# What is the pressure drop in my pipe?

Pressure drop calculator for pipe flow: the Darcy-Weisbach pressure and head loss from the flow rate or velocity, pipe size, length and material, with the friction factor, Reynolds number and flow regime.

- Page: https://www.acalculator.org/physics/pressure-drop-calculator
- JSON spec: https://www.acalculator.org/physics/pressure-drop-calculator.json
- Version: fe60ee7472d4

## Default answer

Example with the default inputs (Flow given as Flow rate, Flow rate (Q) 30, Flow unit US gal/min (gpm), Inside diameter (D) 2 in, Pipe length (L) 100 ft, Pipe material Commercial steel or wrought iron (0.00015 ft), Fluid Water at 20 °C (68 °F)): The pressure drop is 0.91197 psi (2.1074 ft of head), with f = 0.0240783 at Re = 47,277.8.

## Inputs

| Key | Label | Description |
| --- | --- | --- |
| by | Flow given as | Give the flow as a volume rate or as the mean velocity in the pipe. |
| q | Flow rate (Q) | The volume of fluid per unit time, in the unit chosen below. |
| qu | Flow unit | The unit of the flow rate you type. |
| v | Velocity (v) | The mean velocity of the fluid in the pipe. |
| D | Inside diameter (D) | The inside diameter of the pipe. |
| L | Pipe length (L) | The length of straight pipe. |
| mat | Pipe material | The pipe material, which sets the absolute roughness ε (Hydraulic Institute Table 3.A.1). |
| e | Roughness (ε) | The absolute roughness of the pipe wall. |
| fl | Fluid | The fluid, which sets its density and viscosity (NIST Chemistry WebBook for water). |
| rho | Density (ρ) | The density of the fluid. |
| mu | Viscosity (μ) | The dynamic viscosity of the fluid, in the unit chosen below. |
| muu | Viscosity unit | The unit of the viscosity you type. |

## Outputs

| Key | Label | Description |
| --- | --- | --- |
| dp | Pressure drop | Δp = f × (L ÷ D) × ρ × v² ÷ 2 over the length of pipe. |
| pa | Pressure drop (Pa) | The pressure drop in pascals. |
| psi | Pressure drop (psi) | The pressure drop in pounds per square inch. |
| headM | Head loss (m) | h = Δp ÷ (ρg): the same loss as a height of the fluid, in metres. |
| headFt | Head loss (ft) | The head loss in feet of the fluid. |
| velocity | Velocity (m/s) | The mean velocity in the pipe. |
| flowLps | Flow rate (L/s) | The flow rate in litres per second. |
| re | Reynolds number | Re = ρ × v × D ÷ μ. |
| regime | Flow | Laminar below Re = 2,000, turbulent above 4,000, transitional between. |
| f | Friction factor f | The Darcy friction factor used (EPANET method). |
| colebrook | Colebrook friction factor | The Colebrook-White factor, for turbulent flow only, to compare. |
| relRough | Relative roughness ε ÷ D | The roughness as a share of the diameter. |
| rho | Density used (kg/m³) | The fluid density in kilograms per cubic metre. |

## Method

Δp = f × (L ÷ D) × ρv² ÷ 2; h = Δp ÷ ρg; Re = ρvD ÷ μ; f = 64 ÷ Re for Re < 2,000; f = 0.25 ÷ [log10(ε ÷ 3.7D + 5.74 ÷ Re^0.9)]² for Re > 4,000; Dunlop’s cubic between (EPANET).

## Assumptions

- Steady flow of a liquid (incompressible) filling a straight round pipe; fittings, valves and height changes are not included.
- Between Re = 2,000 and 4,000 the flow is unstable and f is uncertain (Hydraulic Institute); the page interpolates as EPANET does.

