A water pump is a device used to transport or pressurize liquids. Pump head refers to the height to which the pump can lift water; pump head is directly proportional to pressure. Based on clear water at 4°C (39.2°F), density 1000 kg/m³: 1 psi = 2.31 ft H₂O = 0.703 m H₂O; 1 ft H₂O = 0.433 psi; 1 m H₂O = 1.422 psi = 9.81 kPa = 0.0981 bar. For other liquids, divide by the specific gravity (SG).
Next, let us examine in detail the relationship and conversion between head (feet or meters) and pressure (psi, bar, or kg/cm²).
Definition of “head”: The pressure exerted by a column of liquid at its base, expressed in terms of the column’s height. Pump head characterizes the kinetic energy generated by the pump; specifically, it is a measurement of the height of an incompressible fluid column that would result from the kinetic energy imparted to the liquid by the pump.
In the pump and liquid level industries, “head” is commonly used instead of psi. This is because, for a given pump, the head remains constant even when handling different liquids, whereas the pressure varies.
Avoiding Confusion: Head vs. Pressure
Head is sometimes confused with pressure simply because the two parameters are closely related; however, there is a fundamental difference between them.
Head is independent of the fluid; in other words, a pump will lift a fluid to the same height regardless of the fluid’s specific gravity. Therefore, the head remains the same whether the fluid is water or a heavier substance like sludge.
Pressure, on the other hand, depends on the fluid and is influenced by gravity. Consequently, the same head will generate different pressures depending on the specific gravity of the fluid.
The pressure at the bottom of a liquid column is determined solely by the liquid’s density, the acceleration due to gravity, and the height of the column; it is independent of the container’s shape:
P = ρ × g × H
To express pressure in bar: P (bar) = ρ × g × H × 10⁻⁵
Using Imperial units, the conversion for clear water at room temperature is:
psi = feet of head × 0.433 × SG
SG is the liquid’s specific gravity (liquid density ÷ water density); for clear water, SG = 1.
100 ft × 0.433 × 1 = 43.3 psi
The density of diesel is approximately 0.85 g/cm³, so SG = 0.85.
100 ft × 0.433 × 0.85 = 36.8 psi
At the same height of 100 feet, the pressure exerted by diesel is lower than that of water. This is because the pressure of a liquid column is directly proportional to its density.
Calculating liquid column height (head) from pressure:
H = P ÷ (ρ × g)
Here, ρ × g represents the specific weight of the fluid.
In Imperial units, the conversion for clear water at ambient temperature is:
Feet of head = psi × 2.31 ÷ SG
60 psi × 2.31 ÷ 1 = 138.6 ft
1 psi = 6894.757 Pa. Substituting this into H = P ÷ (ρ × g) and using a water density of 1000 kg/m³ (at 4°C):
H = 6894.757 ÷ (1000 × 9.80665) = 0.7031 m = 2.307 ft ≈ 2.31 ft
Conversely, 1 foot of water column = 0.433 psi.
The density of water decreases as the temperature rises, reaching its maximum at 4°C. Therefore, for the same pressure of 1 psi, hot water can support a taller column of water:
| Water temperature | Water density (kg/m³) | 1 psi is equivalent to | 1 ft of water column is equivalent to |
| 4℃ | 1000 | 2.307 ft（0.703 m） | 0.4335 psi |
| 20℃ | 998.3 | 2.311 ft（0.704 m） | 0.4328 psi |
| 60℃ | 983 | 2.347 ft（0.715 m） | 0.4262 psi |
| 80℃ | 972 | 2.373 ft（0.723 m） | 0.4214 psi |
| 100℃ | 958 | 2.408 ft（0.734 m） | 0.4153 psi |
The commonly used values of 2.31 and 0.433 apply to clear water at ambient temperature. If calculations are based on ambient-temperature density when the water temperature rises to 80°C, the liquid level error is approximately 2.7%. The value 2.31 can be used directly when temperature fluctuations are minimal; however, when high precision is required, calculations should be based on the density at the actual temperature.
Calculation basis: water density 1000 kg/m³, gravitational acceleration 9.80665 m/s²; 1 psi = 6894.757 Pa, 1 ft = 0.3048 m.
