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Piping

Pipe Pressure Drop Calculator

This calculator estimates the pressure drop and head loss in a straight circular pipe from the flow rate, internal diameter, length, fluid density and viscosity, and pipe roughness, computing the Darcy friction factor (laminar f = 64/Re or the Swamee-Jain approximation) and applying the Darcy-Weisbach equation with optional minor-loss and elevation terms.

This calculator gives a preliminary estimate of the pressure drop in a straight circular pipe carrying an incompressible single-phase liquid. From the flow rate and internal diameter it computes the velocity, then the Reynolds number from the fluid density and viscosity, then the Darcy friction factor (laminar f = 64/Re, or the Swamee-Jain explicit turbulent approximation), and finally the Darcy-Weisbach friction head. An optional total K-value adds minor (fitting) losses and an optional elevation change adds static head. Unlike the Darcy-Weisbach and Pipe Head Loss calculators, you do not supply the friction factor — it is calculated from the flow conditions.

TypeInteractive engineering calculator

Calculator

Velocity:
Head:
Pressure:
Result
Velocity (v)1.5719 m/s
Reynolds number (Re)235785
Flow regimeTurbulent
Darcy friction factor (f)0.0174724
Friction head (h_f)1.46745 m
Minor-loss head (h_m)0 m
Elevation head (h_z)0 m
Total head loss (h_total)1.46745 m
Pressure drop (ΔP)14.3907 kPa

Formulas

Flow area
A = πD²/4
Velocity
v = Q / A
Reynolds number
Re = ρvD/μ
Friction factor (laminar)
f = 64 / Re
Friction factor (turbulent, Swamee-Jain)
f = 0.25 / [log₁₀(ε/(3.7D) + 5.74/Re⁰·⁹)]²
Friction head
h_f = f × (L/D) × v²/(2g)
Minor-loss head
h_m = K × v²/(2g)
Elevation head
h_z = Δz
Total head
h_total = h_f + h_m + h_z
Pressure drop
ΔP = ρ × g × h_total

Diagram

Pipe Pressure Drop: ΔP = ρg(h_f + h_m + h_z)KQ, v = Q/AΔzh_f (friction)h_mh_total = h_f + h_m + h_zΔP = ρ × g × h_total

Worked example

Water flows at 100 m³/h through a 150 mm internal-diameter pipe, 100 m long. Density = 1000 kg/m³, dynamic viscosity = 1 cP, absolute roughness = 0.045 mm, total K = 0, no elevation change, g = 9.80665 m/s².

  1. 01A = π × 0.15² / 4 = 0.017671 m²
  2. 02v = (100 / 3600) / 0.017671 = 1.572 m/s
  3. 03Re = 1000 × 1.572 × 0.15 / 0.001 = 235,785 → turbulent
  4. 04ε/D = 0.000045 / 0.15 = 0.0003
  5. 05f = 0.25 / [log₁₀(0.0003/3.7 + 5.74/235785⁰·⁹)]² ≈ 0.01747 (Swamee-Jain)
  6. 06v²/(2g) = 1.572² / (2 × 9.80665) = 0.1260 m
  7. 07h_f = 0.01747 × (100 / 0.15) × 0.1260 = 1.467 m
  8. 08ΔP = 1000 × 9.80665 × 1.467 = 14,390 Pa ≈ 14.4 kPa
Result

Velocity ≈ 1.572 m/s, Re ≈ 235,785 (turbulent), f ≈ 0.0175, friction head ≈ 1.47 m, pressure drop ≈ 14.4 kPa for the 100 m run. (A 1,000 m run of the same pipe would give ≈ 14.7 m / ≈ 144 kPa.)

FAQ

Do I need to supply the friction factor?
No. This calculator computes the Darcy friction factor for you from the Reynolds number and the relative roughness (laminar f = 64/Re, or the Swamee-Jain turbulent approximation). If you would rather supply your own friction factor, use the Darcy-Weisbach Pressure Drop or Pipe Head Loss calculator instead.
What is the difference between pressure drop and head loss?
Head loss (h, in metres of fluid) is the energy lost per unit weight of fluid; pressure drop (ΔP) is the corresponding loss in pressure, related by ΔP = ρ·g·h. Head loss is independent of fluid density for a given velocity and geometry, while pressure drop scales with density.
How do I include fittings, valves, and bends?
Sum the loss coefficients (K-values) of all the fittings on the run and enter the total in the optional "Total K" field. The calculator adds K × v²/(2g) as minor-loss head. K-values are not looked up — consult Crane TP-410 or the Pipe Fitting K-Values reference. Alternatively convert fittings to an equivalent length of pipe and add it to the pipe length.
How do I handle a rise or fall in elevation?
Enter the net static elevation change (outlet elevation minus inlet elevation) in the optional elevation field. A rise adds head; enter a fall as a negative value to subtract static head. The pressure drop reported includes this static term.
Can I use this for gas, steam, or slurry?
No. This is an incompressible single-phase liquid estimate. Compressible gas flow with significant density change, two-phase or flashing flow, slurry, and non-Newtonian fluids each need different methods and are out of scope.
Is this enough for final line sizing?
No. It is a preliminary estimate. Final hydraulic design needs verified pipe internal diameter and roughness, accurate fluid properties, the full fittings and valve schedule, elevation profile, the min/normal/max operating cases, the applicable project standards, and qualified engineering review.

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