Pumps - Head vs. Pressure

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

Pumps - Water Head vs. Pressure

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

Head vs. Pressure - 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.

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Length m
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in
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yards
miles
naut miles

Area m²
km²
in²
ft²
miles²
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Volume m³
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in³
ft³
us gal

Weight kg_f
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Velocity m/s
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Pressure Pa
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mm H₂O
kg/cm²
psi
inches H₂O

Flow m³/s
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