Pipe Friction Loss Calculator Formula

Understand the math behind the pipe friction loss calculator. Each variable explained with a worked example.

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Formulas Used

Head Loss per 100 ft

head_loss_per_100 = c_factor > 0 && diameter > 0 ? 10.67 * pow(flow_gpm / c_factor, 1.852) / pow(diameter / 12, 4.8655) : 0

Total Head Loss

total_head_loss = c_factor > 0 && diameter > 0 ? 10.67 * pow(flow_gpm / c_factor, 1.852) / pow(diameter / 12, 4.8655) * pipe_length / 100 : 0

Pressure Loss

pressure_loss_psi = c_factor > 0 && diameter > 0 ? 10.67 * pow(flow_gpm / c_factor, 1.852) / pow(diameter / 12, 4.8655) * pipe_length / 100 * 0.4333 : 0

Variables

VariableDescriptionDefault
flow_gpmFlow Rate(GPM)15
diameterInside Diameter(in)1
c_factorHazen-Williams C130
pipe_lengthPipe Length(ft)100

How It Works

Hazen-Williams Formula

h_f = 10.67 x (Q / C)^1.852 / D^4.8655 x L

This empirical formula is widely used for water distribution systems. C is the roughness coefficient (150 for new PVC, 130 for copper, 100 for old steel). Results are in feet of head; multiply by 0.4333 for psi.

Worked Example

15 GPM through 1-inch copper (C=130) for 100 feet.

flow_gpm = 15diameter = 1c_factor = 130pipe_length = 100
  1. 01h_f/100ft = 10.67 x (15/130)^1.852 / (1/12)^4.8655
  2. 02Compute numerator and denominator using the formula
  3. 03Total head loss for 100 ft is approximately 12.5 ft
  4. 04Pressure loss = 12.5 x 0.4333 = 5.42 psi

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