It depends. The following comments apply to the usual household rulers and tape measures less than 50 m long. Measurement issues apply to rulers in any measurement system, not just metric.
If you are making a millimeter measurement using a ruler graduated in millimeters, perhaps 1 or 2 significant figures. If you are making a kilometer measurement using a "ruler" graduated in millimeters, perhaps 6 or 7 significant figures (see below about accuracy).
Unless a vernier or magnifying glass is involved, you can usually use a ruler to measure to a resolution that is half of the smallest marked interval on the ruler. For a ruler graduated in mm, that would be to the nearest .5 mm. How many significant figures that might represent depends on the size of the measurement being made.
Once you start trying to make measurements of 3 or 4 or more significant figures, accuracy and repeatability issues come into play. These, too, depend on a number of factors, including the material the ruler is made of, temperature and humidity, force used to hold the ruler in place, whether the measurement is longer than the ruler, initial calibration accuracy of the ruler, etc. It is not uncommon to find that two rulers made to primary school standards differ by more than 1 mm over their length of less than 1 m.
If you are making a millimeter measurement using a ruler graduated in millimeters, perhaps 1 or 2 significant figures. If you are making a kilometer measurement using a "ruler" graduated in millimeters, perhaps 6 or 7 significant figures (see below about accuracy).
Unless a vernier or magnifying glass is involved, you can usually use a ruler to measure to a resolution that is half of the smallest marked interval on the ruler. For a ruler graduated in mm, that would be to the nearest .5 mm. How many significant figures that might represent depends on the size of the measurement being made.
Once you start trying to make measurements of 3 or 4 or more significant figures, accuracy and repeatability issues come into play. These, too, depend on a number of factors, including the material the ruler is made of, temperature and humidity, force used to hold the ruler in place, whether the measurement is longer than the ruler, initial calibration accuracy of the ruler, etc. It is not uncommon to find that two rulers made to primary school standards differ by more than 1 mm over their length of less than 1 m.
