Pipe Diameter Calculator
Estimate the required inside diameter from flow rate and target velocity, or check the flow velocity in an existing pipe.
Choose a unit system and calculation mode. Results update automatically as you change your measurements.
Required Pipe Diameter
The result is a calculated inside diameter, not a nominal pipe size.
How to Use the Calculator
- Select SI / Metric or US / Imperial units. Existing values are converted when you switch systems.
- Choose Required Pipe Diameter to calculate inside diameter from flow rate and target velocity.
- Choose Check Existing Pipe to calculate actual velocity from flow rate and the measured inside diameter.
- Results update automatically when valid inputs or units change. Use Reset Calculator to restore the defaults.
Assumptions and Limitations
This calculator uses the continuity equation for steady flow through a circular pipe: flow rate equals cross-sectional area multiplied by average velocity. It calculates the theoretical inside diameter or average velocity only. It does not calculate pressure loss, pump requirements, or compliance with plumbing codes. The calculated diameter is not automatically a commercially available nominal pipe size; actual inside diameter depends on pipe material, standard, and wall thickness. Choose a suitable design velocity for the specific application and confirm final sizing against applicable engineering guidance.
