Identified content errors were corrected on 2026-10-25. Sources and scope for this page: San Diego State University — explicit Darcy–Weisbach friction factor (accessed 2026-10-25), covering swamee–Jain equation and its turbulent application. Unit conversions and the fixed-f diameter scaling are derived explicitly above. What the 2026-10-25 correction changed is stated with the source above; nothing beyond that scope was re-checked. Full sources, values and limits on this page.
Page last changed 2026-10-25 · based on Public reference accessed 2026-10-25; scope stated on page
Quick Answer
Pipe Pressure Drop (Darcy-Weisbach)
The Formulas Used
Q: US gpm × 0.003785411784/60 → m³/s; D: inches × 0.0254 → m; μ: cP × 0.001 → Pa·s; ρ = 1000 × SG kg/m³. v = 4Q/(πD²); Re = ρvD/μ. In these input units, Re ≈ 3162.56 Q·SG/(D·μ).
Darcy f = 64/Re for laminar flow below Re 2300. For Re ≥ 5000 the selected Swamee–Jain approximation is f = 0.25/[log₁₀(ε/(3.7D) + 5.74/Re⁰·⁹)]². The intermediate range is withheld. Head loss = f(L/D)v²/(2g), with g = 9.80665 m/s².
Diameter, flow and operating conditions
At fixed Q, doubling D reduces velocity by four and L/D by two. With f held fixed, the combined reduction is 16 × 2 = 32. The result is not a universal prediction because Reynolds number and relative roughness also change.
Likewise ΔP ∝ Q² holds only with f fixed. Enter liquid properties at the operating temperature and actual pipe roughness. The default inputs are arithmetic examples. This calculation excludes fittings, valves, entry losses, elevation, gas compressibility and non-Newtonian flow.
Basis and scope
San Diego State University — explicit Darcy–Weisbach friction factor: Swamee–Jain equation and its turbulent application. Unit conversions and the fixed-f diameter scaling are derived explicitly above.
Frequently Asked Questions
- What was corrected in the Reynolds number?
- The previous coefficient 50.6 did not match the displayed US gpm, inch, specific-gravity and cP units. Direct SI conversion gives approximately 3162.56 for these units.
- Why is there no result for transitional flow?
- Neither laminar f = 64/Re nor the selected turbulent approximation is used to fill the intermediate range. The calculator does not force Reynolds number upward to manufacture a turbulent result.
- Does this calculate pump head?
- It calculates straight-pipe friction only. Add the other applicable system terms and assess the pump and operating point separately.
Related
Value Sources
References and calculation methods used for this page are listed below. Numbered markers connect table columns where present. A reference listing does not itself establish that every value has been independently checked.
| # | Source | Type | Revision / method |
|---|---|---|---|
| [1] | San Diego State University — explicit Darcy–Weisbach friction factor | reference | Public reference accessed 2026-10-25 — source |
Data Sources
| Standard | Revision | What it covers on this page |
|---|---|---|
| San Diego State University — explicit Darcy–Weisbach friction factor | Public reference accessed 2026-10-25; scope stated on page | Swamee–Jain equation and its turbulent application. Unit conversions and the fixed-f diameter scaling are derived explicitly above. |
Verification scope
- 2026-10-25 — Content corrected 2026-10-25. Swamee–Jain equation and its turbulent application. Unit conversions and the fixed-f diameter scaling are derived explicitly above. Evidence
Content corrected 2026-10-25. Swamee–Jain equation and its turbulent application. Unit conversions and the fixed-f diameter scaling are derived explicitly above.
Cite This Page
Read the source notes, calculation methods and verification scope above before reusing a value. To cite this page: