Square Feet Of Rectangle Calculator - Area, Perimeter, and Diagonal in Feet

A square feet of rectangle calculator turns any rectangle's length and width in feet into the rectangle's area in square feet (ft²). Type the two sides in feet and the calculator returns the area in ft² plus the perimeter and the diagonal in feet, so a tape measure reading drops straight into a flooring, paint, or yard-takeoff number.

• • Flooring and tile takeoffs: Measure a rectangular room in feet, type the length and width, and the calculator returns the floor area in ft² for ordering flooring, tile, or underlayment.
• • Wall and paint coverage: Translate a rectangular wall's length and height in feet into the wall area in ft² for paint, primer, or wallpaper coverage estimates.
• • Yard, deck, and patio planning: Find the area of a rectangular yard, deck, or patio in ft² to order sod, mulch, pavers, or deck stain in the right amount.
• • Geometry homework and quick sketches: Verify textbook answers and quick sketches by switching the length and width around and reading the area, perimeter, and diagonal in the same units.

The calculator is built for the most common tape-measure case: a single rectangle whose length and width are known in feet.

Keep both inputs in feet. Mixing feet with inches, centimeters, or meters will mix the systems and the resulting square feet number will be off by a square of the unit ratio.

For a rectangle where only the area and perimeter (or area and aspect ratio) are known, the Length Width Area Rectangle Calculator solves length and width back out from those constraints.

The calculator reads the length and width in feet, multiplies them for the area, and uses standard geometry formulas for the perimeter and diagonal. Every result drops out in feet or square feet, so the output panel reads as a single tape-measure-style takeoff.

• length (feet): Length of the rectangle in feet, used as one side of the multiplication and one term in the perimeter.
• width (feet): Width of the rectangle in feet, used as the second side of the multiplication and the second term in the perimeter.
• diagonal (feet): Diagonal of the rectangle in feet, returned from the Pythagorean identity diagonal^2 = length^2 + width^2.

The area is a simple product. The perimeter and diagonal are extra rows that turn the same length-and-width pair into a small set of useful tape-measure values.

The diagonal uses the Pythagorean theorem, so a 3-4-5 rectangle gives a 5 ft diagonal and a 5-12-13 rectangle gives a 13 ft diagonal. That makes the diagonal a quick sanity check on whether the room is actually rectangular.

According to Wolfram MathWorld, the area of a rectangle is the product of its length and width, the perimeter is twice the sum of the two sides, and the diagonal is the square root of the sum of the squares of the two sides.

When you also need perimeter, diagonal, or side formulas in one place, the Rectangle reference page covers the full rectangle math.

These four ideas decide how the length and width map onto the result panel and how to read each output.

Length is usually the longer side of the rectangle, but the formulas are symmetric, so swapping length and width returns the same area, perimeter, and diagonal.

Square feet is the unit that links this calculator to flooring, paint, and yard takeoffs. Most trades quote material in ft², so the area result drops straight into a material order.

If one of the source measurements comes in yards, meters, or acres, the Square Feet Converter moves that number into square feet before the area step.

Type the rectangle's length and width in feet, then read the area, perimeter, and diagonal in the result panel. The result updates in real time as the inputs change.

1. 1 Measure the rectangle in feet: Use a tape measure to find the length and width of the rectangle in feet. Round each measurement to the nearest 0.01 ft or better before you type it.
2. 2 Enter the length and width: Type the longer side into Length (feet) and the shorter side into Width (feet). Both inputs use the same unit, so the area in ft² is a true square feet number.
3. 3 Read the area in ft²: The primary result is the rectangle's area in square feet. Use it directly for flooring, paint, yard, or homework orders.
4. 4 Use the perimeter and diagonal as checks: The perimeter tells you the total length of trim, edging, or fencing around the rectangle. The diagonal is a quick tape-measure check that the corners really form a rectangle.
5. 5 Fix validation errors: If the calculator shows an error, check that the length and width are both positive finite numbers. Empty or negative inputs are caught and explained.

A bedroom floor is 14 ft by 11 ft. Type length = 14 and width = 11, and the calculator returns area = 154 ft², perimeter = 50 ft, and diagonal = sqrt(317) = 17.80 ft. Order carpet for 154 ft² and use the diagonal to confirm the corners form a true rectangle.

If you only know the area and the width and need the matching length, the Length of a Rectangle solves the long side from any two of width, area, perimeter, or diagonal.

Keeping the inputs and outputs in feet and square feet shortens the path from a tape measure to a material order.

• • Length times width with no mental math: The area, perimeter, and diagonal update as you type, so there is no pencil-and-paper step between the tape measure and the takeoff.
• • Three results from two inputs: A single length and width pair returns the area in ft², the perimeter in feet, and the diagonal in feet, so one entry covers most tape-measure questions.
• • Honest support values: The perimeter and diagonal are geometric values, not rounded guesses. The diagonal uses the Pythagorean theorem, so a 3-4-5 rectangle returns exactly 5 ft.
• • Validation built in: Empty fields and negative sides are caught with a named validation message, so a missed measurement does not produce a phantom area.
• • Unit-consistent workflow: Both inputs are in feet and every output is in feet or ft², so the result drops straight into a flooring, paint, or yard takeoff without a unit conversion step.

The scope is intentionally small. The most common tape-measure case is a single rectangle whose length and width are known in feet, so the tool is built for that case.

For mixed shapes, add a rectangle plus a triangle or a second rectangle together using the same square feet. The unit stays the same across shapes, so the totals line up.

When the diagonal is the main tape-measure number you already have, the Diagonal of Rectangle Calculator returns the matching length and width from the diagonal and one other side.

The math is stable, but the measurement precision and the unit used on the tape measure decide how trustworthy the ft² number is.

• • The calculator assumes a perfect rectangle. Walls at an angle, alcoves, or curved corners change the true area, and the simple multiplication will overstate it.
• • Both inputs must be in feet. Inches or centimeters need to be converted to feet (divide inches by 12, multiply centimeters by 0.0328084) before the area is a true square feet number.
• • The calculator returns a geometric area only. Real-world takeoffs usually add waste, overlap, or seam allowances, or subtract openings, before they become a material order.

If the area looks too high, the most common cause is that one side was measured in inches instead of feet. Re-measure both sides and confirm the unit before changing the inputs.

Floating-point precision keeps the multiplication exact for normal flooring, paint, and yard takeoffs. Very large inputs (over a million feet) can lose a few digits of precision, but that is rare in everyday work.

According to Khan Academy, the area of a rectangle equals length times width, and the perimeter equals twice the length plus twice the width.

When the layout mixes the rectangle with a triangle or a circle, the Area Calculator keeps the area number and switches the shape without leaving square feet.