Instructions: enter values for salinity, temperature and pressure and click on the 'Calculate' button. (Note: a decibar is a unit of pressure corresponding to an increment of approximately 1 metre of depth.)

Example from Adrian E. Gill (1982) Atmosphere-Ocean Dynamics, Academic Press, New York. You should check the calculator I've provided here against Gill's reference value of rho=1027.67547 at 35 PPT, 5 degC and 0 dbar (in his appendix A3.1).