How to calculate water pressure loss with plumbing tubes

Calculating the loss of water pressure moving through plumbing pipes is significantly less involved then calculating more pressure loss. Plumbing systems work similarly enough that you can make assumptions about how they operate. Plumbing pipes carry liquid water through a smooth pipe in less than two meters per second. Since almost all plumbing pipes obey these simple rules, you do not have to deal with issues like unusual fluid properties, strange fluid or how rough the pipe is.


  1. Divide the density of the water by the dynamic viscosity of the water. Viscosity is the amount of resistance that a fluid has to be deformed. You need to know the temperature of the water in the pipeline to determine the dynamic viscosity. Once the temperature, you can see the viscosity in a table of water viscosity. Make sure you use dynamic viscosity, not kinematic viscosity. The density of water can be estimated 1000 kg per cubic meter. Regardless of the temperature, liquid water in a plumbing pipe will always be about this density.
  2. Multiply the density and viscosity dividend by the velocity of the water through the pipe and the internal diameter of the pipe to obtain the Reynolds number of the pipe. The Reynolds number is a value that is used in the pressure loss equations. The internal diameter of the pipe is not necessarily equal to the extent that the pipe is sold low. The internal diameter should be measured or refer to the specifications of internal diameter pipe. Make sure you use consistent units. For example, if the water goes 1 meter per second in a 10 mm tube, it should multiply 1 meter per second by 0.01 meter tube, conversion of 10 mm in meters.
  3. Look at a graph by comparing the Reynolds number against the friction factor. Locate the calculated Reynolds number on the horizontal axis of the graph. Trace that point vertically to the table line. The point of the line passing over Reynolds is used to determine the friction factor. Read the value on the vertical axis to obtain the pipe friction factor.
  4. Divide the length of the pipe by the internal diameter of the pipe.
  5. Multiply the value of the previous step by the friction factor.
  6. Multiply the value of the previous step by the density of the water.
  7. Multiply squared the value of the previous step by the speed of the water. The square speed of water is obtained by multiplying the speed of the water with itself.
  8. Divide the value of the previous step by two. The given number is the loss of pressure. If you use metric units, the pressure loss is in Newton’s per square meter.

Tips & Warnings

  • You need to use the same type of units through the equation. You cannot mix actual Engineering (feet, pounds, gallons, pound’s strength) and metrics (meters, kilograms, liters, Newton) or the equation will give the wrong number. It does not matter which system you use, but it must be consistent. Metric units are generally going to be easier since most charts and tables tend to be metric out charts made specifically for industries predominantly using real engineering units.