Rectangle Area Calculator

The area of a rectangle is A = l × w, length multiplied by width. Enter both above and the calculator returns the area with the substitution shown, converted into square metres, square feet, acres and hectares.

What the formula means

A rectangle is a four-sided shape whose corners are all right angles. Multiplying the two side lengths counts how many unit squares fit inside it. A rectangle 30 units long and 20 wide holds 30 rows of 20 squares, which is 600 squares, so the area is 600 square units.

That is why area is measured in squared units rather than plain ones. The number answers the question "how many unit squares cover this", and a unit square is one unit on each side.

The square is the special case

When length and width are equal, the rectangle is a square and the formula collapses to A = a². This is where the word "squared" for a second power comes from. A square 5 m on each side covers 25 m².

A worked example

Take a length of 30 units and a width of 20.

• Multiply the two: 30 × 20 = 600 square units

Reversing it is just as common. If you know the area and one side, divide. A room covering 24 m² that measures 6 m along one wall must be 4 m along the other.

Perimeter is a different question

Perimeter is the distance round the outside, 2(l + w). Area is the space inside. Skirting board, fencing and edging are bought by perimeter; carpet, turf and paint are bought by area. Mixing them up is the most expensive mistake on this page.

The two do not track each other. Fix the perimeter at 40 m, so the two sides must add to 20, and the area still varies enormously: 10 × 10 covers 100 m², 15 × 5 covers 75 m², and 19 × 1 covers just 19 m². For a fixed perimeter the square encloses the most area, which is why animals curl up to conserve heat and why compact fields cost less to fence per hectare than long thin ones.

Rooms that are not rectangles

Most real floors are rectangles with pieces added or removed. The method is to break the shape into rectangles, work out each, then add or subtract.

An L-shaped room 5 m by 4 m with a 2 m by 1.5 m alcove removed covers 20 − 3 = 17 m². A room with a bay window is the main rectangle plus the bay. Doing this on paper before measuring saves the second trip to the shop.

Adding for waste

The geometric area is never the quantity to order. Flooring, tiles and wallpaper all need an allowance for cuts, breakage and pattern matching.

Material	Typical allowance	For a 17 m² room
Laminate, straight lay	7%	18.2 m²
Tiles, straight lay	10%	18.7 m²
Tiles, diagonal lay	15%	19.6 m²
Patterned wallpaper	15 to 20%	19.6 to 20.4 m²

Buy the whole packs that cover the adjusted figure, and keep the offcuts. Matching a batch later is often impossible, since dye lots vary between production runs.

Units and land measure

Area conversions square the linear factor, which catches people out. One metre is 3.2808 feet, so one square metre is 10.764 square feet.

Unit	Equals
1 m²	10.764 ft²
1 hectare	10,000 m²
1 acre	43,560 ft², or 4046.86 m²
1 square mile	640 acres

An acre was originally the area a team of oxen could plough in a day, which is why it is an awkward number rather than a round one. A hectare is the metric replacement and is about 2.47 acres.

Where it gets used

Building and decorating. Paint coverage, flooring, plasterboard and insulation are all quoted per square metre.

Land and property. Floor area drives valuation, rent and tax in most markets, and plot area drives what can be built.

Printing and materials. Paper and sheet goods are priced per square metre, and cutting plans are area problems with constraints.

Agriculture. Seed, fertiliser and spray rates are quoted per hectare or per acre, so the field area sets the order.

Paint, and why coverage is quoted oddly

Paint is sold by the litre and covers by the square metre, so the area of the walls decides the order. Walls are rectangles, so measure each, add them, then subtract the doors and windows.

A room 4 m by 3 m with a 2.4 m ceiling has a wall area of 2 × (4 + 3) × 2.4 = 33.6 m². Take off a standard door at about 1.6 m² and a window at 1.5 m² and you have 30.5 m² to cover.

Coverage is typically quoted at 10 to 12 m² per litre per coat on a smooth sealed surface, and considerably less on bare plaster, which drinks the first coat. Two coats over 30.5 m² at 11 m² per litre needs about 5.5 litres, so a 5 litre tin plus a 1 litre, or one 5 litre tin if the surface is already painted in a similar shade.

Floor area definitions differ

Property listings quote floor area, but not always the same one, and the differences are large enough to matter when comparing prices.

Measure	What it counts
Gross external	Everything, measured to the outside of the walls
Gross internal	Inside the external walls, including internal walls and stairs
Net internal	Usable floor only, excluding walls, stairs and plant

Net internal can be 15 to 25% below gross external in a building with thick walls and a stairwell. Comparing a price per square metre from one basis against another is how two similar properties appear to differ in value when they do not.

Estimating without a tape measure

A pace is close to 0.75 m for most adults, so pacing a room gives an area within about 10%, which is enough to decide whether something will fit. Standard sizes help too: a single bed is about 0.9 by 1.9 m, a door is 0.76 to 0.84 m wide, and a floor tile is commonly 300 or 600 mm square. Counting tiles is often the fastest way to measure a floor you are standing on.

Diagonals, and checking a room is square

The diagonal of a rectangle is √(l² + w²) by Pythagoras. A room 4 m by 3 m has a diagonal of exactly 5 m, one of the rare cases that lands on a whole number.

Builders use this in reverse. Measure both diagonals of a room or a frame, and if they are equal the corners are true right angles. If they differ, the shape is a parallelogram rather than a rectangle, and any flooring laid to one wall will drift visibly against the other. The check takes ten seconds and saves relaying a floor.

The diagonal also decides what fits. A 2.2 m sheet of plasterboard will not turn a corner in a 2 m corridor, but it will pass diagonally through a doorway wider than its thickness, which is why sheets are carried edge on.

Common mistakes

Mixing units. A length in metres and a width in centimetres gives an answer a hundred times out. Convert first.

Using perimeter where area is wanted. Fencing and flooring answer different questions.

Forgetting the waste allowance. Ordering the exact geometric area leaves you short after the first cut.

Not squaring conversion factors. Square metres to square feet is 10.764, not 3.28.