What is the Water Flow Calculator?
Calculate volumetric flow rate with this free water flow calculator. Use Q = A × v for circular pipes (diameter + velocity) or rectangular open channels (width × depth × velocity). Estimate flow with Manning's equation for sloped channels — enter width, depth, slope %, and roughness n. Convert between m³/s, L/s, L/min, m³/hr, GPM, and CFS. Metric and imperial units. Runs instantly in your browser.
How to use the Water Flow Calculator
- Choose Pipe Flow, Channel Flow, Manning, or Unit Converter mode.
- Select metric or imperial units (not needed for converter).
- For pipe flow, enter diameter and velocity; for channels, enter width, depth, and velocity.
- For Manning mode, add channel slope (%) and roughness n, or use a preset.
- Review Q in L/s, GPM, CFS, m³/s, and other units with calculation steps.
Common use cases
- Sizing flow through a supply pipe from diameter and velocity
- Estimating discharge in an irrigation ditch or stream channel
- Drainage design with Manning's equation on a concrete swale
- Converting pump rating from GPM to L/s or m³/hr
- Hydrology homework on continuity and open-channel flow
Frequently asked questions
- How do I calculate water flow rate in a pipe?
- Volumetric flow rate Q = cross-sectional area × velocity. For a circular pipe, A = π(D/2)². Enter internal diameter and mean flow velocity; the calculator returns Q in L/s, m³/s, GPM, and more.
- What is Manning's equation?
- Manning's formula estimates open-channel flow: Q = (1/n) × A × R^(2/3) × S^(1/2), where n is roughness, A is area, R is hydraulic radius (A/wetted perimeter), and S is slope (m/m). It is widely used in hydrology and drainage design.
- What is GPM vs L/s?
- GPM is US gallons per minute; L/s is liters per second. 1 L/s ≈ 15.85 GPM. This tool converts between GPM, L/s, CFS (ft³/s), m³/s, L/min, and m³/hr.
- What Manning n value should I use?
- Roughness depends on lining: smooth concrete ~0.013, earth channel ~0.025, natural stream ~0.035. Use the presets or consult a Manning n table for your channel material.
- Does this account for pipe friction losses?
- No — pipe and channel modes use the continuity equation Q = Av or Manning's equation. For pressure pipe head loss, use a Hazen-Williams or Darcy-Weisbach tool; this calculator gives volumetric flow from geometry and velocity or slope.
