How much paint does the room need
The living room walls are bare and the paint can says one litre covers 10 square meters. The room is a plain rectangle, 5.5 meters long and 4 meters wide, but the floor is not what you are painting, the walls are. Still, every wall is a rectangle, and the whole job starts with the same simple question: what is the area of a rectangle?
The rectangle area formula
Formula: A = length x width
Length and width are the two perpendicular sides, measured in the same units. It does not matter which side you call length; the product is the same. The answer comes out in square units: square meters, square feet, square inches, matching whatever units you measured in.
How to use this calculator
- Measure the two sides in the same units.
- Enter the length and the width.
- Run the calculation to get the area in square units.
- For walls, repeat for each wall and add the results, then subtract doors and windows.
Worked example
One wall is 5.5 m long and 2.8 m high. Step 1: multiply, 5.5 x 2.8 = 15.4 square meters. Step 2: subtract the door, 2.0 m x 0.9 m = 1.8 square meters, leaving 15.4 - 1.8 = 13.6 square meters to paint. Step 3: divide by coverage, 13.6 / 10 = 1.36 litres, so two 1-litre cans cover it with a little to spare.
Where this comes in handy
Flooring installers price by the square meter, tilers count tiles per square foot, gardeners order topsoil by bed area, and carpet layers cut to room dimensions. The rectangle is the workhorse shape of every renovation, and nearly every irregular room can be split into rectangles to estimate it.
Real estate listings quote rooms in square feet or square meters because buyers instantly grasp what twenty square meters feels like. Event planners work the same way, dividing a hall's total floor area by the space allowed per guest to set a safe and comfortable capacity for the venue.
Mistakes people often make
- Mixing units, feet for length and inches for width. Convert everything to one unit first.
- Forgetting to subtract doors, windows, and fixtures from wall areas.
- Measuring the diagonal by mistake. Area needs the sides, not the corner-to-corner distance.
What this number does not tell you
The formula assumes a true rectangle with four right angles. Older rooms are often slightly out of square, so a wall that is 5.5 m at the floor might be 5.45 m at the ceiling. For paint and flooring the difference is negligible, but for fitted furniture measure in several places and use the smallest reading.
Measuring less regular spaces? The trapezoid area calculator handles tapered rooms, the sector area calculator covers curved bays, and the area under the curve calculator tackles irregular plots.