Area Calculator for Rectangle: Formula & Examples

This calculator works out the area of a rectangle from its length and width. You enter the two side measurements in any unit (meters, feet, inches, etc.), and you get the area in square units of that same measurement. The result is the total space inside the rectangle, expressed as length × width.

Formula

The area of a rectangle is the product of its two adjacent sides:

A = l × w

• A = area, in square units (m², ft², cm², etc.)
• l = length of one side, in linear units
• w = width of the adjacent side, in the same linear units

The only rule is that both measurements must use the same unit before multiplying. If you mix meters and centimeters, the result is meaningless. The perimeter, shown as a secondary output, is the total distance around the rectangle: P = 2(l + w).

Worked Example

Using the default values from the calculator above (length = 10 m, width = 5 m):

1. Write the formula: A = l × w
2. Substitute the values: A = 10 × 5
3. Multiply: A = 50
4. Attach the square unit: A = 50 m²

The calculator returns 50.00 m² for the area and 30.00 m for the perimeter. If your room is 10 meters long and 5 meters wide, you need 50 square meters of flooring or carpet.

When to Use a Rectangle Area Calculator

Any time you need the surface area of a shape with four right angles and opposite sides equal, this formula applies. Common real-world uses include:

• Floor area for tiles, carpet, or hardwood
• Wall area for paint or wallpaper
• Garden or lawn area for sod or fertilizer
• Land parcel area (if the plot is a true rectangle)
• Sheet material like plywood, glass, or drywall

The rectangle formula also works for squares, since a square is just a rectangle with equal sides. For a square, A = s², which is the same as l × w when l = w.

How to Measure Length and Width Correctly

Accuracy starts before you type numbers into the calculator. Follow these steps:

1. Use a tape measure or laser distance meter.
2. Measure the longest side first (call it length) and the shorter side second (width).
3. Measure at the midpoint of each wall if the sides are not perfectly straight.
4. For irregular rooms, divide the space into rectangles and calculate each separately, then add the areas.
5. Round to the nearest centimeter or half-inch for most projects; round up when buying materials.

If the shape has a bay window, a curved wall, or an angled corner, it is not a rectangle. In that case, break it into smaller rectangles and triangles, or use a different method.

Common Mistakes and How to Avoid Them

The most frequent error is mixing units. If you measure length in meters and width in centimeters, the product is neither square meters nor square centimeters. Always convert both to the same unit first:

• 1 m = 100 cm, so 2.5 m = 250 cm
• 1 ft = 12 in, so 6 ft = 72 in
• 1 m ≈ 3.28084 ft

Another common mistake is confusing area with perimeter. Area is the space inside (square units); perimeter is the boundary length (linear units). For a 10 m × 5 m rectangle, the area is 50 m² but the perimeter is 30 m. They are different quantities and different units.

A third error is rounding too early. If you measure 9.87 m and 4.23 m, multiply first (41.75 m²) and then round. Rounding each side to 10 and 4 gives 40 m², which is 4% off.

Unit Conversions for Area

Area results come out in square units of whatever you put in. If you need a different unit, convert before multiplying or convert the final area:

• 1 m² = 10.7639 ft²
• 1 ft² = 0.092903 m²
• 1 m² = 10,000 cm²
• 1 ft² = 144 in²
• 1 acre = 43,560 ft²
• 1 hectare = 10,000 m²

For example, a room 12 ft by 10 ft has an area of 120 ft². To express that in square meters, multiply: 120 × 0.092903 = 11.15 m². The calculator gives you the area in the unit you entered; use the conversion factors above if you need another unit.

Limits of the Rectangle Formula

The formula A = l × w only works for true rectangles. It fails for:

• Parallelograms (slanted sides) — use base × height instead
• Triangles — use ½ × base × height
• Circles — use Ï€ × r²
• Irregular quadrilaterals — divide into triangles

Also, the formula assumes a flat, two-dimensional surface. For a curved surface like a cylinder wall, you need a different formula (2Ï€rh). For a 3D box, the surface area is 2(lw + lh + wh), which adds the top, bottom, and four sides.

If you are measuring a physical space, remember that walls may not be perfectly square. A room labeled 10 × 5 might actually be 10.05 × 4.97. For materials you cannot return, measure twice and add a small waste factor (typically 5–10% for flooring).

Frequently Asked Questions

What is the formula for the area of a rectangle?

The area equals length multiplied by width: A = l × w. Both measurements must be in the same unit, and the result is in square units of that measurement. For a rectangle 8 cm by 3 cm, the area is 24 cm².

How do I find the area if I only know the perimeter?

You cannot determine the area from the perimeter alone. Many different rectangles share the same perimeter; for example, a 6×4 rectangle and a 7×3 rectangle both have a perimeter of 20, but their areas are 24 and 21 respectively. You need at least one side length in addition to the perimeter.

What is the difference between area and perimeter?

Area measures the space inside the rectangle, in square units (m², ft²). Perimeter measures the distance around the outside, in linear units (m, ft). For a 10 m × 5 m rectangle, the area is 50 m² and the perimeter is 30 m.

Can I use this calculator for a square?

Yes. A square is a rectangle with all four sides equal. Enter the same number for length and width, and the area will be the side length squared. A square with sides of 6 m has an area of 36 m².

How do I convert square feet to square meters?

Multiply the area in square feet by 0.092903 to get square meters. For example, 200 ft² × 0.092903 = 18.58 m². To go the other way, multiply square meters by 10.7639 to get square feet.

What if my room is L-shaped?

Divide the L-shape into two rectangles. Measure each rectangle separately, calculate each area, then add them together. For example, a 6 m × 4 m section plus a 3 m × 2 m section gives 24 m² + 6 m² = 30 m² total.

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in length | Length | m | 10 | 0
in width | Width | m | 5 | 0
out area | Area | m² | length * width | 2
out perimeter | Perimeter | m | 2 * (length + width) | 2