Calculus formation on tooth surfaces is analogous to the formation of lime and scale deposits in plumbing. Magnetic water devices have been shown to significantly reduce scale deposits in industry; therefore an oral irrigator with a magnetic water device may have a similar effect on calculus. To test this hypothesis, a double-blind clinical study was established using 64 irrigators, 30 of which had their magnetic devices removed. 54 patients with heavy supragingival calculus were given irrigators at random after prophylaxis. Instructions were given to irrigate twice a day, particularly the lower 6 anterior teeth. The patients were also told not to floss these 6 teeth which were to be the study teeth. They were examined after 3 months and measurements were taken of the accretions adhering to the study teeth. No attempt was made to determine whether the adhering material was hard or soft so it must be assumed that at least some of the measured material was also plaque. The measurements of the group using an irrigator with a magnetic device showed a 44% greater reduction in calculus volume (p < 0.0005) and a 42% greater reduction in area (p < 0.0001) over the group using an unmagnetized irrigator. There appears to be a statistically significant difference in supragingival accretion volumes between conventional irrigation and using an irrigator with a magnetic water treatment device.