Water Pressure Drop Calculator Formula

Understand the math behind the water pressure drop calculator. Each variable explained with a worked example.

Use the Water Pressure Drop Calculator

Formulas Used

Flow Velocity

velocity_fps = vel

Head Loss

head_loss_ft = diameter_ft > 0 ? friction_factor * (pipe_length / diameter_ft) * pow(vel, 2) / (2 * 32.174) : 0

Pressure Drop

pressure_drop_psi = diameter_ft > 0 ? friction_factor * (pipe_length / diameter_ft) * pow(vel, 2) / (2 * 32.174) * 0.4333 : 0

Variables

VariableDescriptionDefault
pipe_lengthPipe Length(ft)100
diameterInside Diameter(in)1
flow_gpmFlow Rate(GPM)10
friction_factorFriction Factor (f)0.02
diameter_ftDerived value= diameter / 12calculated
flow_cfsDerived value= flow_gpm / 448.831calculated
areaDerived value= pi * pow(diameter_ft / 2, 2)calculated
velDerived value= area > 0 ? flow_cfs / area : 0calculated

How It Works

Darcy-Weisbach Pressure Drop

h_f = f x (L / D) x (V squared / 2g)

Where f is the Darcy friction factor, L is pipe length, D is diameter, V is velocity, and g is gravitational acceleration (32.174 ft/s squared). Multiply head loss in feet by 0.4333 to convert to psi.

Worked Example

100 ft of 1-inch pipe carrying 10 GPM, friction factor 0.02.

pipe_length = 100diameter = 1flow_gpm = 10friction_factor = 0.02
  1. 01D = 1/12 = 0.0833 ft
  2. 02Area = pi x 0.04167 squared = 0.005454 sq ft
  3. 03V = (10/448.831) / 0.005454 = 4.09 ft/s
  4. 04h_f = 0.02 x (100/0.0833) x (4.09 squared / 64.348) = 6.24 ft
  5. 05Pressure drop = 6.24 x 0.4333 = 2.70 psi

Ready to run the numbers?

Open Water Pressure Drop Calculator