Reference
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. So the familiar rule of thumb of one bar per ten metres is about half a percent low. That does not matter in conversation, but it does in a 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. Treat that as margin. It does not mean a part qualified in fresh water has been tested to its seawater rating.
Absolute versus gauge catches people out. Gauge pressure is what the housing wall has to hold back: the difference between the water outside and the air 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. Keep the MPa figure in mind when you read foam datasheets, because syntactic foam compressive strength is quoted in MPa. A grade rated at ≥12 MPa sees about 1.0 MPa at 100 m, a comfortable margin. A 6,000 m grade rated at ≥70 MPa sees about 60 MPa, which is a thin margin, and is why those 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 pressure the part was tested to, and for how long.
Sustained load is not the same as a spike. A housing that survives a momentary excursion to its rated depth has not demonstrated it will hold that depth for a six-hour dive, and creep in polymers over a long soak is real. A pressure test should hold at pressure for a set time. Reaching it for a moment proves little.
Cycling matters more than peak. Seals and penetrators that will be deployed many times should be pressure-cycled several times; one hold is not enough. 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 changes with temperature, salinity and, slightly, pressure. 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, use the higher density figure. It costs nothing and it fails in the safe direction.
Run the numbers. The method above is built into our free Depth to pressure calculator, which lists its assumptions and what it leaves out.
Sources & basis
What this is based on.
- Hydrostatic relation p = ρgh with g = 9.80665 m/s²; densities taken as 1025 kg/m³ seawater and 1000 kg/m³ fresh water.
- 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 as listed in the Vebix Automation shop.
Published 16 August 2026. Last revised 16 August 2026. Corrections to sales@
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