Water Head to Pressure Conversion

Use the hydrostatic relationship

For a static fluid column, pressure difference is p = ρgh. Here ρ is the fluid density in kilograms per cubic metre, g is gravitational acceleration in metres per second squared, and h is the vertical height in metres. The result is pascals. Divide by 1000 for kilopascals, by 100,000 for bar, or by 6,894.757 for pounds per square inch. The U.S. Army Corps of Engineers describes hydraulic head and pressure through the fluid’s specific weight, ρg.

The table uses a reference density of 1000 kg/m³ and standard gravity of 9.80665 m/s². Under those assumptions, one metre of water produces 9.80665 kPa of static gauge pressure, approximately 0.09807 bar or 1.422 psi. These are calculated reference values; the displayed columns are rounded. Real water density changes with temperature and dissolved substances, and local gravity varies slightly by location.

Head is a vertical elevation difference, not the length of a pipe. In an open tank, the pressure below the free surface rises with depth. The table reports pressure difference relative to the fluid surface. It does not add atmospheric pressure, so values are gauge pressure. For absolute pressure, add the local atmospheric pressure to the gauge result.

Convert a known head

To convert ten metres of reference water head, multiply 10 by 9.80665 kPa per metre to obtain 98.0665 kPa. That is 0.980665 bar or approximately 14.22 psi. Conversely, to find the equivalent water head from a pressure difference, rearrange the equation: h = p/(ρg). With p in pascals, density in kg/m³ and gravity in m/s², h is in metres.

For psi inputs, first convert pressure to pascals by multiplying by 6,894.757. For bar, multiply by 100,000. Do not use the numerical value in psi as though it were pascals. Dimensional analysis is a fast check: kg/m³ × m/s² × m reduces to kg/(m·s²), which is a pascal. A result that does not reduce to pressure signals a unit or variable error.

The equation depends on vertical head. A 10 m long horizontal pipe does not automatically create 10 m of pressure head. Pipe friction and fitting losses consume head when fluid flows, and a pump’s total dynamic head includes elevation, pressure and velocity terms as appropriate. This simple conversion covers hydrostatic pressure from an equivalent column; it does not calculate a flowing system’s pressure drop or pump duty.

Keep fluid properties visible

The same head produces different pressure for fluids with different density. A 10 m column of a fluid with relative density 0.8 produces about 80% of the pressure of the reference water column at the same gravity. Use the actual density at the operating temperature and composition when precision matters. A water-based mixture, saline solution, oil or process liquid should not be treated as pure reference water without checking its density.

The reference density in this table is intentionally set to 1000 kg/m³ as a convenient engineering approximation. It is not a claim that water has exactly that density at every temperature. For high-accuracy conversion, obtain density from a recognized property source for the fluid condition and use the local gravitational acceleration if the tolerance requires it. Report the assumptions beside the result.

Head can be reported as metres of water, feet of water, or another reference fluid. Always identify the fluid used in a head convention. A pressure transducer reading in metres of water is a converted pressure signal, whereas a mechanical elevation measurement is a physical level. Their interpretation can differ when density, calibration or reference conditions change.

Distinguish pressure from equipment limits

A pressure conversion does not establish a pipe, valve, tank or pump rating. Maximum allowable pressure depends on the component design, material, temperature, corrosion allowance, manufacturing standard, installation and transient loads. Water hammer, pump shutoff pressure, elevation changes and blocked-in thermal expansion can create conditions beyond the static column calculation. Use the applicable design code and product documentation for equipment selection.

For a tank open to atmosphere, hydrostatic gauge pressure at depth h is calculated relative to the free surface. For a pressurized tank, add the gas pressure above the liquid to the hydrostatic contribution at the point of interest. For a closed system, use a consistent datum and include all relevant pressure boundary conditions. Clarify whether a reported value is gauge or absolute before comparing it with an instrument or rating.

This table provides a unit conversion for static reference water. It excludes dynamic pressure, friction, velocity head, elevation datum changes, compressibility and pump performance. When a design depends on those effects, use the full energy equation and a qualified hydraulic model. Keep fluid density, gravity, head, output units and reference pressure explicit so the calculation can be checked and repeated.

Source and scope

Equation source: U.S. Army Corps of Engineers, EM 1110-1-4001. Table values are calculated with the reference assumptions stated above.