What velocity should I pick?
Water lines often use about 1–2.5 m/s; confirm with your application.
Pipe Size Calculator
Back-calculate a required inside pipe diameter from target flow and design velocity.
Estimate pipe diameter
Enter the volumetric flow and a design velocity, then choose units.
About this calculator
How to use Pipe Size Calculator, plus the formulas and logic behind the results.
Tool description
The Pipe Size Calculator estimates internal diameter needed to carry a given flow at a chosen average velocity.
How to use
- Select the flow unit and enter flow rate.
- Enter a design velocity in m/s.
- Select Calculate to view required inside diameter in mm and inches.
Formulas & reference
Q_m3s from L/s, m³/h, or GPMD = sqrt(4Q ÷ (πv))D_in = D_m × 39.3701
Sizing relation
- D = sqrt(4Q ÷ (πv))
- Higher velocity → smaller diameter
- Lower velocity → larger diameter, less friction
Calculation logic
Flow is converted to m³/s. Diameter follows from rearranging Q = A·v for a circular section. Results are theoretical IDs before commercial sizing.
This is a first-pass hydraulic estimate. Choose commercial pipe IDs, check pressure drop, and follow applicable codes.
Pipe Size Calculator FAQ
Common questions about this calculator and how to use the results.
Is the result a nominal pipe size?
No—it is an internal diameter estimate. Pick the next suitable commercial size.
Does schedule/wall thickness matter?
Yes for OD/ID. This tool sizes the flow ID only.
Can I enter GPM?
Yes—choose the GPM flow unit.
What if velocity is very high?
Friction and noise rise; consider a larger pipe / lower velocity.
Is sludge or slurry included?
No. Non-Newtonian or solids-laden flows need specialist methods.