Water Pipe Pressure Drop: engineer's guide
Pressure drop across a pipe run drives pump head selection and fixture pressure at outlet. It depends on flow, diameter, length, and pipe roughness.
Open the Water Pipe Pressure Drop →
What this calculator does
The calculator applies the Hazen-Williams equation (preferred for water at typical temperatures) and reports pressure drop per 100 ft, velocity, and total head loss.
Formulas used
- Hazen-Williams:
hf = 10.67 × L × (Q/C)1.852 / D4.87 (SI, metres) - Imperial form:
ΔP (psi) = 0.002083 × L × (100/C)1.852 × Q1.852 / D4.8655
Variables
- Q = flow rate, m³/s or GPM
- D = internal diameter
- L = length
- C = roughness coefficient (150 copper, 140 PEX, 120 steel new, 100 steel aged)
Worked example
Scenario: 50 GPM through 100 ft of 2 in copper (C = 140).
- ΔP = 0.002083 × 100 × (100/140)1.852 × 501.852 / 24.8655
- ΔP ≈ 2.1 psi over 100 ft
- Add elevation and fitting losses for total pump head
Common mistakes to avoid
- Wrong C-factor for aged pipe
- Using nominal instead of actual internal diameter
- Ignoring fittings (equivalent length is often 30 to 50% of pipe length)
- Applying Hazen-Williams outside typical water temperatures
Frequently asked questions
Hazen-Williams or Darcy-Weisbach?
Hazen-Williams for water at room temperature, closed-form and quick. Darcy-Weisbach for hot water, oil, or non-Newtonian fluids.
Maximum velocity?
8 fps for cold, 5 fps for hot to limit erosion.
Standards and references
ASPE Data Book, ASHRAE Fundamentals Ch. 22
Related tools on this site
Open the calculator that pairs with this guide: Water Pipe Pressure Drop.
Verification note
This guide is planning support only. Final design must be verified against the applicable project specification, authority requirements, manufacturer data, and the standards cited above. Consult a licensed engineer for construction-issue calculations.