The isothermal compressibility of water from 0 to 100 °C and 0 to 1000 bar has been determined from Wilson's sound velocity measurements which have been normalized to Kell's 1 atm values. The isothermal compressibilities determined from the sound velocities have been fit, with a maximum deviation in compressibility of ± 0.016 × 10-6 bar-1, to an extended bulk modulus equation V 0P/(V0-VP) = B + A1P + A2P2, where V0 and VP are the specific volume at an applied pressure of zero and P; and B, A1, and A2 are temperature dependent constants. Our specific volume results are in reasonable agreement with the work of Kell and Whalley at low pressures; however, our results at high pressures (1000 bar) disagree by as much as 169 ppm (the average deviation is approximately 115 ppm). A comparison of the compressibilities indicates a parabolic shift in Kell and Whalley's work with a maximum of approximately 0.205 × 10-6 bar-1 at 400 bar and 5 °C. Since the velocity of sound data is extremely reliable (± 0.2 m/sec) and the maximum error in the compressibilities derived from the sound data is within ± 0.016 × 10-6 bar-1, our PVT results based upon the sound data are more accurate than any direct measurements made to date.