Quick answer
The Darcy–Weisbach equation gives ΔP = f × (L ÷ D) × ρv² ÷ 2, with f = 64 ÷ Re for laminar flow and the Swamee–Jain formula for turbulent flow. Water at 5 L/s through a 50 mm pipe 100 m long drops 139.2 kPa, at 2.55 m/s with Reynolds number 126,841 and friction factor 0.02151.
Enter the flow, inside diameter, length, roughness, density and viscosity. The calculator returns pressure drop, velocity, Reynolds number and friction factor.
Related tools: Gallons per Minute Calculator: Flow in gallons per minute from pipe size and velocity, or volume and time. Friction Loss Calculator: Pipe friction head loss with the Hazen–Williams equation.
How to use the Pipe Pressure Drop Calculator
- Enter flow (L/s) (above 0).
- Enter inside diameter (mm) (above 0).
- Enter length (m) (above 0).
- Enter absolute roughness (mm) (0 or more).
- Enter fluid density (kg/m³) (above 0). It starts at 998.2.
- Enter dynamic viscosity (mPa·s) (above 0). It starts at 1.002.
- Press Calculate to see the result, and use Reset to start again.
| Input | What to enter | Allowed values |
|---|---|---|
| Flow | A number, in L/s | > 0 |
| Inside diameter | A number, in mm | > 0 |
| Length | A number, in m | > 0 |
| Absolute roughness | A number, in mm | ≥ 0 |
| Fluid density | A number, in kg/m³ | > 0, starts at 998.2 |
| Dynamic viscosity | A number, in mPa·s | > 0, starts at 1.002 |
Formula and method
Formula
ΔP = f × (L ÷ D) × ρ v² ÷ 2; f = 64 ÷ Re (laminar), Swamee–Jain otherwise
IEEE-754 double precision; shown to the decimals indicated.
Defaults are water at 20 °C. Fittings and elevation changes are not included.
Worked examples
These examples are verified: each one is an automated test case that the live calculator must reproduce exactly.
Example 1
Inputs: Flow = 5; Inside diameter = 50; Length = 100; Absolute roughness = 0.045; Fluid density = 998.2; Dynamic viscosity = 1.002.
| Result | Value |
|---|---|
| Pressure drop | 139.2326 kPa |
| Velocity | 2.5465 m/s |
| Reynolds number | 126,841 |
| Friction factor | 0.02151 |
Example 2
Inputs: Flow = 2; Inside diameter = 25; Length = 30; Absolute roughness = 0.0015; Fluid density = 998.2; Dynamic viscosity = 1.002.
| Result | Value |
|---|---|
| Pressure drop | 180.6658 kPa |
| Velocity | 4.0744 m/s |
| Reynolds number | 101,473 |
| Friction factor | 0.01817 |
Example 3
Inputs: Flow = 0.2; Inside diameter = 50; Length = 20; Absolute roughness = 0.045; Fluid density = 870; Dynamic viscosity = 40.
| Result | Value |
|---|---|
| Pressure drop | 1.043 kPa |
| Velocity | 0.1019 m/s |
| Reynolds number | 111 |
| Friction factor | 0.57776 |
The first line is the headline result (pressure drop). Below it the pipe Pressure Drop Calculator lists velocity, reynolds number and friction factor, which describe the same case from other angles. IEEE-754 double precision; shown to the decimals indicated.
Laminar or turbulent
Reynolds number below about 2,300 is laminar; above 4,000 it is turbulent. At Re = 126,841 the flow is fully turbulent, and the friction factor depends on pipe roughness (0.045 mm for commercial steel).
Accuracy
Swamee–Jain is accurate to about 1 to 2% of the Colebrook equation for turbulent flow. Fittings, valves and elevation changes add loss and are not included here. Works for any Newtonian fluid, unlike Hazen–Williams.
How the result changes with flow
Holding every other input at the first worked example, this table changes only flow. It shows how sensitive the answer is to that one value; each row was produced by running this calculator.
| Flow | Pressure drop | Velocity | Reynolds number |
|---|---|---|---|
| 2.5 | 37.4493 kPa | 1.2732 m/s | 63,421 |
| 5 | 139.2326 kPa | 2.5465 m/s | 126,841 |
| 10 | 530.6868 kPa | 5.093 m/s | 253,682 |
| 20 | 2,060.6758 kPa | 10.1859 m/s | 507,364 |
What happens with invalid input
These messages are produced by the calculator itself, so you know what to fix.
