To promote uniformity in the calibration and reporting of low-level tritium assays in waters, a tritium measurement scale is proposed for adoption in both β-particle counting and 3He ingrowth mass-spectrometric methods; the scale is based on the certified specific activity of the new U.S. National Bureau of Standards tritiated-water standard SRM-4926C (3.406 × 10 6 Bq·kg −1 on 3 September 1978) and a half life of tritium of 4540 days (12.43 yr). For the mass spectrometric method, the new scale replaces that based on the 3He/ 4He ratio and 4He concentration in air, but routine calibrations are still made using these parameters. To assist in application of the new scale, equations are developed for calibrating tritium assays by both methods, and for converting results based on earlier measurement scales. The ICRU 1963 recommendation to express tritium concentrations as Tritium Ratios (TR) rather than Tritium Units (TU) is recommended for use with the new measurement scale, but a conversion factor to SI specific activity units (Bq·kg −1) is listed.