How big is 3 centimetres? Think about the end of your thumb to figure out how big 3 centimetres (cm) is. On average, there is one inch between the first joint in the tip, and one inch is 2.54 centimetres. This means that 1 3/16 inches is about 3 centimetres, and 3 centimetres is just a little bit longer than the average thumb from the first knuckle to the tip.
Three centimetres is about the length of a dollar bill folded in half lengthwise, which is about 3.2 centimetres, or the diameter of a quarter, which is 2.5 centimetres.