Square Footage Calculator
Work out the area of a room or plot, in square feet, yards, metres or acres.
Last reviewed September 19, 2026
Result
Buy 180 ft² square feet
No allowance is added. This is a measurement, not a material estimate. The calculators that buy something add their own waste on top of an area like this one.
- Perimeter
- 54 ft (16.4592 m)
https://calcgallery.com/area-volume/square-footage-calculator/
Results are estimates for planning only. Actual needs vary with site conditions, products and workmanship. Always confirm quantities with your supplier and follow local building codes and manufacturer instructions. Read the full disclaimer
How to use it
- Pick the shape that matches what you are measuring.
- Enter the dimensions. For a triangle or trapezoid the height is measured square to the base, not along a sloping side.
- For an awkward room, choose several rectangles and add a row for each part.
- Read the area in square feet, yards, metres or acres.
How it is calculated
Each shape has its own formula, and all of them reduce to a length times a length:
rectangle A = length × width
circle A = π × radius²
triangle A = ½ × base × height
trapezoid A = ½ × (side a + side b) × height
L-shape A = first rectangle + second rectangleFor a triangle or a trapezoid, the height is the perpendicular distance between the base and the opposite point or side. It is not the length of a sloping edge, and using the sloping edge always gives an answer that is too large.
The L-shape and several rectangles options do the same thing: they let you break an awkward space into parts that do not overlap and add them up. Most real rooms are two rectangles.
Practical tips
Sketch it first and write the numbers on the sketch. The mistakes in area calculations are almost never arithmetic. They are a wall measured twice, a section missed, or two rectangles that overlap in the corner.
Decide whether alcoves and bays count. A bay window can add several square feet, and whether it belongs depends entirely on what the number is for. Flooring includes it. A rough paint estimate usually does not.
Measure the same way twice. Once along each wall, then check by measuring the diagonals. In a true rectangle the diagonals are equal, and if they are not, the room is out of square and you should treat it as a trapezoid.
Do not subtract small obstructions. A kitchen island or a hearth takes area out on paper but the material still has to be cut around it, and the offcuts are rarely usable. Leave them in.
Switch units for large plots. Once you are past a few thousand square feet, acres or hectares are far easier to sanity check, and errors of a factor of ten become obvious.
With your numbers
- Area = 12 ft × 15 ft = 180 ft²
Quick reference
No allowance is added. This is a measurement, not a material estimate. The calculators that buy something add their own waste on top of an area like this one.
| Job | Area |
|---|---|
| 12 × 15 ft room | 180 ft² |
| 10 × 10 ft room | 100 ft² |
| 20 × 30 ft garage | 600 ft² |
| Circle, 5 ft radius | 78.5398 ft² |
| Triangle, 12 ft base, 10 ft high | 60 ft² |
| L-shape, 12 × 15 plus 8 × 6 ft | 228 ft² |
| 4 × 5 m room | 20 m² |
| Circle, 1.5 m radius | 7.0686 m² |
Common questions
- How do I calculate square footage of a room?
- Multiply the length by the width. A 12 by 15 ft room is 180 ft². If the room is not a rectangle, split it into rectangles, work each out and add them, which is what the several rectangles option does.
- How many square feet are in a square yard?
- Nine, because a square yard is three feet by three feet. So 180 ft² is 20 yd². Carpet is often still sold by the square yard, which is why the conversion matters.
- How do I work out an irregular room?
- Break it into rectangles that do not overlap, measure each and add them. Almost every awkward room is two or three rectangles. For a genuinely curved edge, approximate it with several narrow rectangles, or treat it as part of a circle.
- How do I measure a triangle or a gable end?
- Half the base times the height, where the height is measured square to the base rather than along the sloping edge. Using the slope length instead of the true height is the usual mistake and it overstates the area.
- How many square feet are in an acre?
- 43,560. That is a plot of about 209 ft square. Switch the display to acres once an area gets large enough for square feet to stop being useful.
- Should I measure to the wall or to the skirting?
- Depends what you are buying. Flooring goes to the wall, under the skirting, so measure wall to wall. Paint and wallpaper follow the visible surface. For a room being sold or let, the measuring convention is set locally and is worth checking.