About the Hydrostatic Pressure Calculator
This hydrostatic pressure calculator finds the pressure exerted by a column of liquid at a given depth using P = ρgh. Pick a fluid (fresh water, seawater, mercury, ethanol or oil) or enter your own density, set the depth in metres or feet, and you get the gauge pressure from the liquid alone and the absolute pressure including the air above it, in kilopascals, psi, bar and atmospheres.
Divers use it to see why every 10 m of seawater adds roughly one atmosphere, pool and tank builders use it to estimate wall and bottom loads, and students use it for fluid statics problems. The chart shows how pressure climbs in a straight line with depth, and the table lists the pressure at regular intervals down to your chosen depth.
The calculation assumes a still, incompressible liquid with uniform density. Gravity defaults to standard gravity (9.80665 m/s²) and the surface pressure to one standard atmosphere (101.325 kPa); change them for other planets, altitudes or a pressurised tank. Real seawater density varies with temperature, salinity and depth, so 1,025 kg/m³ is a typical surface value.
With the default inputs, the gauge pressure (from the fluid) is 98.067 kPa. Change any value above to recalculate instantly.
How to use the hydrostatic pressure calculator
- 1Pick the fluid, or choose custom and enter its density.
- 2Enter the depth and choose metres or feet.
- 3Leave gravity and surface pressure at the defaults for Earth at sea level, or change them.
- 4Read the gauge pressure from the liquid and the absolute pressure including the atmosphere.
- 5Use the chart and table to see how pressure grows with depth.
Formula and method
In a liquid at rest, the pressure at depth h equals the weight of the liquid column above each square metre: density ρ (kg/m³) times gravitational acceleration g (m/s²) times depth h (m), giving pascals. This is gauge pressure — what a pressure gauge zeroed at the surface reads. Absolute pressure adds whatever presses on the surface, normally the atmosphere (101,325 Pa at sea level).
Pressure depends only on depth, density and gravity, not on the shape or width of the container (the hydrostatic paradox). Feet are converted at exactly 0.3048 m. Conversions use 1 psi = 6,894.757 Pa, 1 bar = 100,000 Pa and 1 atm = 101,325 Pa. The model ignores the small compressibility of water and density changes with temperature.
- ρ
- Fluid density (kg/m³)
- g
- Gravitational acceleration (m/s²)
- h
- Depth below the surface (m)
- P_surface
- Pressure acting on the liquid surface (usually 1 atm)
Worked examples
10 m of fresh water
P = 1,000 × 9.80665 × 10 = 98,066.5 Pa, or about 98.1 kPa (14.2 psi) from the water alone. Adding 101.325 kPa of air gives an absolute pressure of 199.4 kPa — almost exactly two atmospheres.
Scuba diver at 30 m in seawater
P = 1,025 × 9.80665 × 30 = 301,554 Pa ≈ 3.02 bar of water pressure. With the atmosphere on top, the diver experiences about 402.9 kPa, or just under 4 atmospheres absolute.
Bottom of a 20 ft deep water tank
20 ft is 6.096 m, so P = 1,000 × 9.80665 × 6.096 ≈ 59,781 Pa = 8.67 psi gauge, which matches the familiar rule of about 0.433 psi per foot of water.
Frequently asked questions
How much does water pressure increase with depth?+
In fresh water, pressure increases by about 9.81 kPa per metre, or 0.433 psi per foot. In seawater it is about 10.05 kPa per metre, so roughly every 10 m adds one atmosphere.
What is the difference between gauge and absolute pressure?+
Gauge pressure is measured relative to the surrounding atmosphere, so it is zero at the surface. Absolute pressure is measured from a perfect vacuum and equals gauge pressure plus atmospheric pressure (about 101.3 kPa at sea level).
Does the shape of the container change hydrostatic pressure?+
No. At a given depth the pressure depends only on density, gravity and depth. A narrow pipe and a wide lake produce the same pressure at the same depth — the so-called hydrostatic paradox.
What density should I use for seawater?+
About 1,025 kg/m³ is a common average for surface seawater. Actual values range from roughly 1,020 to 1,030 kg/m³ depending on temperature and salinity, and increase slightly with depth.
Why is mercury used in barometers?+
Mercury is about 13.5 times denser than water, so one atmosphere supports a column only about 760 mm tall instead of over 10 m of water, which makes a practical instrument.