To use a printable metric ruler, go to Vendian.org and select the ruler size that you would like. Then press print and disable any 'shrink to fit' options before printing.
Sometimes a printed ruler is not accurate because your printer or computer has re-sized your image. To measure the accuracy of your printable ruler, obtain a small object that you know the exact size of and match it against your printed ruler. If the numbers match, then your ruler is accurate. Your printed metric ruler should only be used for projects where a high degree of accuracy is not required.