Enter fluid depth and density to instantly calculate hydrostatic pressure in PSI, Bar, kPa, Pascal, and metres of water column — for tanks, pipes, pools, and reservoirs.
Calculating hydrostatic pressure takes under 30 seconds:
Enter the fluid depth or height
Measure the vertical distance from the water surface down to the point where you need to know the pressure — not the length of a sloping pipe. Use the unit selector to match your measurement (feet, inches, metres, cm or mm).
Select your fluid type
Use the quick-select dropdown to pick fresh water, seawater, diesel, or another common fluid. This automatically fills the density field. For unusual fluids, choose "Custom" and enter the density in kg/m³ manually — if you know the specific gravity (SG), multiply it by 1,000; if you know lb/ft³, multiply by 16.02.
Hit Calculate
Results appear instantly in PSI, Bar, kPa, Pascal, metres water column (mWC), atmospheres and mmHg. The note under the results shows the working in US units (feet × 0.433 psi per foot × SG) and the absolute pressure. Copy or print the full output with one click.
Check the pressure diagram
The visual shows the depth-pressure relationship — the longer the water column, the higher the gauge pressure at the bottom.
Important: This calculator gives gauge pressure (pressure above atmospheric) — what an ordinary pressure gauge reads. Absolute pressure = gauge pressure + about 14.7 psi (101.325 kPa, 1 atm) at sea level, a little less at altitude. For most tank and pipe problems, gauge pressure is what you need.
📐 The Formula
Hydrostatic Pressure Formula
Hydrostatic pressure depends on three things: fluid density, gravitational acceleration, and depth:
P = ρ × g × h
P = hydrostatic pressure (Pascal, Pa)
ρ = fluid density (kg/m³)
g = standard gravity = 9.80665 m/s² (9.81 is fine for hand sums)
h = depth below the fluid surface (metres)
The result in Pascal can then be converted to any other pressure unit. In US units the same formula becomes P (psi) = 0.433 × SG × h (ft), because a 1 ft column of water weighs 62.4 lb over 144 sq in. Fresh water has SG = 1.0; seawater about 1.025. Put the other way, every 2.31 ft of water adds 1 psi, and every 10.2 m adds 1 bar. This calculator performs all the conversions automatically.
Unit Conversion Reference
Unit
Equals Pascal
From Pascal
1 Pascal (Pa)
1 Pa
× 1
1 Kilopascal (kPa)
1,000 Pa
÷ 1,000
1 Bar
100,000 Pa
÷ 100,000
1 PSI
6,894.76 Pa
÷ 6,894.76
1 Atmosphere (atm)
101,325 Pa
÷ 101,325
1 mmHg (Torr)
133.322 Pa
÷ 133.322
1 m Water Column (mWC)
9,806.65 Pa
÷ 9,806.65
Pressure vs. Depth Reference (Fresh Water)
Depth
Pascal (Pa)
kPa
PSI
Bar
1 m
9,807
9.81
1.42
0.098
5 m
49,033
49.03
7.11
0.490
10 m
98,066
98.07
14.22
0.981
20 m
196,133
196.13
28.45
1.961
50 m
490,332
490.33
71.12
4.903
100 m
980,665
980.66
142.23
9.807
Pressure vs. Depth in Feet (Fresh Water)
Depth
Metres
PSI
kPa
Note
1 ft
0.30 m
0.43
3.0
—
2.31 ft
0.70 m
1.00
6.9
1 psi
5 ft
1.52 m
2.17
14.9
—
10 ft
3.05 m
4.34
29.9
—
20 ft
6.10 m
8.67
59.8
—
33.9 ft
10.33 m
14.70
101.3
≈ 1 atmosphere
50 ft
15.24 m
21.68
149.5
—
100 ft
30.48 m
43.35
298.9
—
231 ft
70.41 m
100.14
690.5
100 psi
Values use standard gravity (9.80665 m/s²) and water at 1,000 kg/m³. Real fresh water is slightly lighter when warm (about 998 kg/m³ at 20 °C / 68 °F), which changes the result by less than 0.3%.
🏗️ Use Cases
When to Use a Hydrostatic Pressure Calculator
🏠
Rooftop Water Tanks
Calculate the static water pressure at ground-floor taps from a rooftop tank — the starting point for deciding whether a gravity feed will work or a pump is needed.
🏊
Swimming Pools
Find the pressure on pool walls and floor at maximum depth — the load a pool structure or liner must be designed for.
🚰
Pipe & Valve Sizing
Determine inlet pressure to correctly specify valves, gaskets, and pipe class ratings before installation.
🛢️
Underground Tanks
Assess the pressure on buried tank walls from a high water table — important for buoyancy (flotation) checks and structural design.
