The physics in one line
A column of liquid presses down with a pressure that depends on only three things — how dense the liquid is, how tall the column is, and the local gravity:
With density in kg/m³, g in m/s² and height in metres, the result is in pascals. Using specific gravity (SG) instead of density:
Notice what is not in the formula: tank diameter, tank shape, liquid volume. Ten metres of water in a drinking straw and ten metres of water in a storage tank exert exactly the same pressure at the bottom. This is the principle every hydrostatic level measurement is built on — a pressure transmitter at the tank bottom is really a height gauge, as long as density stays constant.
Worked example — ranging a level transmitter
A diesel storage tank (SG = 0.85) needs level measurement from the transmitter tapping up to 8 m. What DP range should be configured?
- P = 9.80665 × 0.85 × 8 = 66.7 kPa
- Configure the transmitter 0 to 66.7 kPa → 4–20 mA covers 0–8 m of diesel
- Cross-check: in mmWC, that's 0.85 × 8000 = 6800 mmWC
And the reverse problem: a gauge at the bottom of a water tank reads 45 kPa — the level is 45 ÷ 9.807 = 4.59 m. Type 45 into the pressure field above to see it.
Handy equivalents
| Water column | Pressure |
|---|---|
| 1 mm | 9.807 Pa |
| 1 cm | 0.098 kPa |
| 1 m | 9.807 kPa ≈ 0.098 bar ≈ 1.422 psi |
| 10 m | 98.07 kPa ≈ 0.981 bar ≈ 14.22 psi |
| 10.20 m | exactly 1 bar |
Field notes
- Density changes = level errors. A transmitter ranged for SG 0.85 reads 5.9% high if the product arrives at SG 0.80. Temperature alone shifts hydrocarbon density noticeably.
- Measure from the tapping, not the tank floor. The height in the formula is liquid above the sensing point.
- Closed tanks need the vapour space handled — that's a DP measurement with a compensating leg; see our DP level calculators.
- Local gravity matters at precision levels: g varies ~0.5% between equator and poles. Deadweight testers care; ordinary level measurement doesn't.
Frequently asked questions
What is the formula for hydrostatic pressure?
P = ρ × g × h, where ρ is the liquid density in kg/m³, g is gravitational acceleration (9.80665 m/s²) and h is the liquid column height in metres. The result is in pascals.
How much pressure does 1 metre of water produce?
One metre of water at 4 °C produces 9.807 kPa, which is 0.09807 bar or 1.422 psi. A convenient rule of thumb: 10 m of water ≈ 1 bar.
Does tank shape or width affect hydrostatic pressure?
No — only the vertical height of liquid above the measurement point matters, along with density. A narrow 5 m column and a wide 5 m tank of the same liquid produce identical pressure at the bottom.
How do I use specific gravity instead of density?
Multiply specific gravity by 1000 to get density in kg/m³ (water = 1.0 = 1000 kg/m³). This calculator accepts either.
Provided for reference and education. Verify independently before use in safety-critical work. See our disclaimer.