According to research carried out at the University of New Jersey, digital bathroom scales are quite accurate down to .95 pounds. This does not reflect the accuracy of mechanical scales, which often vary slightly. Usually inaccuracies occur because of human error, rather than a problem with the scales.
Whether or not bathroom scales are accurate depends on how you use them. When using old scales, inaccuracies often arise due to not setting them back to zero before each use. Generally, electronic scales measure accurately in the right conditions. Good Housekeeping found that using scales on carpet rather than hard flooring may make a person seem as though he weighs less than usual, so always the scales on hardwood flooring for the most accurate resuts.