Use the calculator on the right to calculate the properties of a rectangle.

Enter the two side lengths and the rest will be calculated.
For example, enter the two side lengths. The area, perimeter and diagonal lengths will be found.

Try this

In the figure above, click on "hide details"

Drag the orange dots on the vertices to make a random-size rectangle.

Now try to estimate the area of the rectangle just looking at the squares inside it

When you done click "show details" to see how close you got.