Area Calculator
Calculate the area and perimeter of common shapes — rectangle, circle, triangle, trapezoid, and ellipse.
This calculator finds the area and perimeter of five shapes — rectangle, circle, triangle, trapezoid, and ellipse — reading only the dimension fields that apply to whichever shape you select and quietly ignoring the rest. With the default shape set to Rectangle, the Radius, Base, Height (Triangle), and every advanced trapezoid or ellipse field play no role whatsoever in the reported Area or Perimeter, since the switch statement that computes them for a rectangle only ever reads Length and Width. Length and Width pull on Area in exactly the same proportion, since area is a straight product of the two — a ten percent change to either one moves the area by the same ten percent — but they don't tie on Perimeter, where Length carries noticeably more weight simply because it starts out larger (20 versus 15 at the defaults) in a formula that adds the two sides together rather than multiplying them. This page performs no cross-shape unit conversion: swapping from Circle to Trapezoid mid-session keeps whatever numbers were already sitting in each dimension field, so a Radius value left over from a Circle calculation has no bearing on a Trapezoid's result, and nothing warns you that fields you can no longer see are still holding stale values.
Inputs
Results
Area
300 sq ft
≈ 7 king-size beds
How to Use This Calculator
- Select the shape from the dropdown (rectangle, circle, triangle, trapezoid, or ellipse).
- Enter the required dimensions that appear for your chosen shape (e.g., length and width for a rectangle).
- Area formulas used: rectangle = l × w; circle = πr²; triangle = ½ × base × height; trapezoid = ½(a+b)×h; ellipse = πab.
- Perimeter or circumference is also computed alongside area for each shape.
- All dimension inputs must be positive; units are not converted — ensure all dimensions use the same unit.
- For irregular polygons or composite shapes, break the shape into known sub-shapes and sum their areas.
How the result changes with Length
| Length | Area |
|---|---|
| 10,000 | 150,000 sq ft |
| 35,000 | 525,000 sq ft |
| 65,000 | 975,000 sq ft |
| 90,000 | 1,350,000 sq ft |
What each input means
- Shape
- The geometric shape to calculate.
- Length
- Length of the shape or side.
- Width
- Width of the shape or side.
- Radius
- Distance from center to edge.
- Base
- Base length of the shape.
How this is calculated
Worked example, using the default values
- Identify Input Parameters4 parametersShape = 0, Length = 20, Width = 15, Radius = 10 = 11 input(s) provided
- Calculate AreaArea300 = 300
- Calculate PerimeterPerimeter70 = 70
- Calculate AreaArea27.87 = 27.87
Engine last updated . Checked against 2 independently-derived tests — how we verify calculators.
Frequently Asked Questions
Why doesn't changing the Radius affect the Area when Rectangle is selected?
Each shape's area and perimeter are computed by a separate branch of code that only reads the dimension fields relevant to that specific shape. The rectangle branch reads only Length and Width, so Radius — along with every trapezoid and ellipse field — sits completely unused whenever Shape is set to Rectangle, no matter what value you type into it.
Why does Length affect the Perimeter more than Width does?
Perimeter is twice the sum of Length and Width, an additive formula rather than a multiplicative one, so each side's pull on the total scales with how large that side already is relative to the other. At the default values of 20 and 15, Length makes up a larger share of the sum, so an equal percentage nudge to Length moves the perimeter by more. That's specific to Perimeter, not a pattern that spills over into Area: every one of this calculator's five shapes — rectangle, circle, triangle, trapezoid, ellipse — computes Area as a product of its governing dimensions rather than a sum, which is exactly why Length and Width tie perfectly on Area but pull apart on Perimeter.
Do Length and Width affect Area equally?
Yes — area is Length multiplied by Width, and for a pure product like that, a given percentage change to either factor produces the identical percentage change in the result, regardless of which one starts out larger. That symmetry is specific to Area; it doesn't carry over to Perimeter, which adds the two sides instead of multiplying them.
If I switch from Circle back to Rectangle, does the old Radius value get cleared?
No — the calculator keeps whatever value was last entered in every dimension field, even the ones that stop being relevant once you pick a different shape. Switching Shape back to a setting that used that field, like Circle, will pick the stale value right back up rather than resetting it to a fresh default.
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