Blast pressure measurements of a controlled underwater explosion in the sea were carried out. An explosive of 25-kg trinitrotoluene (TNT) equivalent was detonated, and the blast pressures were recorded by eight different high-performance pressure sensors that work at the nonresonant high-voltage output in adverse underwater conditions. Recorded peak pressure values are used to establish a relationship in the well-known form of empirical underwater explosion (UNDEX) loading formula. Constants of the formula are redetermined by employing the least-squares method in two different forms for best fitting to the measured data. The newly determined constants are found to be only slightly different from the generally accepted ones.