## Worked examples

1. by = velocity, v = 1, D = 0.05, L = 10, mat = drawn, fl = custom, rho = 900, mu = 0.1, muu = Pa-s gives re = 450, f = 0.142222, pa = 12,800, regime = Laminar. Source: Hydraulic Institute, HI Data Tool, Pipe Frictional Losses (Eq. 3.A.1 h_f = f × (L ÷ D) × v² ÷ 2g; Eq. 3.A.2 Colebrook; Eq. 3.A.3 explicit approximation; Eq. 3.A.5 f = 64 ÷ Re below 2,000; Table 3.A.1 roughness), https://datatool.pumps.org/fluid-flow-iii/general (retrieved 2026-10-18) (f = 64 ÷ Re); OpenStax, University Physics Volume 1, §14.7 Viscosity and Turbulence (Poiseuille’s law, Equation 14.19: Q = (p₂ − p₁)πr⁴ ÷ 8ηl), https://openstax.org/books/university-physics-volume-1/pages/14-7-viscosity-and-turbulence (retrieved 2026-10-18): Δp = 8ηlv ÷ r² = 8 × 0.1 × 10 × 1 ÷ 0.025² = 12,800 Pa, the same as Darcy-Weisbach with f = 64 ÷ 450.
2. by = flow, q = 30, qu = gpm, D = 0.0508, L = 30.48, mat = steel, fl = water20 gives v = 0.933826, re = 47,277.787126, f = 0.024078, psi = 0.911971, colebrook = 0.023936. Source: Hydraulic Institute, HI Data Tool, Pipe Frictional Losses (Eq. 3.A.1 h_f = f × (L ÷ D) × v² ÷ 2g; Eq. 3.A.2 Colebrook; Eq. 3.A.3 explicit approximation; Eq. 3.A.5 f = 64 ÷ Re below 2,000; Table 3.A.1 roughness), https://datatool.pumps.org/fluid-flow-iii/general (retrieved 2026-10-18); US EPA, EPANET Users Manual, 13. Analysis Algorithms, §13.1 (Hagen-Poiseuille below Re = 2,000, Swamee and Jain above 4,000, Dunlop’s cubic interpolation between), https://epanet-manual.readthedocs.io/en/latest/analysis_algorithms.html (retrieved 2026-10-18); NIST Chemistry WebBook, SRD 69, Isobaric Properties for Water at 0.101325 MPa (20 °C: 998.21 kg/m³, 0.0010016 Pa·s), https://webbook.nist.gov/cgi/fluid.cgi?Action=Load&ID=C7732185&Type=IsoBar&Digits=5&P=0.101325&THigh=100&TLow=10&TInc=10&RefState=DEF&TUnit=C&PUnit=MPa&DUnit=kg%2Fm3&HUnit=kJ%2Fkg&WUnit=m%2Fs&VisUnit=Pa*s&STUnit=N%2Fm (retrieved 2026-10-18).
3. by = velocity, v = 0.3, D = 0.01, L = 1, mat = drawn, fl = water20 gives re = 2,989.846246, regime = Transitional (critical zone), f = 0.033043. Source: US EPA, EPANET Users Manual, 13. Analysis Algorithms, §13.1 (Hagen-Poiseuille below Re = 2,000, Swamee and Jain above 4,000, Dunlop’s cubic interpolation between), https://epanet-manual.readthedocs.io/en/latest/analysis_algorithms.html (retrieved 2026-10-18) (Dunlop cubic interpolation).
4. by = velocity, v = 2, D = 0.2, L = 1,000, mat = castIron, fl = water10 gives re = 306,210.276438, f = 0.021877, colebrook = 0.021731. Source: Hydraulic Institute, HI Data Tool, Pipe Frictional Losses (Eq. 3.A.1 h_f = f × (L ÷ D) × v² ÷ 2g; Eq. 3.A.2 Colebrook; Eq. 3.A.3 explicit approximation; Eq. 3.A.5 f = 64 ÷ Re below 2,000; Table 3.A.1 roughness), https://datatool.pumps.org/fluid-flow-iii/general (retrieved 2026-10-18) (Colebrook and the explicit form); US EPA, EPANET Users Manual, 13. Analysis Algorithms, §13.1 (Hagen-Poiseuille below Re = 2,000, Swamee and Jain above 4,000, Dunlop’s cubic interpolation between), https://epanet-manual.readthedocs.io/en/latest/analysis_algorithms.html (retrieved 2026-10-18); NIST Chemistry WebBook, SRD 69, Isobaric Properties for Water at 0.101325 MPa (20 °C: 998.21 kg/m³, 0.0010016 Pa·s), https://webbook.nist.gov/cgi/fluid.cgi?Action=Load&ID=C7732185&Type=IsoBar&Digits=5&P=0.101325&THigh=100&TLow=10&TInc=10&RefState=DEF&TUnit=C&PUnit=MPa&DUnit=kg%2Fm3&HUnit=kJ%2Fkg&WUnit=m%2Fs&VisUnit=Pa*s&STUnit=N%2Fm (retrieved 2026-10-18).

