Estimating

• 1). Gather a dime, penny and quarter along with any marbles to be measured.
  
  
• 2). Place each marble on the dime, the penny and then the quarter in turn, looking down at it from straight overhead.
  
  
• 3). Set each marble near the coin it most closely matches in size. If the marble is noticeably smaller than the dime, it's 5/8-inch or smaller; the most common size is 5/8-inch. If the marble is the same size as the penny or just slightly smaller, it's 3/4-inch--the standard size for shooters in the United States. If the marble is just slightly larger than the quarter it's likely to be 1 inch in diameter, the standard size for a shooter elsewhere in the world.
  
  

Exact Sizing

• 1). Purchase a marble sizing template (see Resources for an example) or create your own by using a sharp craft knife to trim circles of 9/16, 5/8, 3/4, 1, 1-3/8, 1-1/2 and 1-5/8 inches in diameter from a piece of stiff plastic or cardboard.
  
  
• 2). Place each marble through each hole of the template, starting with the largest hole and working toward the smallest hole.
  
  
• 3). Note the measurement of the smallest hole each marble successfully passed through; this is the marble's designated size, even if its actual size is slightly larger or smaller.