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Grab a normal credit card. Stand the card up on a piece of paper beside the object you’re measuring. Use a pen or pencil to draw a thin line along the inner edge of the card. Slide the card down to the other side of the line and repeat the process.
Most credit cards (and other types of plastic cards) have a thickness of 30 mil, which comes out to around 0.76 millimeters (0.762 mm, to be exact). It isn’t the most exact measuring tool, but it may be close enough for tasks that require you to have a rough idea of how something measures up in millimeters.  If you don’t have a credit card handy, stack 10 sheets of 8 1⁄2 in (22 cm) x 11 in (28 cm) printer paper on top of each other to get a layer that’s about 1 millimeter thick. This may be harder to work with than a single plastic card, though. A “mil” is a little-used imperial unit that corresponds to one thousandth of an inch, and is not to be confused with millimeters. Align the outer edge of the card with your chosen starting point on the object. Imagine that the card is a ruler, and that the edge is the 0mm line. For this method, you’ll essentially be adding up 1 millimeter at a time in order to find the one of the object’s given dimensions. Run the tip of your writing utensil down the card to trace a line long enough to be plainly visible. This marks a distance of 0.762 millimeters between the end of the object and your first line. You’ll be drawing a number of lines quite close together, so use light pressure to make the line as thin as possible. Sharpening your pencil or using a pen with an ultra-fine point will help. This line will be 1.52 millimeters from your starting point. Reset your card on the far edge of your second line and draw another. Continue measuring and marking in small increments until you reach the end of the object, then count up the number of individual spaces.  Make sure you count the spaces between the lines and not the lines themselves, since there will be 1 too many. To increase your precision a bit, count every 4 lines as 3 millimeters total. This will help make up the difference, since the card isn’t exactly 1mm thick.