Square Feet of a Rectangle Calculator
Enter the length and width of your rectangle to get the area in square feet. Supports feet, inches, yards, metres, centimetres, and millimetres. Also outputs the perimeter, the diagonal, and an optional material cost estimate. Perfect for flooring, painting, landscaping, carpet, and any home improvement project.
Formula
Worked example
A room 12 ft x 10 ft: Area = 12 x 10 = 120 sq ft. In square yards: 120 / 9 = 13.33 sq yd. Perimeter = 2 x (12+10) = 44 ft. Diagonal = sqrt(144+100) = sqrt(244) = 15.62 ft. With 10% waste: 120 x 1.10 = 132 sq ft to order. At $3.50/sq ft: 132 x $3.50 = $462.
How to calculate square feet of a rectangle
The area of a rectangle in square feet is simply its length multiplied by its width, both measured in feet. If your measurements are in other units, convert them first: divide inches by 12 to get feet, divide centimetres by 30.48, multiply metres by 3.28084, and multiply yards by 3. For example, a room that is 12 feet long and 10 feet wide has an area of 120 square feet. This single number is what contractors, flooring suppliers, and paint manufacturers use when quoting materials.
Perimeter, diagonal, and why they matter
Beyond the area, two other measurements are useful on most projects. The perimeter (the total distance around all four sides) tells you how much baseboard, edging strip, or border tile you need. For a rectangle it is simply 2 x (length + width). The diagonal - the distance from one corner to the opposite corner, calculated with the Pythagorean theorem as sqrt(length squared + width squared) - is used to check whether a room is truly square: if you measure both diagonals on site and they match, the corners are right angles. These two figures cost nothing extra to calculate alongside the area, and the calculator above includes both.
Ordering materials: waste factors and overage
You should never order exactly the net area of material, because every installation involves cuts, breakage, and trim waste. The industry standard for most flooring projects is 10% extra: multiply the net area by 1.10 and round up to the next full box or roll. Diagonal tile and patterned wallpaper typically need 15-20% because every row starts with an angled cut, and repeat patterns require aligning seams across rolls. The waste field in this calculator lets you set any percentage; the resulting "area with waste" is the quantity to put on your order form. If you also enter a price per square foot, the calculator shows the total estimated material cost.
Rooms with multiple rectangles
Most real rooms are not perfect rectangles: they have alcoves, bay windows, closets, or L-shaped footprints. The easiest approach is to divide the floor plan into non-overlapping rectangles, calculate the area of each one, then add them together. For a closet you want to include, add its area; for a kitchen island or built-in wardrobe you want to exclude, subtract it. When calculating paint area for a wall with a window or door, measure the full wall as one rectangle, then subtract the area of the opening. This calculator handles one rectangle at a time, so run it for each section and add up the results.
Typical waste allowances by project type
| Project type | Recommended waste % | Reason |
|---|---|---|
| Hardwood flooring (straight lay) | 10% | End cuts and minor defects |
| Hardwood flooring (diagonal) | 15% | Longer diagonal cuts increase waste |
| Ceramic or porcelain tile (straight) | 10% | Breakage and edge cuts |
| Tile (diagonal / pattern) | 15-20% | Increased angle cuts at edges |
| Carpet | 10% | Seams and pattern matching |
| Vinyl plank / LVP | 10% | Staggered row offsets |
| Exterior decking | 15% | Ripped boards and fascia |
| Wall paint | 0% | No waste - measure net wall area |
| Wallpaper (plain) | 10% | Overlap and alignment |
| Wallpaper (repeat pattern) | 20-30% | Pattern matching at seams |
Use these percentages as a starting point when estimating how much extra material to order.
Frequently asked questions
How do I calculate the square footage of a rectangle?
Multiply the length by the width, both in feet. For example, a space 15 feet long and 8 feet wide is 15 x 8 = 120 square feet. If your measurements are in inches, divide each one by 12 first. If they are in metres, multiply each by 3.28084 before multiplying length by width, or just use the metre option in this calculator and it converts automatically.
How many square feet are in a square yard?
One square yard equals 9 square feet (because 1 yard = 3 feet and 3 x 3 = 9). To convert square feet to square yards divide by 9. Carpet is often sold by the square yard, so a 120 sq ft room needs 120 / 9 = 13.33 sq yd of carpet.
How much extra material should I order?
For most flooring and tile projects, add 10% for waste, cuts, and breakage. Add 15-20% when laying tile on the diagonal or matching a wallpaper repeat pattern. For exterior decking add about 15% for ripped boards and fascia. Never order exactly the net area - you will almost certainly end up short.
What is the formula for the perimeter of a rectangle?
The perimeter is 2 x (length + width). A 12 ft x 10 ft room has a perimeter of 2 x (12+10) = 44 ft. Use this for baseboard, edging strip, or any border material that runs around the outside of the space.
How do I find the diagonal of a rectangle?
Use the Pythagorean theorem: diagonal = square root of (length squared + width squared). For a 12 ft x 10 ft room that is sqrt(144 + 100) = sqrt(244) = about 15.62 ft. Measuring the diagonals on site is a quick way to check that corners are truly square: if both diagonals match, the room is a rectangle.
How do I calculate square footage for an L-shaped room?
Split the L-shape into two or more non-overlapping rectangles. Measure the length and width of each rectangle separately, calculate the area of each, and add the results together. This calculator handles one rectangle at a time, so run it for each section.
Is square footage the same as square feet?
Yes, "square footage" simply means an area measured in square feet. The two terms are used interchangeably in construction, real estate, and home improvement contexts in the United States. Outside the US, the equivalent metric unit is square metres (m2).