Pipe Sizing Calculator Formula

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

Use the Pipe Sizing Calculator

Formulas Used

Minimum Diameter

min_diameter_in = 2 * sqrt(area_needed / pi) * 12

Cross-Section Area

area_sqin = area_needed * 144

Variables

VariableDescriptionDefault
flow_gpmRequired Flow(GPM)20
max_velocityMax Velocity(ft/s)6
flow_cfsDerived value= flow_gpm / 448.831calculated
area_neededDerived value= flow_cfs / max_velocitycalculated

How It Works

Pipe Sizing

Rearrange the continuity equation Q = A x V to solve for the minimum cross-sectional area, then back-calculate the diameter.

D = 2 x sqrt( Q / (V x pi) )

Always round up to the next standard pipe size available from your supplier.

Worked Example

Need 20 GPM at a maximum velocity of 6 ft/s.

flow_gpm = 20max_velocity = 6
  1. 01Q = 20 / 448.831 = 0.04456 cu ft/s
  2. 02A = 0.04456 / 6 = 0.007427 sq ft
  3. 03D = 2 x sqrt(0.007427 / pi) x 12 = 1.17 in
  4. 04Choose the next standard size: 1-1/4 inch pipe.

Ready to run the numbers?

Open Pipe Sizing Calculator