## FAQ

### How do you calculate pressure drop in a pipe?

Use the Darcy-Weisbach equation: Δp = f × (L ÷ D) × ρv² ÷ 2, with f the friction factor, L the pipe length, D the inside diameter, ρ the density and v the mean velocity. 30 gpm of water in 100 ft of 2 in steel pipe loses about 0.91 psi.

### How is the friction factor found?

From the Reynolds number Re = ρvD ÷ μ and the relative roughness ε ÷ D. In laminar flow (Re below 2,000), f = 64 ÷ Re. In turbulent flow (above 4,000) the page uses the Swamee-Jain formula 0.25 ÷ [log10(ε ÷ 3.7D + 5.74 ÷ Re^0.9)]², as the EPA’s EPANET does, and shows the Colebrook value beside it.

### What happens between Re = 2,000 and 4,000?

The flow is unstable and can switch between laminar and turbulent, so the friction factor is uncertain. The page follows EPANET and joins the two curves with a smooth cubic, so the answer does not jump.

### What is head loss?

The pressure drop written as a height of the flowing fluid: h = Δp ÷ ρg. A head loss of 2.11 ft of water is the same as about 0.91 psi.

### How rough are common pipes?

From the Hydraulic Institute: plastic and drawn tubing 0.000005 ft (0.0015 mm), stainless steel 0.00005 ft, commercial steel 0.00015 ft (0.046 mm), galvanized iron 0.0005 ft, uncoated cast iron 0.00085 ft (0.26 mm), and concrete 0.001 to 0.01 ft. The HI adds that commercial installations may need 15% more.

### Does the pressure drop depend on the fluid?

Yes. Density and viscosity set the Reynolds number and the friction factor. Warm water is thinner than cold water, so it loses a little less pressure at the same flow.

## Sources

- Hydraulic Institute, HI Data Tool, Pipe Frictional Losses: Eq. 3.A.1, h_f = f × (L ÷ D) × v² ÷ 2g; Eq. 3.A.2, Colebrook; Eq. 3.A.3, the explicit approximation; Eq. 3.A.4, Re = ρvD ÷ μ; Eq. 3.A.5, f = 64 ÷ Re; Table 3.A.1, pipe roughness, retrieved 2026-10-18. https://datatool.pumps.org/fluid-flow-iii/general
- US EPA, EPANET Users Manual, 13. Analysis Algorithms, §13.1 Hydraulics: Hagen-Poiseuille below Re = 2,000, Swamee and Jain above 4,000, and Dunlop’s cubic interpolation from the Moody diagram between, retrieved 2026-10-18. https://epanet-manual.readthedocs.io/en/latest/analysis_algorithms.html
- OpenWaterAnalytics, EPANET source code, src/hydcoeffs.c, frictionFactor(): the turbulent factor 1 ÷ (−0.8686 ln y)² and the transition terms taken at Re = 4,000, retrieved 2026-10-18. https://github.com/OpenWaterAnalytics/EPANET/blob/dev/src/hydcoeffs.c
- NIST Chemistry WebBook, SRD 69, Isobaric Properties for Water at 0.101325 MPa: density and viscosity at 10, 20, 40, 60 and 80 °C, retrieved 2026-10-18. https://webbook.nist.gov/cgi/fluid.cgi?Action=Load&ID=C7732185&Type=IsoBar&Digits=5&P=0.101325&THigh=100&TLow=10&TInc=10&RefState=DEF&TUnit=C&PUnit=MPa&DUnit=kg%2Fm3&HUnit=kJ%2Fkg&WUnit=m%2Fs&VisUnit=Pa*s&STUnit=N%2Fm
- OpenStax, University Physics Volume 1, §14.7 Viscosity and Turbulence: Poiseuille’s law, Equation 14.19, Q = (p₂ − p₁)πr⁴ ÷ 8ηl, retrieved 2026-10-18. https://openstax.org/books/university-physics-volume-1/pages/14-7-viscosity-and-turbulence
- Colebrook, C. F. (1939). Turbulent flow in pipes, with particular reference to the transition region between the smooth and rough pipe laws. Journal of the Institution of Civil Engineers 11(4), 133–156. https://doi.org/10.1680/ijoti.1939.13150
- Swamee, P. K., and Jain, A. K. (1976). Explicit equations for pipe-flow problems. Journal of the Hydraulics Division (ASCE) 102(5), 657–664. https://doi.org/10.1061/JYCEAJ.0004542