Area Calculator
The following are calculators to evaluate the area of 7 most common shapes. The area of other shape usually can be obtained by cutting it into many simple shapes and adding the areas up. This calculator is especially useful for estimating the land area.
Rectangle

Triangle

Use the triangle calculator to convert if you only know other parameters of a triangle instead of the length of the three edges. 
Trapezoid

Circle

Sector

Ellipse

Parallelogram

Reference
Area Formulas of Simple Shapes
Rectangle Area

total area = lw
Where l is length and w is width
Triangle Area

total area = (s(sx)(sy)(sz))^{½}
where s = ½(x + y + z)
Trapezoid Area

total area = ½(w1 + w2)h
Where h is the height and w1 and w2 are the top and bottom edge length
Circle Area

total area = πr^{2}
Where r is the radius
Sector Area

total area = ½(θ/π)r^{2}
Where r is the radius and θ is the angle
Ellipse Area

total area = πab
Where a and b are the length of the major and minor axes
Parallelogram Area

total area = bh
Where h is the height and b is the base length
Common Area Units
Unit  Area in m^{2} 
square meter  SI Unit 
hectare  10,000 
square kilometre (km2)  1,000,000 
square foot  0.0929 
square yard  0.8361 
acre  4,046.9 (43,560 square feet) 
square mile  2,589,988 (640 acres) 