Online geometry calculators

Calculate the area of the main plane figures: rectangle, triangle, circle, pentagon, hexagon, rhombus, ellipse and trapezoid. Pick a tool from the menu on the left or from the cards below; formulas, notes and frequently asked questions are further down.

Featured tools

CalculatorRectangle areaA = b · h, the product of base and height. CalculatorTriangle areaA = b · h / 2, half the product of base and height. CalculatorCircle areaA = π · r², from the radius.

Area calculators

Eight calculators for the area of the most common plane figures. Choose a shape: the calculator page opens.

CalculatorRectangle areaA = b · h. Quadrilateral with four right angles. CalculatorTriangle areaA = b · h / 2. Polygon with three sides. CalculatorCircle areaA = π · r². Points whose distance from the center does not exceed the radius. CalculatorPentagon areaFive-sided polygon. If regular, A = p · a / 2. CalculatorHexagon areaSix-sided polygon. If regular, A = (3√3 / 2) · l². CalculatorRhombus areaA = D · d / 2. Quadrilateral with four equal sides. CalculatorEllipse areaA = π · a · b. Closed curve defined by two semi-axes. CalculatorTrapezoid areaA = (B + b) · h / 2. Quadrilateral with two parallel sides.

Main formulas

The reference formulas for each figure are summarized below. For regular polygons the area can also be calculated as half the product of the perimeter and the apothem.

Shape	Area	Symbols
Rectangle	A = b · h	b base, h height
Triangle	A = b · h / 2	b base, h height relative to the base
Circle	A = π · r²	r radius, π ≈ 3.14159
Regular pentagon	A = p · a / 2 ≈ 1.7205 · l²	p perimeter, a apothem, l side
Regular hexagon	A = (3√3 / 2) · l² ≈ 2.598 · l²	l side
Rhombus	A = D · d / 2	D longer diagonal, d shorter diagonal
Ellipse	A = π · a · b	a and b semi-axes
Trapezoid	A = (B + b) · h / 2	B longer base, b shorter base, h height

Reference formulas for the area of the main plane figures.

Practical notes

Use the same unit of measurement for all dimensions: if lengths are in centimeters, the area is in square centimeters (cm²). To convert between area units, remember that 1 m² = 10,000 cm². Round only the final result, not the intermediate values, to avoid accumulating errors.

Frequently asked questions

How do you calculate the area of a triangle?

The area is half the product of base and height, A = b · h / 2. The height is the perpendicular distance between the base and the opposite vertex. Example: base 8 cm and height 5 cm give A = 8 · 5 / 2 = 20 cm².

What is the formula for the area of a circle?

A = π · r², where r is the radius. If you know the diameter, the radius is half of it. Example: with r = 3 m, A = π · 9 ≈ 28.27 m².

How do you calculate the area of a trapezoid?

Add the two bases, multiply by the height and divide by two: A = (B + b) · h / 2. Example: B = 10, b = 6, h = 4 give A = 16 · 4 / 2 = 32.

How do you calculate the area of a regular hexagon?

With side l, A = (3√3 / 2) · l² ≈ 2.598 · l². Example: for l = 4 cm, A ≈ 2.598 · 16 ≈ 41.57 cm².