Table 1: Conversion of psi to feet of head and meters of water column (1–100 psi)
| psi | ft H₂O | m H₂O |
| 1 | 2.31 | 0.703 |
| 2 | 4.61 | 1.406 |
| 3 | 6.92 | 2.109 |
| 4 | 9.23 | 2.812 |
| 5 | 11.53 | 3.515 |
| 6 | 13.84 | 4.218 |
| 7 | 16.15 | 4.921 |
| 8 | 18.45 | 5.625 |
| 9 | 20.76 | 6.328 |
| 10 | 23.07 | 7.031 |
| 11 | 25.37 | 7.734 |
| 12 | 27.68 | 8.437 |
| 13 | 29.99 | 9.14 |
| 14 | 32.29 | 9.843 |
| 15 | 34.6 | 10.546 |
| 16 | 36.91 | 11.249 |
| 17 | 39.21 | 11.952 |
| 18 | 41.52 | 12.655 |
| 19 | 43.83 | 13.358 |
| 20 | 46.13 | 14.061 |
| 25 | 57.67 | 17.577 |
| 30 | 69.2 | 21.092 |
| 35 | 80.73 | 24.607 |
| 40 | 92.27 | 28.123 |
| 45 | 103.8 | 31.638 |
| 50 | 115.33 | 35.153 |
| 55 | 126.87 | 38.669 |
| 60 | 138.4 | 42.184 |
| 65 | 149.93 | 45.7 |
| 70 | 161.47 | 49.215 |
| 75 | 173 | 52.73 |
| 80 | 184.53 | 56.246 |
| 85 | 196.07 | 59.761 |
| 90 | 207.6 | 63.276 |
| 95 | 219.13 | 66.792 |
| 100 | 230.67 | 70.307 |
Table 2: Conversion of feet of head to psi and bar (1–300 ft)
| ft H₂O | psi | bar |
| 1 | 0.434 | 0.0299 |
| 2 | 0.867 | 0.0598 |
| 3 | 1.301 | 0.0897 |
| 4 | 1.734 | 0.1196 |
| 5 | 2.168 | 0.1495 |
| 6 | 2.601 | 0.1793 |
| 7 | 3.035 | 0.2092 |
| 8 | 3.468 | 0.2391 |
| 9 | 3.902 | 0.269 |
| 10 | 4.335 | 0.2989 |
| 15 | 6.503 | 0.4484 |
| 20 | 8.671 | 0.5978 |
| 25 | 10.838 | 0.7473 |
| 30 | 13.006 | 0.8967 |
| 35 | 15.173 | 1.0462 |
| 40 | 17.341 | 1.1956 |
| 45 | 19.509 | 1.3451 |
| 50 | 21.676 | 1.4945 |
| 55 | 23.844 | 1.644 |
| 60 | 26.012 | 1.7934 |
| 65 | 28.179 | 1.9429 |
| 70 | 30.347 | 2.0923 |
| 75 | 32.515 | 2.2418 |
| 80 | 34.682 | 2.3913 |
| 85 | 36.85 | 2.5407 |
| 90 | 39.017 | 2.6902 |
| 95 | 41.185 | 2.8396 |
| 100 | 43.353 | 2.9891 |
| 110 | 47.688 | 3.288 |
| 120 | 52.023 | 3.5869 |
| 130 | 56.359 | 3.8858 |
| 140 | 60.694 | 4.1847 |
| 150 | 65.029 | 4.4836 |
| 160 | 69.364 | 4.7825 |
| 170 | 73.7 | 5.0814 |
| 180 | 78.035 | 5.3803 |
| 190 | 82.37 | 5.6792 |
| 200 | 86.706 | 5.9781 |
| 210 | 91.041 | 6.277 |
| 220 | 95.376 | 6.5759 |
| 230 | 99.711 | 6.8749 |
| 240 | 104.047 | 7.1738 |
| 250 | 108.382 | 7.4727 |
| 260 | 112.717 | 7.7716 |
| 270 | 117.052 | 8.0705 |
| 280 | 121.388 | 8.3694 |
| 290 | 125.723 | 8.6683 |
| 300 | 130.058 | 8.9672 |
Downloadable PDF conversion chart:
Common Approximate Conversions (Metric)
| Conversion Equivalents | Approximate values | Exact value |
| 1 MPa is equivalent to a water column height of… | Approx. 100 m | 101.97 m |
| 1 kgf/cm² (1 kg) | Approx. 0.1 MPa, approx. 10 m water column | 0.0981 MPa |
| The height of a water column supported by 1 standard atmosphere | Approx. 10 m | 10.33 m |
| 1 MPa | Approx. 145 psi | 145.04 psi |
| 1 psi | Approx. 6.9 kPa | 6.8948 kPa |
| 1 mmH₂O | Approx. 9.8 Pa | 9.8067 Pa |
“1 MPa ≈ 100 m” and “1 kg ≈ 0.1 MPa” are both engineering approximations with an error of approximately 2%, suitable for rough estimates; precise values should be used when creating conversion tables or calculators.
