What this calculator does
Hydrostatic pressure is the pressure a fluid exerts purely from the weight of the fluid sitting above the point being measured. It depends on three things only: how dense the fluid is, how deep the point is, and gravity, captured in the formula P = ρgh. Unlike pressure from a pump or a mechanical load, it says nothing about the shape or size of the container, only the vertical depth of fluid above.
This calculator works for any fluid, not just water, so it covers seawater, oils, fuels or mercury by entering the correct fluid density. It reports both gauge pressure (the pressure due to the fluid alone) and absolute pressure (gauge pressure plus the atmosphere pressing down from above), since which figure is relevant depends on whether the surrounding atmospheric pressure has already been accounted for elsewhere in a calculation.
The formula
Multiply fluid density by standard gravity (9.80665 m/s²) and by depth to get gauge pressure. Add atmospheric pressure at the surface to get absolute pressure, which is what a point at that depth actually experiences once the weight of the air above the fluid surface is included.
| Term | Meaning |
|---|---|
| Gauge pressure | Pressure due to the fluid column alone, ignoring the atmosphere above the fluid surface: P = ρgh. |
| Absolute pressure | Gauge pressure plus atmospheric pressure: the total pressure actually acting at that depth. |
| ρ (rho) | Fluid density, which varies by fluid: about 1,000 kg/m³ for fresh water, 1,025 kg/m³ for seawater, and much higher for dense fluids like mercury. |
The inputs explained
| Field | What to enter |
|---|---|
| Fluid density (kg/m³) | The density of the fluid: roughly 1,000 for fresh water, 1,025 for seawater, 800-900 for typical oils, or 13,600 for mercury. |
| Depth (m) | Vertical depth below the fluid surface to the point being measured. |
| Atmospheric pressure at the surface (kPa) | Atmospheric pressure at the fluid surface, usually close to standard sea-level pressure unless working at altitude. |
When to use it
Diving and underwater equipment
Divers and submersible equipment are rated against absolute pressure at depth, since it is the total pressure, not just the water’s own contribution, that acts on the body and any sealed equipment.
Dam, tank and pipeline design
The pressure a dam wall or storage tank wall must withstand rises directly with depth, so hydrostatic pressure at the deepest point sets the structural load the design has to handle.
Comparing different fluids at the same depth
Because pressure scales directly with density, the same depth produces very different pressures in water, oil or mercury, which matters when specifying gauges, seals or manometers for a particular fluid.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How does hydrostatic pressure change with depth in fresh water?
A fixed fluid density, across a range of depths.
| Depth | Gauge pressure at this depth | Absolute pressure (incl. atmosphere) |
|---|---|---|
| 5 m | 49.0 kPa | 150.4 kPa |
| 10 m | 98.1 kPa | 199.4 kPa |
| 20 m | 196.1 kPa | 297.5 kPa |
| 30 m | 294.2 kPa | 395.5 kPa |
| 50 m | 490.3 kPa | 591.7 kPa |
| 100 m | 980.7 kPa | 1,082.0 kPa |
How does hydrostatic pressure differ between fluids, at the same depth?
A fixed depth, across a range of fluid densities.
| Fluid density | Gauge pressure at this depth |
|---|---|
| 800 kg/m³ (oil) | 78.5 kPa |
| 1,000 kg/m³ (fresh water) | 98.1 kPa |
| 1,025 kg/m³ (seawater) | 100.5 kPa |
| 13,600 kg/m³ (mercury) | 1,333.7 kPa |
Questions
Does the shape or width of the container matter?
No. Hydrostatic pressure depends only on the vertical depth of fluid above the point, the fluid’s density and gravity. A narrow tube and a wide tank filled with the same fluid to the same depth produce identical pressure at the bottom, which is sometimes called the hydrostatic paradox.
Should I use gauge or absolute pressure?
Use gauge pressure when comparing against equipment already referenced to atmospheric pressure, such as most pressure gauges. Use absolute pressure when the calculation needs the true total pressure, such as when working with gas laws or checking a diver’s exposure to total ambient pressure.
How is this different from the pressure under a load?
Pressure from a mechanical load is force divided by the area it acts over, and depends on the object pressing down, not on any fluid. Hydrostatic pressure comes purely from the weight of a fluid column and depends only on fluid density, depth and gravity.
Why does atmospheric pressure matter if it is not part of the fluid?
The atmosphere itself is a fluid pressing down on the surface of whatever liquid is being measured, so its weight adds to the total pressure at any depth beneath it. That is why absolute pressure, not gauge pressure, is the true total pressure acting at a point.
For pressure from a mechanical load rather than a fluid column, and a fixed-to-water version of this depth calculation, see the pressure calculator. For whether an object floats in a given fluid, see the buoyancy calculator.