = 186 sq ft of coverage needed. A gallon does 350 sq ft, so one gallon is plenty for two coats with leftover for touch-ups.

Aspect ratio matters for some things, not others:

For flooring or paint, only area matters. For carpets and rugs, the aspect ratio of the room dictates which rug sizes work. A 9 × 12 rug fits beautifully in a 14 × 16 room but looks stranded in a 30 × 10 hallway.

Diagonal: d = √(l² + w²). Useful for sizing the longest item you can fit on the diagonal, or for checking if a sheet is actually square (diagonals of a true rectangle are equal).