Geometry calculators
Calculators for measuring flat shapes and solids, and for working out every property of a triangle, circle, rectangle or polygon from the parts you know. Most draw the shape with your values on it, which helps with homework, DIY and flooring estimates.
Which calculator do I need?
| You have or want | Use |
|---|---|
| The floor area of a room or the size of a plot, in m², ft² or acres | Area calculator |
| The length of fencing or trim needed to go round a shape | Perimeter calculator |
| How much a tank, box or cylinder holds, in litres or gallons | Volume calculator |
| A triangle’s area, heights, medians or inradius from three parts | Triangle calculator |
| Circumference and area from a radius, or an arc or segment | Circle calculator |
| The diagonal of a rectangle, screen or square | Rectangle and square calculator |
| The interior angle of a regular hexagon, or the area of any polygon from its corners | Polygon calculator |
Area, perimeter and volume
Measure many shapes at once: pick the shape, enter its dimensions, and read off the result in the units you need.
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Area calculator
The area of thirteen shapes, with a labelled diagram and conversions to ft², acres and hectares.
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Perimeter calculator
The distance round any common shape, including the ellipse by Ramanujan’s approximation.
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Volume calculator
Volume and surface area of eleven solids, in cubic units, litres and gallons.
Triangles
Every side, angle and centre of a triangle from any three independent parts.
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Triangle calculator
Area, angles, type, heights, medians and radii of a triangle from any three parts.
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Triangle solver
Solve any triangle from three parts, with the ambiguous SSA case and a drawing to scale.
Circles, rectangles and polygons
One shape in depth: enter any one or two measurements and get all the others.
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Circle calculator
Radius, diameter, circumference and area from any one, plus arcs, chords and segments.
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Rectangle and square calculator
Length, width, area, perimeter and diagonal from any two known values.
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Polygon calculator
Regular polygons from any one measurement, and any polygon’s area from coordinates.
General calculators and single-shape calculators
The area, perimeter and volume calculators each cover many shapes and answer one question about them. The shape calculators work the other way round: one shape, every question. If you know a circle’s area and want its radius, or a rectangle’s diagonal and one side and want the other, use the circle or rectangle calculator. They solve from whichever values you have.
Worked comparison: one set of measurements, three answers
A 3 m by 4 m rectangle has a perimeter of 2 × (3 + 4) = 14 m, an area of 3 × 4 = 12 m², and a diagonal of √(3² + 4²) = 5 m. A circle of radius 5 m has a circumference of 10π ≈ 31.42 m and an area of 25π ≈ 78.54 m². Perimeter is a length, area is a length squared, and volume is a length cubed. That is why scaling a shape by 2 doubles the perimeter, multiplies the area by 4 and the volume by 8.
Units are where most mistakes happen
Converting lengths is linear, but converting areas and volumes is not. One metre is 3.2808 feet, but one square metre is 10.764 square feet (3.2808²), and one cubic metre is 35.315 cubic feet. Convert the lengths before multiplying, or use the unit selectors on the calculators, which apply the squared or cubed factor for you. A litre is 1,000 cm³, so a 50 cm × 30 cm × 20 cm box holds 30,000 cm³ = 30 L.
Common mix-ups
- Radius versus diameter. Entering a diameter as the radius makes the area four times too large.
- Slant height versus height. A cone’s volume uses the perpendicular height. Its curved surface area uses the slant height.
- Triangle calculator versus triangle solver. Both take three parts. The solver focuses on the sine and cosine rules and the ambiguous SSA case. The calculator adds heights, medians and the inscribed and circumscribed circles.
Guides to read alongside
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Methodology
How results are rounded and how many significant figures are kept.
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Learn
All of the site’s guides in one place.