Rectangle Area Calculator
Calculate the area of a rectangle by entering length and width. Fast and clear result with A = length x width.
Rectangle area, and why the simple cases still go wrong
Length times width is the least controversial formula in geometry. The errors happen elsewhere: mixed units, confusing area with perimeter, and assuming a room is rectangular when it has an alcove, a bay, or walls that are not quite square.
Formula
A = length x width
Both measurements must be in the same unit. A length in metres and a width in centimetres gives a meaningless number.
Area and perimeter are independent
Two rectangles can share a perimeter and have very different areas. A 1 x 9 rectangle and a 5 x 5 square both have a perimeter of 20, but areas of 9 and 25.
For a fixed perimeter, the area is maximised when the shape is square, and it approaches zero as the rectangle becomes long and thin. This is why field boundaries, packaging, and floor plans tend toward squarer proportions when material cost is the constraint.
The practical consequence: you cannot estimate paint or flooring from a perimeter measurement, and you cannot estimate skirting board or fencing from an area. They are different quantities requiring different measurements.
Measuring rooms that are not really rectangles
Very few real rooms are true rectangles. The reliable method is to divide the space into rectangles, calculate each, and add.
- Sketch the floor plan and split it into rectangles along the walls.
- Measure each rectangle at the widest points, in the same unit throughout.
- Calculate each area separately and sum them.
- Subtract permanent fixtures only if they reach the floor - a kitchen island counts, a wall cabinet does not.
- Add a wastage allowance: 5 to 10% for straight-lay flooring, 15% for diagonal or patterned lay.
Unit conversion is where the money is lost
Area conversions square the linear factor, which catches people out. One metre is 100 centimetres, but one square metre is 10,000 square centimetres, not 100.
The same trap applies between imperial and metric. One foot is 0.3048 m, so one square foot is 0.09290 m^2. A 400 sq ft room is 37.16 m^2, not 121.9 - the difference is a factor of 3.28 applied once too many times.
Worked example
An L-shaped living room: the main section is 5.2 m by 3.8 m, with an alcove 1.6 m by 2.1 m off one side. A chimney breast 0.9 m by 0.3 m projects into the main section.
Main: 5.2 x 3.8 = 19.76 m^2. Alcove: 1.6 x 2.1 = 3.36 m^2. Chimney breast: 0.9 x 0.3 = 0.27 m^2, subtracted because it reaches the floor.
Net floor area: 19.76 + 3.36 - 0.27 = 22.85 m^2. Ordering engineered flooring at 10% wastage means buying for 25.14 m^2, which at typical pack sizes of 2.2 m^2 means 12 packs.
Area unit conversions
| From | To | Multiply by |
|---|---|---|
| square metres | square feet | 10.7639 |
| square feet | square metres | 0.092903 |
| square metres | square centimetres | 10,000 |
| square yards | square metres | 0.836127 |
| acres | square metres | 4,046.86 |
| hectares | square metres | 10,000 |
Linear factors must be squared for area. Metres to feet is 3.2808, so square metres to square feet is 3.2808^2 = 10.7639.
Frequently Asked Questions
Related calculators
References
- NIST Handbook 44 - specifications for measuring devices
- BSI - BS 7976 and dimensional measurement guidance
Last reviewed: 2026-08-07. This page is informational. For legal, medical, tax, or financial decisions, confirm the result with a qualified professional.