Pipe Size Calculator (Velocity Method)
Estimate the minimum internal pipe diameter from your flow rate and target velocity using d = √(4Q / πv).
What this calculator does
This pipe size calculator helps you quickly choose a practical line size by using a target fluid velocity. In early design, velocity-based sizing is one of the fastest ways to get to a realistic pipe diameter before running detailed pressure drop modeling.
You enter:
- Flow rate (US gpm, L/s, m³/h, or m³/s)
- Design velocity (ft/s or m/s)
The tool returns:
- Minimum required internal diameter in mm and inches
- A nearest larger Schedule 40 nominal pipe size suggestion
- The resulting velocity in that suggested size
Formula used
The calculator uses the continuity equation rearranged for diameter:
d = √(4Q / πv)
- d = internal diameter (m)
- Q = volumetric flow rate (m³/s)
- v = flow velocity (m/s)
This is ideal for first-pass sizing and comparison of alternatives.
Typical design velocity ranges
Water systems (general guidance)
- Building branches: 0.9 to 1.8 m/s (3 to 6 ft/s)
- Mains: 1.5 to 2.4 m/s (5 to 8 ft/s)
- Pump suction: often lower, around 0.6 to 1.5 m/s
Compressed air (general guidance)
- Main headers: 6 to 10 m/s
- Branches/drops: 4 to 8 m/s
Use project standards, local code, and equipment manufacturer requirements whenever they differ from rules of thumb.
Worked example
Suppose you need to carry 120 gpm of water at a target velocity of 5 ft/s.
- Convert flow to m³/s
- Convert velocity to m/s
- Apply
d = √(4Q / πv)
You will get a minimum internal diameter around 77 mm (about 3.0 in), and the nearest practical Schedule 40 choice is typically a 3" nominal line.
Important notes before final design
1) Pressure drop still matters
Velocity-based sizing does not replace a full friction loss calculation. Long pipe runs, high roughness, many fittings, and control valves can change final size decisions.
2) Internal diameter varies by schedule and material
A 2" nominal Schedule 40 steel pipe has a different internal diameter than another schedule/material. Always size against actual inside diameter (ID), not just nominal size.
3) Check noise, erosion, and water hammer risk
Higher velocity can increase noise and wear, especially at elbows, tees, and valves. Critical systems should be reviewed for transient events and surge protection.
FAQ
Is this calculator for liquids only?
It works best for incompressible-flow quick estimates (water-like services). It can be used for gases as a rough first pass, but gas systems generally need compressible-flow methods for final sizing.
Does this include elevation and pump head?
No. This tool calculates diameter from flow and velocity only. Pump TDH and system curve calculations should be done separately.
Can I use this for PVC, copper, and steel?
Yes for minimum ID estimation. For nominal size suggestions, this page uses a Schedule 40 steel reference table. If you use another material, compare the required ID to that material's pipe chart.