Pumps - Head vs. Pressure
Converting head (ft or m) to pressure (psi or bar, kg/cm2) and vice versa.
Pumps characteristic curves are often in head - feet or metres - and a conversion to pressure scales commonly used in pressure gauges - like psi or bar - may be required.
Converting Head to Pressure
Converting head in feet to pressure in psi
Pumps characteristic curves in feet of head can be converted to pressure - psi - by the expression:
p = 0.433 h SG (1)
where
p = pressure (psi)
h = head (ft)
SG = specific gravity of the fluid
Converting head in metre to pressure in bar
Pumps characteristic curves in metre of head can be converted to pressure - bar - by the expression:
p = 0.0981 h SG (2)
where
h = head (m)
p = pressure (bar)
Converting head in metre to pressure in kg/cm2
Pumps characteristic curves in metre of head can be converted to pressure - kg/cm2 - by the expression:
p = 0.1 h SG (2b)
where
h = head (m)
p = pressure (kg/cm2)
Converting Pressure to Head
Since pressure gauges often are calibrated in pressure - psi or bar - a conversion to the heads commonly used in pump curves - like feet or meter - may be required.
Converting pressure in psi to head in feet
h = 2.31 p / SG (3)
where
h = head (ft)
p = pressure (psi)
Converting pressure in bar to head in metre
h = p 10.197 / SG (4)
where
h = head (m)
p = pressure (bar)
Converting pressure in kg/cm2 to head in metre
h = p 10 / SG (4b)
where
h = head (m)
p = pressure (kg/cm2)
Example - Converting Pump Head - feet - to Pressure - psi
The pressure - psi - of a water pump operating with head 120 ft can be calculated with eq. 1 as:
p = 0.433 (120 ft) (1)
= 52 psi
Example - Converting Pressure - psi - to Pump Head - feet
The head in feet water column can be calculated from pressure 100 psi with eq. 3 as:
h = 2.31 (100 psi) / (1)
= 231 ft
Where specific gravity of water = 1.
Feet of Head Water to psi
| Head (ft) | Pressure (psi) |
|---|---|
| 1 | 0.43 |
| 2 | 0.87 |
| 3 | 1.30 |
| 4 | 1.73 |
| 5 | 2.17 |
| 6 | 2.60 |
| 7 | 3.03 |
| 8 | 3.46 |
| 9 | 3.90 |
| 10 | 4.33 |
| 20 | 8.66 |
| 30 | 13.0 |
| 40 | 17.3 |
| 50 | 21.7 |
| 60 | 26.0 |
| 70 | 30.3 |
| 80 | 34.7 |
| 90 | 39.0 |
| 100 | 43.3 |
| 120 | 52.0 |
| 140 | 60.6 |
| 160 | 69.3 |
| 180 | 78.0 |
| 200 | 86.6 |
| 250 | 108 |
| 300 | 130 |
| 350 | 152 |
| 400 | 173 |
| 500 | 217 |
| 600 | 260 |
| 700 | 303 |
| 800 | 346 |
| 900 | 390 |
| 1000 | 433 |
- water at 62°F (17°C)
Download pressure vs head imperial units as pdf file
Download water pressure vs head - SI units as pdf file
Head vs. Pressure when handling Liquids of different Density
| Liquid | Specific Gravity | Head (ft) | Pressure (psi) |
|---|---|---|---|
| Water | 1.0 | 100 | 43.3 |
| Kerosene | 0.8 | 100 | 36.6 |
| Sulphuric Acid | 1.8 | 100 | 77.9 |
Identical pumps will develop the same head when running at the same speed but the discharge pressure is quite different due to the different densities of the liquids. It can be simpler to discuss the performance pumps in terms of head rather than pressure since the use of head makes the pump curve applicable to any liquid regardless of density.
