This online Pressure to Depth online calculator is based on the UNESCO technical papers in marine science Algorithms for computation of fundamental properties of seawater, page 25-27, 1983.

The calculation includes three formulas. The first one is used to calculate a depth from the pressure, but without considering the gravity:

D(P) = -1.82E-15*P^{4} + 2.279E-10*P^{3} - 2.2512E-5*P^{2} + 9.72659*P (1)

Where

D(P) - Depth, meters;

P - Pressure, decibars.

The second formula is used to calculates the gravity at the given latitude, it also considers the pressure:

G(φ, P) = 9.780318*(1.0 + 5.2788E-3*sin^{2}φ + 2.36E-5*sin^{4}φ) + 1.092E-6*P (2)

Where

G(φ, P) - Gravity variation with latitude, m/s^{2};

φ - Latitude, radians;

D - Depth, meters.

The third formula combines the first (1) and the second (2) formula:

D = D(P) / G(φ, P) (3)

It gives the depth D in meters for the pressure (decibars) at the latitude (radians). This is so called "Saunders and Fofonoff" method (Peter M. Saunders, N.P. Fofonoff - Deep Sea Research and Oceanographic Abstracts, 1976, Volume 23, Issue 1, pages 109-111) based on the Knudsen-Ekman equation of state (the formula is refitted for 1980 equation of state).

All units used in this online calculator are Degree for the Latitude, Decibars and Atmosphere for the Pressure, Meters for the Depth. Results are correct for the temperature of 0° Celsius and Salinity = 35 (sea water).