Transparent method
The formula
Worked example
Example inputs
How to interpret the result
Area and perimeter answer two different questions about the same rectangle, and a project that needs more perimeter, baseboard, fencing, doesn't necessarily need more area, or the reverse. A long, narrow rectangle can have a small area and a large perimeter, while a square with the same area has a much smaller perimeter, because perimeter responds to shape and area doesn't.
Before you rely on it
What to check
Confirm length and width are recorded in the same unit before multiplying. Mixing feet and inches silently produces an answer that's off by a factor of twelve.
The common error
Where people go wrong with rectangle area & perimeter calculator
Assuming two rectangles with equal area have equal perimeter. A 2-by-50 rectangle and a 10-by-10 rectangle both cover 100 square units, but one needs far more material to fence than the other.
Sensitivity evidence
How length changes the area
| Length | Area | Perimeter |
|---|---|---|
| 10 | 80 | 36 |
| 11 | 88 | 38 |
| 12worked example | 96 | 40 |
| 13 | 104 | 42 |
| 14 | 112 | 44 |
Every input, tested
Which input moves the area most
| Input | Tested from | To | Area at each end | Swing |
|---|---|---|---|---|
| Width | 7 | 9 | 84 to 108 | 24 (25%) |
| Length | 11 | 13 | 88 to 104 | 16 (17%) |
Two variables at once
Area by length and width
| Length \ Width | 6 | 8 | 10 |
|---|---|---|---|
| 10 | 60 | 80 | 100 |
| 11 | 66 | 88 | 110 |
| 12 | 72 | 96 | 120 |
| 13 | 78 | 104 | 130 |
| 14 | 84 | 112 | 140 |
Step by step
The worked example, input by input
| Input | Value used |
|---|---|
| Length | 12 |
| Width | 8 |
| Area | 96 |
| Perimeter | 40 |
Where the inputs come from
Enter the length and the width of the rectangle in the same unit, such as feet, meters or inches. Measure the two sides that meet at a corner, not a diagonal, and use the inside dimensions for a room's floor area if you are buying flooring. Area is in square units and perimeter is in the same units as the sides. If the shape is a square, enter the same number for both.
When the result stops applying
Units must match, since multiplying feet by inches gives a meaningless number. Materials come in fixed sizes, so the amount you buy is larger than the area. Curved and angled walls need other formulas.
What to compare it with
Handle other shapes on the triangle, trapezoid, parallelogram and circle calculators, and volumes on the rectangular prism page. The unit conversion calculators change square feet to square meters, and the concrete, paint and flooring calculators in the trades categories use area. The land acreage page converts to acres.
People also ask
Frequently asked questions
How do I calculate the area of a rectangle?
Multiply the length by the width. A rectangle 12 units long and 8 wide has an area of 96 square units.
What is the difference between area and perimeter?
Area is the surface inside the shape, measured in square units, and perimeter is the distance around it, measured in linear units.
How much extra flooring should I buy?
Add about 5% to 10% for cuts and waste, more for diagonal patterns or rooms with many corners.