How the calculator picks a formula
Area measures the flat space a shape covers, always in square units. Every shape has its own formula, so the first thing the calculator needs is which shape you mean. Once you pick one from the dropdown, only the fields that formula requires are read — the rest are quietly ignored.
That is why you can leave the defaults in fields you are not using. A circle never looks at the width box, and a square never looks at the radius. The selected shape alone decides which numbers feed the result.
The seven formulas at a glance
Each supported shape reduces to a short multiplication once you have the right measurements:
- Square: side × side. One length is all you need because every side is equal.
- Rectangle and parallelogram: base × height. For a parallelogram the height is the perpendicular distance between the parallel sides, not the slanted side.
- Triangle: half of base × height, again using the perpendicular height.
- Trapezoid: average the two parallel sides, then multiply by the height between them.
- Circle and ellipse: π times the radius squared, or π times the two semi-axes for an ellipse.
Getting an accurate answer
The most common error is mixing units. If one side is in centimetres and another in metres, convert both to the same unit before entering them, or the area will be off by a large factor.
The second trap is using a slanted edge where the formula wants a perpendicular height. For triangles, parallelograms and trapezoids the height must drop straight down to the base at a right angle. If you only have a sloped side and an angle, solve for the true height first with a triangle or trigonometry tool.
Reading and using the result
The figure is reported in square units that match whatever unit you entered: square metres for metres, square feet for feet, and so on. It is rounded for display but computed at full precision.
For real-world jobs like flooring, paint or fabric, add a waste allowance on top of the bare area — typically five to ten percent — to cover cuts, overlaps and mistakes.
Formula
square s²; rectangle w·h; triangle ½·b·h; circle π·r²; trapezoid ½·(b₁+b₂)·h; parallelogram b·h; ellipse π·a·bFrequently asked questions
- Which fields matter?
- Only the dimensions of the shape you select are used. Extra fields are ignored, so you can leave their defaults.