- Flow: entering "0" shows "Flow must be greater than 0." beside that field and no result.
- Absolute roughness: entering "-1" shows "Absolute roughness must be at least 0." beside that field and no result.
Choosing between the Pipe Pressure Drop Calculator and similar calculators
Each row shows what a related calculator on this site asks for and what it returns, so you can pick the one that matches your question.
| Calculator | You enter | It returns |
|---|---|---|
| [Gallons per Minute Calculator](/engineering/gallons-per-minute/) | use | Flow |
| [Friction Loss Calculator](/engineering/friction-loss/) | flow, inside diameter and pipe length | Head loss |
| [Hydraulic Cylinder Force Calculator](/engineering/hydraulic-cylinder-force-calculator/) | units, pressure and bore diameter | Extend force |
| [Pump Flow Calculator](/engineering/pump-flow-calculator/) | displacement, speed and volumetric efficiency | Flow |
| [Voltage Drop Calculator](/engineering/voltage-drop-calculator/) | current, one-way length and wire gauge | Voltage drop |
Background: fluid flow
Volumetric flow rate is Q = A × v (cross-sectional area times average velocity). Bernoulli's equation, P + ½ρv² + ρgh = constant, holds along a streamline for steady, incompressible, frictionless flow, so real pipes need friction corrections. The Reynolds number Re = ρvL/μ compares inertia with viscosity.
Laminar flow in a pipe is usually quoted below Re of about 2,100 to 2,300 and fully turbulent above about 4,000, with an uncertain transition between; treat the thresholds as approximate.
Assumptions and limitations
- Swamee–Jain is accurate to about 1–2% for turbulent flow
Use results as engineering estimates and check them against the governing standards, safety factors and a qualified engineer's judgement before relying on them.
If a required value is empty, not a number, or outside its allowed range, the form marks that field with a message and shows no result instead of a misleading number. The automated checks deliberately try invalid entries for flow and absolute roughness to confirm this.
Frequently asked questions
What is roughness?
The average height of surface irregularities in the pipe.
What does Reynolds number tell me?
Whether flow is laminar or turbulent.
Does it include bends?
No, add minor losses separately.
What does the Pipe Pressure Drop Calculator calculate?
Pressure drop in a straight pipe from the Darcy–Weisbach equation. It returns Pressure drop, Velocity, Reynolds number and Friction factor.
What formula does the Pipe Pressure Drop Calculator use?
ΔP = f × (L ÷ D) × ρ v² ÷ 2; f = 64 ÷ Re (laminar), Swamee–Jain otherwise. Defaults are water at 20 °C.
What do I need to enter in the pipe Pressure Drop Calculator?
You enter Flow (L/s), Inside diameter (mm), Length (m), Absolute roughness (mm), Fluid density (kg/m³) and Dynamic viscosity (mPa·s).
How is the result rounded or limited?
IEEE-754 double precision; shown to the decimals indicated. Swamee–Jain is accurate to about 1–2% for turbulent flow.
What is an example of a pipe Pressure Drop result?
With flow 5, inside diameter 50, length 100, absolute roughness 0.045 and fluid density 998.2, the pressure drop is 139.2326 kPa.
Where does the method come from?
The method follows: Darcy–Weisbach equation; Swamee & Jain, J. Hydraulics Div. ASCE 102 (1976) 657.
Can I rely on this calculator for a decision?
Use results as engineering estimates and check them against the governing standards, safety factors and a qualified engineer's judgement before relying on them. For this calculator in particular: Swamee–Jain is accurate to about 1–2% for turbulent flow.
Related calculators
- Gallons per Minute Calculator — Flow in gallons per minute from pipe size and velocity, or volume and time.
- Friction Loss Calculator — Pipe friction head loss with the Hazen–Williams equation.
- Pump Flow Calculator — Pump output from displacement, speed and volumetric efficiency.
- Pressure Converter — Convert pascals, bar, psi, atmospheres, torr and mmHg.
Sources
- Darcy–Weisbach equation; Swamee & Jain, J. Hydraulics Div. ASCE 102 (1976) 657
Content last reviewed 2026-11-04. Formulas and worked examples are checked by automated tests against independently computed values.