Calculate the pressure of a fluid at rest at any depth with P = ρgh. Choose water, seawater, fuels, oils or a custom density, get gauge and absolute pressure in psi, kPa, bar and atm, or work backwards to the depth that gives a certain pressure.
Last updated:
What do you want to find?
Surface conditions
Standard sea-level atmosphere = 14.696 psi = 101.325 kPa
m/s²
Standard Earth gravity; the Moon is 1.62, Mars 3.71
Enter your values to see the result.
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How to use the Hydrostatic Pressure Calculator
Choose the mode: pressure at a depth, or depth for a pressure.
Select the fluid, or choose Custom and enter its density or specific gravity.
Enter the depth (or the pressure and whether it is gauge or absolute).
Adjust the surface pressure if you are at altitude or in a closed, pressurized tank.
Read gauge and absolute pressure in every common unit and see how it builds with depth.
Hydrostatic pressure formulas
Gauge pressure: P = ρ × g × h
Absolute pressure: P_abs = P_surface + ρ × g × h
Depth for a pressure: h = P_gauge ÷ (ρ × g)
Using specific gravity (US): P (psi) = 0.4335 × SG × h (ft); h (ft) = 2.307 × P (psi) ÷ SG
Standard values: g = 9.80665 m/s²; 1 atm = 101.325 kPa = 14.696 psi = 760 mmHg
These hold for a fluid at rest with constant density. For gases, or for very deep liquids where compressibility matters, density changes with depth and the pressure must be integrated instead.
Pressure per foot and per meter of depth
Fluid
Density (kg/m³)
psi per ft
kPa per m
Depth for +1 atm
Gasoline
740
0.321
7.26
13.96 m (45.8 ft)
Diesel
840
0.364
8.24
12.30 m (40.4 ft)
Fresh water (20 °C)
998.2
0.433
9.79
10.35 m (34.0 ft)
Seawater
1025
0.444
10.05
10.08 m (33.1 ft)
Saturated brine
1200
0.520
11.77
8.61 m (28.2 ft)
Mercury
13546
5.87
132.8
0.763 m (2.50 ft)
Where hydrostatic pressure matters
Diving: the absolute pressure doubles at about 33 ft (10 m) of seawater, which is why air volume and gas absorption change so quickly near the surface.
Water supply: a tank or tower 100 ft above a faucet gives about 43 psi of static pressure there before any flow losses.
Tanks and dams: walls must resist the pressure at the deepest point, which is the largest; the average pressure on a vertical wall is half the bottom value.
Level sensing: a pressure sensor at the bottom of a tank measures liquid level as h = P ÷ (ρg), so the right density setting is essential.
Frequently asked questions
What is the hydrostatic pressure formula?
P = ρ × g × h, where ρ is the fluid density (kg/m³), g the gravitational acceleration (9.80665 m/s²) and h the depth below the free surface (m). The result is the gauge pressure in pascals. Add the pressure acting on the surface (usually the atmosphere) to get absolute pressure.
What is the difference between gauge and absolute pressure?
Gauge pressure (psig) is measured relative to the surrounding atmosphere, so it reads zero at the surface. Absolute pressure (psia) is measured from a perfect vacuum and includes the atmosphere: about 14.7 psi at sea level. Tire and water gauges show gauge pressure; physics and diving tables often use absolute pressure.
How much does water pressure increase with depth?
In fresh water pressure rises about 0.433 psi per foot (9.79 kPa per meter at 20 °C). In seawater it is about 0.444 psi per foot (10.05 kPa per meter). Every 33 ft (10 m) of seawater adds roughly one atmosphere.
Does the shape or size of the container matter?
No. Hydrostatic pressure depends only on the vertical depth, the density and gravity, not on the width or shape of the tank (the hydrostatic paradox). A narrow 10 ft pipe of water and a 10 ft deep lake have the same pressure at the bottom.
What is pressure head?
Head is a pressure expressed as the height of a fluid column that would produce it: h = P ÷ (ρg). For water, 1 psi equals about 2.31 ft of head and 1 bar about 10.2 m. Pumps and water systems are often rated in feet or meters of head.
How accurate are the fluid densities?
They are typical values near room temperature. Real density changes with temperature, salinity and composition (seawater ranges about 1020–1030 kg/m³, fuels vary by blend). For very deep water, compressibility makes density increase slightly, so the result at full ocean depth is a little low. Use a custom density for precise work.
What pressure is at the bottom of the Mariana Trench?
At about 10,935 m of seawater (1,025 kg/m³) ρgh gives roughly 110 MPa, about 1,085 atm or 15,900 psi of gauge pressure. Measured values are a bit higher because seawater compresses and becomes denser at that depth.
Are all the tools on FreeToolCabin tested for quality and accuracy?
Yes! Every tool on our site undergoes automated Q/A testing in real browser environments to ensure high precision, correct output, and zero errors. If you happen to spot a bug or have a suggestion, please contact us at [email protected] and our team will review it right away.