Area Calculator — 5 Shapes

Pick a shape, enter its dimensions, and the Area Calculator returns the area — plus the perimeter where the formula has enough information to compute it. Five shapes are covered: rectangle, circle, triangle, trapezoid, and parallelogram. Each uses the standard geometric formula, displayed alongside the result so you can see exactly what was applied.

Load Sample pre-fills a rectangle with width 12 and height 8, giving an area of 96 square units and a perimeter of 40 units. Pressing Load Sample again cycles through the other shapes, so you can check formula behaviour on each type before entering your own values.

Getting Your First Result

1. Click a shape tab (Rectangle, Circle, Triangle, Trapezoid, or Parallelogram) — switching tabs clears the inputs, because a radius carried over to a rectangle's width field would silently produce a meaningless answer.
2. Enter each dimension in the labelled fields. The placeholder text shows an example value for every field. Commas are stripped on parse, so 1,200 and 1200 are treated identically.
3. The area appears instantly below the inputs. For rectangle and circle, perimeter or circumference follows automatically in a second card.
4. Copy Result bundles the shape name, formula, and area into one block of text — useful for pasting into a spreadsheet, a document, or a message. The live diagram at the bottom labels each dimension you entered and updates as you type.

The Five Area Formulas

• Rectangle: A = width x height. Perimeter = 2 x (width + height).
• Circle: A = pi x r squared. Circumference = 2 x pi x r. JavaScript's Math.PI supplies pi to 15 significant figures, so rounding is never more than one unit of least precision off at the 6th significant figure.
• Triangle: A = half x base x height. This uses the perpendicular height, not Heron's formula. Perimeter is omitted because the slant sides are unknown.
• Trapezoid: A = half x (a + b) x height, where a and b are the two parallel sides. Perimeter is also omitted here.
• Parallelogram: A = base x height (perpendicular height, not slant side). Identical in form to the rectangle formula but geometrically different.

Precision Limits and What Rounds

• Any positive real number up to 1,000,000,000 (one billion) is accepted. Above that the tool shows "capped at 1,000,000,000 — enter a smaller value" rather than silently computing a product that overflows to Infinity.
• Entering zero or a negative number triggers the message "Width must be greater than 0" (or whichever dimension was invalid). The area stays blank until all required fields pass.
• Results are displayed to 6 significant figures. Very large or very small values switch to exponential notation automatically, for example 3.14159e+18, so the display never clips the number.
• Trapezoid and triangle shapes do not show a perimeter card because the slant-side lengths are not entered and cannot be derived from perpendicular height alone.