Pump Calculations and Equations
Here's a list of some basic equations that are used in pump sizing. The primary question in choosing a pump is “What horsepower do I need?’
Formulas For Reciprocating Pumps
I know what maximum PSI and maximum GPM I need. What size pump do I need?
Reciprocating Pump Formula:Minimum Horsepower Required = Max GPM × Max PSI ÷ 1550
I have a pump with no tags or specifications on it. How do I find out what Gallons Per Minute (GPM) this pump is capable of?
(Plunger Radius × Plunger Radius × 3.142) = Sq. Inches of Circle(Sq. Inches of Circle) × (Stroke Length) × (Number of Plungers) = Cubic Inches of Liquid Per Revolution
Cubic Inches of Liquid Per Revolution ÷ 231 = Gallons Per Revolution
(Gallons Per Revolution) × (Max RPM) = Gallons Per Minute
NOTE: Max RPM in the above equation varies according to type of pump, size of stroke, and other variables. Duplex pumps often run about 100 RPM Max. while triplex pumps will run somewhere between 100 RPM Max and 400 RPM Max.
I have a reciprocating pump and I know what my max rated rod load is (in foot pounds). I also know what size plunger size my pump has. What PSI will my pump produce?
Max. PSI = Rod Load Rating of Pump ÷ (Plunger Radius × Plunger Radius × 3.142)I have a reciprocating pump and I know what my max rated rod load is (in foot pounds). I also know what PSI I need.
What size plungers do I need?
Min. Plunger Size Needed = Square Root of (Rod Load Rating of Pump ÷ Max. PSI ÷ 3.142)
Formulas For Centrifugal Pumps
I know what maximum PSI and maximum GPM I need. What size pump do I need?
Centrifugal Pump Formula:Minimum Horsepower Required = ((Max GPM) × (Max PSI) ÷ 1710) ÷ (Efficiency in Percentage) × (specific gravity of material)
How do I calculate PSI (Pressure in Pounds Per Square Inch) or TDH (Total Dynamic Head)?
Assuming you have one measurement for your pump but not the other:PSI = TDH ÷ 2.31
TDH = PSI × 2.31
How do I calculate Brake Horsepower Required for a centrifugal pump?
Brake Horsepower Required = GPM Required × (Total Dynamic Head) ÷ 3960 ÷ EfficiencyGeneral Equations
1 Barrel = 42 gallons
Gallons Per Minute = Barrels Per Day × .0292
GPM = BPD × .0292
Horsepower = Torque in Inch Pounds × (Revolutions Per Minute) ÷ 63,025
HP = ((Torque in Inch Pounds) × RPM) ÷ 63,025
1 US Liquid Gallon = 231 Cubic Inches
1 US Liquid Gallon = 231 Inches^3
1 US Liquid Gallon = 0.13368 Cubic Feet
1 US Liquid Gallon = 0.13368 Feet^3
1 US Liquid Gallon = 3.78541 Liters
1 Liter = 0.26417 US Liquid Gallons
1 Cubic Foot = 7.84 US Liquid Gallons
1 Foot^3 = 7.84 US Liquid Gallons
1 Cubic Foot = 28.31685 Liters
1 Foot^3 = 28.31685 Liters
Cubic Feet Per Day = Cubic Feet Per Minute × 1440
Feet^3 Per Day = (Feet^3 Per Minute) × 1440
Cubic Feet Per Minute = Cubic Feet Per Day ÷ 1440 Minutes Per Day
Feet^3 Per Minute = (Feet^3 Per Day) ÷ 1440 Minutes Per Day
Chart of Relative Density / Specific Gravity for Common Liquids
Specific Gravity is used when sizing a centrifugal pump. Liquids with a specific gravity greater than 1.0 are heavier than water and conversely, liquids with a specific gravity lower than 1.0 are lighter weight than water and will generally float on water.
| Material | Degrees Celsius | Degress Fahrenheit | Specific Gravity |
|---|---|---|---|
| Alcohol, ethyl (ethanol) | 25 | 77 | 0.787 |
| Alcohol, methyl (methanol) | 25 | 77 | 0.791 |
| Alcohol, propyl | 25 | 77 | 0.802 |
| Butane, liquid | 25 | 77 | 0.601 |
| Crude oil, California | 15.56 | 60 | 0.918 |
| Crude oil, Mexico | 15.56 | 60 | 0.976 |
| Crude oil, Texas | 15.56 | 60 | 0.876 |
| Ethane | -89 | -128.2 | 0.572 |
| Ethylene glycol | 25 | 77 | 1.100 |
| Gasoline, Natural | 15.56 | 60 | 0.713 |
| Gasoline, Vehicle | 15.56 | 60 | 0.739 |
| Kerosene | 15.56 | 60 | 0.820 |
| Methane | -164 | -263.2 | 0.466 |
| Octane | 25 | 77 | 0.701 |
| Oxygen | -183 | -297.4 | 1.14 |
| Propane | -40 | -40 | 0.585 |
| Propane | 25 | 77 | 0.495 |
| Water, pure | 4 | 39.2 | 1.0 |
| Water, sea | 25 | 77 | 1.025 |