The formula for liquid pressure is P = ρ × g × H; to determine the height of the liquid column from the pressure, use H = P ÷ (ρ × g). Here, ρ represents the density of the liquid being measured; calculations must use the specific density of the liquid in question.
Densities of Common Liquids (20°C)
| Liquid | Density (g/cm³) |
| Water | 0.9983 |
| Seawater | 1.025 (commonly used value for engineering calculations) |
| Diesel | 0.85 |
| Gasoline | 0.76～0.78 |
| Milk | 1.03 |
| Hydrochloric acid (37%) | 1.19 |
| Sulfuric acid (98%) | 1.84 |
| Pure ethylene glycol | 1.1135 |
Seawater density depends on salinity and has no fixed value; a standard figure of 1.025 × 10³ kg/m³ is generally used. It increases gradually from the surface to greater depths.
Aqueous ethylene glycol solution: concentration, density, and freezing point (at 20°C)
| Ethylene glycol content (vol%) | Density (g/cm³) | Freezing point (°C) |
| 34.2 | 1.048 | −18 |
| 40.4 | 1.056 | −24 |
| 45.6 | 1.063 | −30 |
| 50 | 1.0671 | −35 |
| 51 | 1.07 | −38 |
The freezing point of an aqueous ethylene glycol solution reaches its minimum (approximately −48°C) at a volumetric concentration of about 56%; beyond approximately 59%, the freezing point actually rises. The freezing point of pure ethylene glycol is −13°C.
Aqueous propylene glycol solution: variation of density with concentration and temperature (kg/m³)
| Temp | 30% | 40% | 50% | 60% |
| 0℃ | 1036.24 | 1045.12 | 1052.71 | 1059 |
| 20℃ | 1028.35 | 1036.24 | 1042.87 | 1048.25 |
| 40℃ | 1018.42 | 1025.3 | 1030.98 | 1035.47 |
| 60℃ | 1006.44 | 1012.3 | 1017.04 | 1020.66 |
| 80℃ | 992.42 | 997.25 | 1001.05 | 1003.81 |
Concentration refers to concentration by volume.
Precautions for Ethylene Glycol Systems
In systems using an ethylene glycol solution as the heat transfer fluid, the solution’s concentration and viscosity must be strictly controlled to prevent changes in the freezing point and increases in the power required for circulation.
Temperature Correction
The variable-frequency constant-pressure water supply system consists of a pressure transmitter, a variable frequency drive (VFD), a programmable logic controller (PLC), control circuitry, water pumps, and a pressure tank.
The pressure transmitter is installed upstream of the check valve. This location offers stable pressure with minimal turbulence and vibration, allowing for accurate monitoring of pump pressure. The transmitter outputs a 4–20 mA signal, which is connected to the VFD’s current input terminal.
Pressure setpoint and feedback signals are fed into the controller; following PID processing, a speed control signal is sent to the VFD to regulate pump speed and maintain constant pressure. The pump stops automatically when the pressure reaches the upper limit and starts automatically when it drops to the lower limit.
A liquid level sensor installed in the water reservoir or collection sump automatically starts and stops the pumps based on the water level. Pumps are shut down sequentially according to preset levels as the water level drops; to prevent the same pump from undergoing frequent start-stop cycles, the pumps are operated in a rotating sequence (first-in, first-out).
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If you require liquid level measurement or pump pressure monitoring, please provide us with details regarding the tank height or well depth, the medium, and the operating temperature, and our sales engineers will recommend the most suitable model for your needs.
Zhang Wei, possesses 20 years of experience as an automation instrumentation engineer, specializing in the research, design, installation, commissioning, and maintenance of automation instruments.
Face to various instrument communication protocols (such as Modbus, Profibus, etc.), with solid hardware circuit design and software programming skills (proficient in C language and PLC programming). Has extensive project experience; projects he has led and participated in have all achieved outstanding results, improving product accuracy, reducing costs, and increasing production efficiency.
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