Geometric Formulas
Circle 

Area= 
Square of Diameter x .7854  
or Square of Radius x 3.1416  
Circumference=  Diameter x 3.1416  
Diameter=  Circumference x .3183 
Doubling diameter increases area four times; tripling diameter increases area nine time, etc. 
Square  
Area=  Square of Side  
Diagonal=  Side x 1.4142  
Side=  Diagonal x .7071 
Square Inscribed In Circle 

Side of Square= 
Diameter of Circle x .7071  
or Circumference of Circle x .2251  
Diameter of Circle=  Side of Square x 1.4142  
Circumference of Circle=  Side of Square x 3.545 
Rectangle  
Area=  Length x Width  
Diagonal=  Square root of sum of squares of Width and Length 
Triangle  
Area=  Base x 1/2 of Perpendicular Height 
Hexagon 
(Equal sides and angles) 

Area=  Square of Distance across Flats x .866  
or Square of Side x 2.598  
Side=  1/2 of Diagonal  
or Distance across Flats x .577  
Diagonal=  Distance across Flats x 1.155 or Side x 2 
Octagon 
(Equal sides and angles) 

Area=  Square of Distance across Flats x .828  
or Square of Side x 4.828  
Side=  Diagonal x .383  
or Distance across Flats x .414  
Diagonal=  Distance across Flats x 1.082 or Side x 2.163 
Sphere  
Area of Surface=  Square of Diameter x 3.1416  
Volume=  Cube of Diameter x .5236 
Cube  
Area of Surface=  Square of Side x 6  
Volume=  Cube of Side  
Diagonal=  Side x 1.732 
Cylinder  
Area of Curved Surface=  Diameter x Length x 3.1416  
Volume=  Square of Diameter x Length x .7854 
Cone  
Area of Curved Surface=  Diameter of Base x Slant Height x 1.5708  
Volume=  Diameter of Base Squared x Perpendicular Height x 1.5708 or Area of Base x 1/3 Perpendicular Height 
Pyramid  
Lateral Surface Area(not incl. base)=  Perimeter of Base x 1/2 Slant Height  
Volume=  Area of Base x 1/3 Perpendicular Height 
NOTE:  
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