Subsea pressure and depth conversion guide

Depth ratings, foam compressive strengths and test chamber capacities are quoted in different units by different manufacturers. This is the conversion, the formula behind it, and the places the arithmetic misleads.

The relation

Gauge pressure at depth is the weight of the water column above: p = ρ × g × h, where ρ is water density, g is gravity at 9.80665 m/s², and h is depth in metres. That gives pascals; divide by 100,000 for bar.

In seawater at 1025 kg/m³, ten metres of depth is 1.005 bar. The familiar rule of thumb — one bar per ten metres — is therefore about half a percent optimistic, which does not matter in conversation and does matter in specification. At 1,000 m the rule of thumb under-reads by more than five bar.

Fresh water at 1000 kg/m³ gives 0.981 bar per ten metres, so a lake-tested vehicle sees about 2% less pressure than the same depth at sea. Don't rely on that margin: a component qualified in fresh water has not been tested to its seawater rating.

Absolute versus gauge catches people out. Gauge pressure is what the housing wall has to hold back — the differential between outside and the atmosphere sealed inside. Absolute pressure is gauge plus one atmosphere, and is what an absolute-referenced sensor reads. Depth ratings are gauge; sensor readings frequently are not.

Reference figures

At 1025 kg/m³ seawater, rounded to two decimals: 10 m = 1.01 bar; 50 m = 5.03 bar; 100 m = 10.05 bar; 200 m = 20.10 bar; 300 m = 30.16 bar; 500 m = 50.26 bar; 1,000 m = 100.52 bar; 2,000 m = 201.04 bar; 3,000 m = 301.55 bar; 6,000 m = 603.11 bar.

In other units, 100 m of seawater is 10.05 bar, 145.8 psi, 1.005 MPa, or 9.92 atmospheres. The MPa figure is the one to keep in mind when reading foam datasheets: syntactic buoyancy foam compressive strength is quoted in MPa, and a grade rated at ≥12 MPa is being asked to survive roughly 1.0 MPa at 100 m — a comfortable margin — while a 6,000 m grade at ≥70 MPa faces about 60 MPa, which is not comfortable at all and is why the grades are specified so carefully.

Where the arithmetic misleads

A depth rating is not a test pressure. Manufacturers rate components with a safety factor over the pressure at the stated depth, and the factor is not standardised across suppliers. Two parts both marked 300 m may have been qualified very differently. Where it matters, ask what the part was tested to and for how long.

Sustained load is not the same as a spike. A housing that survives a moment at its rated depth has not shown it will hold that depth for a six-hour dive, and polymers do creep over a long soak. A pressure test should hold the part at pressure for a period; reaching the pressure once proves little.

Cycling matters more than peak. Seals and penetrators that will see repeated deployments should be cycle-tested rather than held once. A single successful hold says nothing about fatigue behaviour over a season of dives.

The system rating is the lowest component rating. A 1,000 m tube with a 300 m penetrator is a 300 m vehicle, and the label on the tube will still say 1,000 m.

Density is not constant

Seawater density varies with temperature, salinity and — slightly — with pressure itself. Surface open-ocean water sits near 1025 kg/m³; cold deep water is denser, brackish estuarine water considerably less so. Over the depth range these vehicles work in, treating density as constant introduces an error well under one percent, which is smaller than the safety factors involved.

For design margin, work to the higher density figure rather than the average. It costs nothing and it fails in the safe direction.

Run the numbers. The method above is implemented in our free Depth to pressure calculator, which states its assumptions and what it does not model.

Sources.

• Hydrostatic relation p = ρgh, with g = 9.80665 m/s², seawater at 1025 kg/m³ and fresh water at 1000 kg/m³. NOAA puts it as one atmosphere for about every 10 m: how pressure changes with ocean depth.
• Unit conversions: 1 bar = 100,000 Pa; 1 atm = 101,325 Pa; 1 psi = 6,894.757 Pa.
• Depth ratings and foam compressive strength grades from the manufacturer's published specifications.

Published 16 August 2026. Last revised 25 September 2026. Corrections to sales@vebixautomation.com.

Confirming a depth rating?

Tell us the operating depth and we will confirm what every part in the pressure boundary needs to be rated to, and what we would test it at. Ask for a quote.