Pipe Pressure Drop Calculator
Calculate the estimated pressure loss through a straight pipe from the flow rate, pipe diameter, pipe length and fluid properties.
How to use the pipe pressure drop calculator
Enter the liquid flow rate, internal pipe diameter, straight pipe length and absolute pipe roughness. Water at 20°C is available as the default fluid.
Select Custom fluid if you know the density and dynamic viscosity of another incompressible liquid. These fluid properties affect the Reynolds number and friction factor, which in turn affect the calculated pressure loss.
Use the actual internal diameter of the pipe rather than its nominal size. Even a relatively small change in internal diameter can have a significant effect on flow velocity and pressure loss.
Pipe pressure drop formula
CalcAtlas uses the Darcy-Weisbach equation to estimate friction loss through a straight, constant-diameter pipe:
ΔP = f × (L ÷ D) × (ρV² ÷ 2)
In this equation:
- ΔP = pressure drop
- f = Darcy friction factor
- L = pipe length
- D = internal pipe diameter
- ρ = fluid density
- V = average fluid velocity
The Darcy friction factor is not a fixed value. It depends on the Reynolds number and, for turbulent flow, the roughness of the pipe relative to its internal diameter.
Reynolds number and flow regime
Reynolds number is a dimensionless value used to describe the behaviour of fluid flow in a pipe. It compares the effects of fluid inertia with the effects of viscosity.
Re = (ρ × V × D) ÷ μ
Here, ρ is fluid density, V is velocity, D is internal pipe diameter and μ is dynamic viscosity.
Low Reynolds numbers are associated with laminar flow, while higher Reynolds numbers generally indicate turbulent flow. Between these regions is a transitional range where the flow can be less predictable.
This matters because the relationship used to determine the Darcy friction factor changes with the flow regime. The calculator therefore reports Reynolds number and the identified flow regime alongside the pressure-drop result.
Why pipe roughness matters
Real pipe walls are not perfectly smooth. Small surface irregularities interact with the flowing liquid and can increase friction, particularly during turbulent flow.
Absolute roughness is a measure of the characteristic height of these surface irregularities. The effect on friction also depends on pipe diameter, so hydraulic calculations commonly use the ratio of absolute roughness to internal diameter, known as relative roughness.
The calculator starts with an absolute roughness of 0.0015 mm, representing a very smooth pipe. For a specific installation, enter a roughness value appropriate to the pipe material and condition. Ageing, corrosion, deposits and internal surface condition can make an existing pipe behave differently from a new one.
What the results mean
Pressure drop is the estimated reduction in pressure caused by friction as the liquid travels through the specified length of straight pipe.
Head loss expresses the same frictional energy loss as an equivalent height of the flowing liquid. This is often useful when working with pumps and hydraulic systems.
The calculator also reports flow velocity, Reynolds number, flow regime, Darcy friction factor and pressure drop per unit length. Together, these values provide more information about the flow conditions than the pressure-drop figure alone.
Worked example
Suppose water at 20°C flows through a straight pipe with an internal diameter of 50 mm. The flow rate is 100 L/min and the pipe is 20 m long.
The calculator first converts the flow rate and pipe dimensions into consistent units and calculates the cross-sectional area of the pipe. From this it determines the average water velocity.
Water density and viscosity are then used with the velocity and pipe diameter to calculate Reynolds number. The flow regime and pipe roughness are used to determine the Darcy friction factor, which is applied in the Darcy-Weisbach equation to estimate the pressure loss along the 20 m pipe.
Changing the internal diameter while keeping the same flow rate can substantially change the result because it changes both fluid velocity and the friction characteristics of the system.
Straight-pipe loss and fitting losses
This calculator estimates friction loss through straight pipe only. A real piping system may also contain elbows, tees, valves, reducers, entrances, exits and other fittings that create additional pressure losses.
These additional losses are often described using fitting loss coefficients or equivalent lengths. They are not included automatically in the result produced by this calculator.
Research published by the National Institute of Standards and Technology (NIST) discusses pressure-loss measurement in straight pipes and common pipe fittings, including the determination of straight-pipe friction factors.
Important limitations
This calculator is intended for a straight, constant-diameter pipe carrying an incompressible liquid. It is not intended for compressible gas-flow calculations.
Results depend on the accuracy of the entered flow rate, internal diameter, pipe length, roughness, fluid density and viscosity. Temperature can also affect fluid properties, particularly viscosity.
The calculation does not include pressure losses through valves, elbows, tees, reducers, entrances, exits or other fittings unless their effects have been accounted for separately.
Do not use this calculator as a substitute for professional hydraulic or piping design where system safety, equipment selection, applicable standards or regulatory compliance are involved.
Frequently asked questions
What causes pressure drop in a pipe?
Pressure is lost as flowing liquid experiences friction against the pipe wall and internal friction within the fluid. Flow rate, pipe diameter, pipe length, fluid properties and internal surface roughness all influence the resulting pressure loss.
Does a smaller pipe increase pressure drop?
For the same volumetric flow rate, reducing the internal pipe diameter increases fluid velocity. This can produce a substantial increase in frictional pressure loss.
Does a longer pipe increase pressure drop?
Yes. Under otherwise identical conditions, Darcy-Weisbach friction loss is proportional to pipe length. Doubling the length of straight pipe therefore approximately doubles its frictional pressure loss when the other flow conditions remain unchanged.
Are valves and elbows included?
No. The result represents friction loss through the entered length of straight pipe. Valves, elbows, tees and other fittings create additional losses that must be considered separately when analysing a complete piping system.
Can I use the calculator for liquids other than water?
Yes, provided the liquid can reasonably be treated as incompressible and you know its density and dynamic viscosity. Select Custom fluid and enter those properties.