The Rectangle Surface Area Calculator calculates the area of a flat rectangle by multiplying its length by its width (A = l × w), and also shows the perimeter.
Rectangle Surface Area Calculator
Area of a flat rectangle, or surface area of a 3D rectangle (rectangular prism).
If you mean a 3D rectangle — a box or rectangular prism — switch to the 3D mode, which adds the six faces using A = 2(lw + lh + wh). Enter your dimensions below to calculate the area.
Formula
Flat rectangle (2D):
A = l × w and P = 2(l + w)
3D rectangle (rectangular prism):
A = 2(lw + lh + wh)
Where:
- A = area or surface area
- P = perimeter
- l = length, w = width, h = height
How the Calculation Works
Step 1: For a flat rectangle, multiply length by width. That is the whole area.
Step 2: For a 3D rectangle, find the three different face areas: lw, lh and wh.
Step 3: Add them and multiply by 2, because each face has a matching opposite face.
Variables Explained
| Variable | Meaning | Unit |
|---|---|---|
| l | Length | cm, m, in, ft |
| w | Width | cm, m, in, ft |
| h | Height (3D mode only) | cm, m, in, ft |
| A | Area / surface area | cm², m², in², ft² |
| P | Perimeter (2D mode) | cm, m, in, ft |
Example Calculation
Flat rectangle
Given: length = 8 cm, width = 5 cm
Calculation: A = 8 × 5; P = 2 × (8 + 5)
Result: A = 40 cm², P = 26 cm
3D rectangle
Given: length = 8 cm, width = 5 cm, height = 4 cm
Calculation: A = 2(40 + 32 + 20) = 2 × 92
Result: A = 184 cm²
Room floor in feet
Given: 12 ft × 10 ft floor
Result: A = 120 ft² (about 11.15 m²)
Common Area Conversion Table
| Unit | Equivalent |
|---|---|
| 1 m² | 10,000 cm² |
| 1 ft² | 144 in² |
| 1 yd² | 9 ft² |
| 1 m² | 10.7639 ft² |
Shape Properties
A rectangle has four right angles and opposite sides of equal length. A square is a rectangle with all four sides equal, so its area is side². The rectangle’s diagonal is √(l² + w²).
Dimensions Explained
Area vs surface area
A flat rectangle has an area. “Surface area” strictly applies to a solid, so the surface area of a rectangle usually means a rectangular box. This calculator handles both.
Feet and inches
If a side is 12 ft 6 in, enter 12.5 ft, or enter 150 in — the calculator converts units for you.
Frequently Asked Questions
What is the formula for the area of a rectangle?
Area = length × width. An 8 cm by 5 cm rectangle has an area of 40 cm².
How do you find the surface area of a 3D rectangle?
Use A = 2(lw + lh + wh), which adds the six faces of the box.
Is area the same as surface area?
Area is used for flat shapes; surface area is the total area of all faces of a 3D solid.
How do I convert rectangle area from square feet to square meters?
Divide square feet by 10.7639, or switch the result unit to m² in the calculator.
How do you find the area of a rectangle with the perimeter?
You need one more measurement. With the perimeter and one side, width = P ÷ 2 − length, then multiply length × width.