Friction Loss Calculator

Enter flow rate, pipe diameter, pipe length, and a Hazen-Williams C coefficient to get head loss, pressure drop, flow velocity, and friction gradient for water flowing through a pipe.

Quick Facts

Hazen-Williams formula
h_f = 10.67 × L × Q¹·⁸⁵² ÷ (C¹·⁸⁵² × D⁴·⁸⁷⁰⁴)
SI form: Q in m³/s, D and L in meters, h_f in meters of head.
Flow velocity
V = Q ÷ (πD²/4)
Average velocity across the pipe's cross-section.
Typical C values
PVC ≈150, copper ≈140, new steel ≈120, old iron ≈100
Lower C means a rougher pipe and more friction loss.
Valid range
Turbulent water flow, pipe ≥ 2 in (50 mm)
For other fluids or laminar flow, use Darcy-Weisbach instead.

Your Results

Calculated
Head Loss (h_f)
-
Hazen-Williams head loss over the pipe length
Pressure Drop (ΔP)
-
ρ × g × h_f, for water
Flow Velocity (V)
-
Q ÷ pipe cross-sectional area
Friction Loss Gradient
-
Head loss rate per 100 units of pipe

Ready

Enter flow rate, pipe diameter, length, and C coefficient, then press Calculate.

Formula and Method for Pipe Friction Loss

Friction loss (also called head loss) is the drop in a fluid's energy as it flows through a pipe, caused by friction between the moving fluid and the pipe wall. This calculator estimates friction loss for water flowing in a pipe using the Hazen-Williams equation, a well-established empirical formula widely used in plumbing, irrigation, and fire-protection engineering. From the flow rate, pipe diameter, pipe length, and a material-specific roughness coefficient (C), it computes head loss, the equivalent pressure drop, flow velocity, and the friction gradient per 100 units of pipe.

How the calculation works

Enter the flow rate, the pipe's inside diameter, the pipe length, and the Hazen-Williams coefficient C for the pipe material. The calculator converts every value into SI units (cubic meters per second, meters) and applies h_f = 10.67 × L × Q1.852 ÷ (C1.852 × D4.8704) to get the head loss h_f in meters of water. It also computes flow velocity from V = Q ÷ (πD²/4), converts head loss into a pressure drop using ΔP = ρgh_f for water (ρ = 1000 kg/m³, g = 9.80665 m/s²), and expresses the loss as a gradient per 100 units of pipe length so results are easy to compare across projects of different sizes.

Choosing a C coefficient

The Hazen-Williams coefficient C represents how smooth the interior of the pipe is — a higher C means less resistance to flow. New PVC or plastic pipe is typically C ≈ 150, copper and new steel run C ≈ 120-140, and old, corroded, or heavily scaled iron pipe can drop to C ≈ 80-100 or lower. Using an outdated C value for an older, corroded pipe will significantly understate real friction loss, so re-measure or use a conservative (lower) C for aging infrastructure.

Limits of the Hazen-Williams formula

This formula is empirical and calibrated for water at ordinary temperatures flowing turbulently through pipes roughly 2 inches (50 mm) and larger; it is not accurate for other fluids (oil, gas, slurries), for laminar flow, or for very small-diameter tubing. For non-water fluids, or when you already know the Darcy friction factor and Reynolds number, use the Darcy-Weisbach equation (h_f = f × (L/D) × V²/2g) instead.

Frequently Asked Questions

What is friction loss in a pipe?
Friction loss (or head loss) is the energy a fluid loses to friction as it moves through a pipe, caused by resistance between the fluid and the pipe wall. It's expressed as an equivalent height of fluid (head, in meters or feet) or as a pressure drop (in kPa or psi), and it increases with flow rate, pipe length, and pipe roughness, and decreases as pipe diameter increases.
What formula does this calculator use?
It uses the Hazen-Williams equation, h_f = 10.67 × L × Q1.852 ÷ (C1.852 × D4.8704), with Q in m³/s and D and L in meters. This is the standard empirical formula for water flow in pipes used throughout plumbing, irrigation, and fire-protection design.
What Hazen-Williams C value should I use?
Use a C value that matches your pipe material and condition: about 150 for new PVC or plastic pipe, 140 for new copper, 120 for new steel or ductile iron, and as low as 80-100 for old, corroded, or heavily scaled iron pipe. When in doubt, use a lower C — it produces a more conservative (higher) friction loss estimate.
Does friction loss increase a lot with flow rate?
Yes. Because flow rate is raised to the power 1.852 in the Hazen-Williams formula, friction loss grows almost with the square of flow rate — roughly doubling the flow through a pipe increases friction loss by about 3.6×. Small increases in flow rate can noticeably raise pressure loss and pumping energy costs.

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