The simplest item to use would be a plastic identification card, license, credit card, or library card. These plastic cards are usually about 1 mm thick from front to back.A plastic card is probably the easiest item to use. Other items that are approximately 1 millimeter in length or width tend to be more difficult to work with, but may include a grain of salt or sand, 10 sheets of notebook paper stapled together, the thickness of one sheet of heavy printer paper, the thickness of a fingernail, the thickness of a grain or rice, and the thickness of a U.S. dime.
 Lay the edge or line you plan on measuring down on a sheet of white or light colored paper. Try to keep the entire edge on the paper.


If desired, you can use a pencil to trace along the outside of the edge. This allows you to remove the object being measured and work only with a flat line, which can make measuring such small lengths easier. Doing so is only optional, however.
The paper needs to be light in color. If it isn't, you might have difficulty seeing your marks.
 Use a pencil to carefully mark a perpendicular line off one side of the edge you are measuring. This will become your starting edge.
 Place the narrow 1-mm edge of your plastic card down on the paper, placing it directly next to your starting edge line. Draw another small mark with your pencil on the opposite side of the card.


Keep the mark as close to the outer edge of your card as possible.
Note that you can use this procedure for any other estimation tool. It is not exclusive to plastic identification cards.
 Shift the card so that it lies to the outside of your newly created mark, then draw another mark on the opposite side of the card again. Continue moving the card down the line in small increments like this, marking each one, until you reach the very end of the edge you are measuring.


Keep the card perpendicular to the line being measured the entire time you measure out the marks.
Make sure that the ending edge of the line is also marked.
 Remove the edge being measured and the card you are using the measure it. Count the gaps in between each line you marked. The number of gaps will give you your estimated number of millimeters.


Only count the gaps. Do not count the lines, since there will be one too many.
Find an item roughly 1 millimeter long.
Place the object to be measured on a blank sheet of paper.
Mark the starting edge.
Place your measuring item at the starting edge.
Move the measuring item down the full length.
Count the gaps.
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