Pipe Flow Rate Calculator
Enter a pipe's inside diameter and flow rate to get flow velocity, checked against a recommended velocity limit, with a comparison across standard pipe sizes.
Pipe-size comparison
Velocity at your flow rate across standard pipe sizes, with your selected diameter highlighted.
Advanced options (velocity vs. flow-rate chart)
Velocity vs. flow rate (selected diameter)
How it works
Flow velocity follows the continuity equation: velocity equals flow rate divided by the pipe's cross-sectional area.
v = Q / A, where A = π × (D / 2)²
Q is flow rate, D is inside diameter, and A is the circular cross-sectional area. A smaller diameter forces the same flow through a smaller area, so velocity rises.
Worked example
10 GPM through a 1 in inside-diameter pipe: area ≈ 0.785 in², so converting units gives a velocity of about 4.1 ft/s — comfortably under an 8 ft/s guideline. Dropping to a 0.5 in pipe at the same flow rate roughly quadruples the velocity to around 16 ft/s, well over the guideline.
Assumptions
- Flow is treated as steady and fills the full pipe cross-section (no partial flow or free surface).
- "Inside diameter" is the actual bore, which is smaller than the nominal pipe size for most schedules.
- The velocity limit is a general guideline, not a code requirement for every application.
Reference
Typical pipe velocity guidelines: The Engineering ToolBox — Water Velocity in Pipes.
Related tools
Frequently asked questions
How do I calculate flow velocity in a pipe?
Divide the flow rate by the pipe's cross-sectional area. A smaller diameter at the same flow rate always gives a higher velocity.
What is a safe pipe velocity for water?
A common guideline is about 8 ft/s (2.4 m/s) or less in distribution piping, to limit erosion, noise, and water-hammer risk.
Does this calculator account for pressure drop?
No — use the dedicated pressure-drop calculator for Darcy-Weisbach pressure loss and head loss.