🌊
Reservoirs & Dams
Calculate base pressure in reservoirs and behind small dams — a first figure for civil engineering and safety assessments, which also need uplift, waves and other loads.
🔬
Industrial Fluid Systems
Use with any liquid — diesel, oil, chemicals — by entering the correct fluid density to get accurate system pressures.
🧮 Worked Example
Worked example: pressure at the bottom of a 12 ft water tank
A water storage tank holds fresh water 12 ft (3.66 m) deep. What does a gauge at the outlet near the bottom read?
US units: 12 ft × 0.433 psi/ft = 5.20 psi.
Metric: 1,000 kg/m³ × 9.80665 m/s² × 3.658 m = 35,869 Pa = 35.9 kPa (0.359 bar).
The tank's width and shape make no difference — a 2 ft wide standpipe and a 40 ft wide tank of water 12 ft deep both read 5.20 psi at the bottom. What changes is the total force on the base, which is the pressure multiplied by the floor area.
❓ FAQ
Frequently Asked Questions
What is hydrostatic pressure?
Hydrostatic pressure is the pressure exerted by a fluid at rest due to gravity. It increases linearly with depth — every additional foot of fresh water adds about 0.433 psi, and every metre about 9.81 kPa (1.42 psi). It does not depend on the shape of the container, only on the depth and density of the fluid.
How do I calculate water pressure from tank height?
In US units, multiply the water depth in feet by 0.433 to get psi — a full tank 10 ft deep gives 4.33 psi at its base. In metric, use P = ρ × g × h, where ρ is the water density (1,000 kg/m³ for fresh water), g is 9.81 m/s² and h is the depth in metres: a full tank 5 m deep gives about 49,000 Pa (49 kPa, 0.49 bar or 7.1 psi) at its base. This calculator does all the arithmetic for you.
What is the difference between gauge pressure and absolute pressure?
Gauge pressure is the pressure relative to atmospheric pressure (what most pressure gauges show, often written psig). Absolute pressure (psia) includes the weight of the atmosphere on top — at sea level it equals gauge pressure + 14.7 psi (101.325 kPa, 1 atm). For tank and pipe design, gauge pressure is almost always the relevant figure. This calculator outputs gauge pressure.
Does tank shape affect hydrostatic pressure?
No — this is a common misconception. Hydrostatic pressure at any given depth depends only on the fluid density and the vertical height of fluid above that point. A narrow tank and a wide lake produce identical pressure at the same depth. This is known as the hydrostatic paradox.
How much pressure does a rooftop water tank generate?
A tank whose water level is 16 ft (5 m) above the tap delivers about 7.1 psi (49 kPa, 0.49 bar) of static pressure — less once water flows and the pipes take their share. That is low by US standards, where homes on a public supply usually see 40–80 psi. Showers and valves made for low-pressure gravity systems are often rated to work from about 0.1 bar (1.5 psi, a 1 m / 3.3 ft head), but most standard fixtures want much more. Use our Minimum Tank Height for Shower Pressure Calculator to size the height for the pressure you need.
Can I use this calculator for fluids other than water?
Yes — select the fluid from the quick-pick dropdown (seawater, diesel, cooking oil, milk, petrol) or enter any custom density in kg/m³. The formula and all outputs adjust automatically. For example, seawater at 1,025 kg/m³ produces about 2.5% more pressure than fresh water at the same depth, while gasoline (about 745 kg/m³) produces about 25% less.
What is metres water column (mWC) and when is it used?
Metres water column (mWC) is a pressure unit commonly used in plumbing and HVAC. 1 mWC equals the pressure exerted by 1 metre of fresh water — 9,806.65 Pa, or about 0.098 bar and 1.42 psi. The US equivalents are feet of water (1 ft = 0.433 psi) and inches of water column (in. WC, 1 in = 249 Pa), often used for low pressures such as gas and air ducts. Pump datasheets specify head in feet or metres, which makes these units convenient for tank and pump system design.
✅ Sources & Review
Sources and review
The formula and constants on this page come from these references:
USGS Water Science School — Water Density: fresh water is about 1,000 kg/m³ (62.4 lb/ft³), slightly less when warm.
NOAA — Sea Water: typical seawater density about 1,025 kg/m³.
Fuel densities: EN 228 (petrol, 720–775 kg/m³ at 15 °C) and EN 590 (diesel, 820–845 kg/m³); US fuels are similar. Check the supplier's data sheet for an exact figure.
Hydrostatic pressure, P = ρgh, as given in standard fluid mechanics texts.
Reviewed by John Mac, PE, Water Systems Specialist
Texas-licensed Professional Engineer with 13+ years in residential water storage and 180+ tank projects. John writes and reviews every calculator on this site.
