You cannot get an accurate measure of the area without pi.
If you are interested in an approximation, you could divide the circle up into tiny squares of some fixed area (their size would depend on how big the original circle was). Then count the number of squares where half or more is inside the circle and multiply by the area of each square. That will give you an estimate of the area of the circle.
You could make an approximation with inscribed and circumscribed polygons (which are the sum of a number of isosceles triangles) and average the two areas, increasing the number of sides of the polygons to increase accuracy (that is the way the early Greek mathematicians did it).
Much easier and quicker to use pi!
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To find area of a circle using diameter, you use this formuler. Area=pi(diameter/2)(diameter/2)
By using the other information supplied about the circle to calculate either its radius (from which its area can be calculated) or its area (if the circle is similar to another with a given area and some ratio between the two circle is given):If the diameter is given: radius = diameter ÷ 2If the circumference is given: radius = circumference ÷ 2πIf the circle is similar to another circle which has a given area, and the length ratio is given; square the length ratio to get the area ratio and apply to the given area.
Make it a whole circle. Find the area. Divide by two.
With great difficulty because a circle does not have volume but it does have an area. Area of a circle = pi*radius2 and measured in square units
If the radius of the circle is R, then the area of the whole circle is πR2 So the area of the semicircle is 0.5*πR2