Geometry Calculator
Area and perimeter for the four shapes that come up constantly — circle, square, rectangle, triangle — from just one or two dimensions, without hunting down four different formulas.
Inputs
Result
Area 78.5398
Circumference 31.4159 (radius 5)
How the geometry calculator works
Circle: area = πr², circumference = 2πr, where the single dimension entered is the radius.
Square: area = side², perimeter = 4 × side.
Rectangle: area = length × width, perimeter = 2 × (length + width).
Triangle here uses area = ½ × base × height — a right-angle-style calculation, distinct from Heron's formula which uses three side lengths instead.
Worked example: rectangle 8 × 5
- Area = 8 × 5 = 40.
- Perimeter = 2 × (8 + 5) = 2 × 13 = 26.
Common mistakes to avoid
Entering diameter where radius is expected for a circle
This tool's circle mode takes the radius, not the diameter — entering the diameter directly quadruples the computed area instead of the intended value.
Using the triangle mode for three known side lengths
This triangle calculation needs base and height, not three sides. For a triangle where only the three sides are known, use the dedicated Triangle Area Calculator, which applies Heron's formula instead.
Frequently asked questions
Why does the triangle option only ask for two values?
It uses the base-times-height form of the area formula, which is the simplest version when you already know or can measure the perpendicular height.
What if I only know a triangle's three side lengths, not its height?
Use the separate Triangle Area Calculator, which applies Heron's formula and needs only the three sides.
Does this handle 3D shapes like spheres or cubes?
No — this is 2D shapes only. For a sphere's volume and surface area, use the Sphere Volume Calculator.
How precise are the results?
Calculations use full floating-point precision internally and display to 4 decimal places, which is more than enough for almost any practical measurement.
Related calculators
Circle Calculator
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Triangle Area Calculator
Heron's formula area from three sides.
Pythagorean Theorem Calculator
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Sphere Volume Calculator
Volume and surface area of a sphere.
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