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Hydronic Pressure-Loss Calculator

Estimate straight-pipe and minor pressure losses with Darcy–Weisbach using actual fluid properties and a summed fitting-loss coefficient.

Why it matters: Pressure loss determines pump operating requirements and helps diagnose throttling, plugged strainers, undersized piping, and unexpected system resistance.

User inputs

Enter field measurements

8 inputs
Select the unit convention used for all values in this calculation.
gpm or L/sVolumetric liquid flow in the selected unit system.
in. or mmActual pipe inside diameter.
ft or mTotal straight pipe length represented by this segment.
in. or mmAbsolute internal roughness appropriate to the material, condition, and selected units.
—Sum of fitting, entrance, exit, and component loss coefficients represented by K.
lb/ft³ or kg/m³Fluid density at the operating temperature and concentration.
cP or mPa·sDynamic viscosity; cP and mPa·s have the same numerical value.

Calculated result

Engineering evaluation

Within screening range
Total pressure loss (ΔP)3.96 psi
Total head loss (hf)
9.14 ft of fluid
Straight-pipe loss (ΔPf)
2.5 psi
Minor-loss component (ΔPm)
1.46 psi
Velocity (V)
5.21 ft/s
Reynolds number (Re)
123,000
Darcy friction factor (f)
0.018

Engineering interpretation

The entered pipe segment is estimated to lose 3.96 psi (9.14 ft of fluid). The broad 2–8 ft/s velocity screen is displayed as the exact SI equivalent of 0.6096–2.4384 m/s. Use actual manufacturer pressure drops for equipment and valves when available, and verify roughness, inside diameter, glycol properties, and fitting coefficients.

Engineering knowledge center

Understand the calculation—not just the answer

2026-08-07 · Free Library
Governing relationships
A = πD²/4
V = Q/A
Re = ρVD/μ
f = 64/Re (laminar) or 0.25/[log₁₀(ε/(3.7D)+5.74/Re⁰·⁹)]²
ΔPf = f(L/D)ρV²/2
ΔPm = ΣKρV²/2
ΔP = ΔPf + ΔPm
hf = ΔP/(ρg)
Current substitution

ΔP = [f(L/D) + ΣK]ρV²/2 = [0.0175(200 ft / 3.068 in.) + 8]ρV²/2 = 3.96 psi.

Calculation sequence

Step-by-step method

  1. Convert flow, diameter, length, density, and viscosity to consistent SI units.
  2. Calculate area, velocity, and Reynolds number.
  3. Use 64/Re for laminar flow or the Swamee–Jain approximation for turbulent Darcy friction factor.
  4. Add straight-pipe and fitting/minor losses, then report psi and feet of fluid.

Equation legend

Inputs and calculated quantities

Q
Flow — gpm or L/s
D
Inside diameter — in. or mm
L
Straight length — ft or m
ε
Absolute roughness — in. or mm
ΣK
Sum of minor-loss K values — —
ρ
Fluid density — lb/ft³ or kg/m³
μ
Dynamic viscosity — cP or mPa·s
ΔP
Total pressure loss — psi
hf
Total head loss — ft of fluid
ΔPf
Straight-pipe loss — psi
ΔPm
Minor-loss component — psi
V
Velocity — ft/s
Re
Reynolds number
f
Darcy friction factor

Continue the evaluation

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Engineering calculation disclaimer

These engineering tools are provided for educational, preliminary evaluation, field verification, and commissioning support. Results depend on the accuracy of user-entered information and the assumptions stated for each calculation. Every colored status and comparison is a screening indicator only; project criteria, contract requirements, applicable codes, manufacturer instructions, measurement uncertainty, and authorized engineering judgment govern. The tools do not replace project-specific engineering analysis or the judgment of a licensed professional engineer.

Read the full use and limitation statement