Measure flow rate from diameters, heads, and velocities. Switch units instantly and validate assumptions easily. Built for site teams, designers, and maintenance engineers everywhere.
| Scenario | Inputs | Estimated flow (m³/s) |
|---|---|---|
| Pipe, 100 mm, 1.8 m/s | d=0.10 m, V=1.8 m/s | 0.014137 |
| Orifice, 50 mm, H=1.2 m, Cd=0.62 | d=0.05 m, H=1.2 m | 0.004886 |
| V-notch, 90°, H=0.08 m, Cd=0.62 | θ=90°, H=0.08 m | 0.001171 |
In small water systems, a 10–20% flow error can shift pump duty points, raise energy use, and shorten seal life. For pipes, the calculator uses Q = A × V, where area scales with diameter squared. That means a diameter measurement error of 5% can create roughly 10% flow variation before velocity uncertainty is added.
Many building services designs target 0.6–2.5 m/s to balance head loss and noise. In open channels, velocities often sit between 0.3–1.5 m/s depending on lining and slope. Enter your measured or assumed velocity and check the converted outputs in L/s, m³/h, gpm, and cfs for quick cross-team communication.
For sharp-edged orifices, Cd commonly falls near 0.60–0.65, but it changes with Reynolds number, thickness, and approach conditions. The calculator applies Q = Cd × A × √(2 g H) with g = 9.80665 m/s². Increasing head from 0.5 m to 2.0 m multiplies √H by 2, doubling flow for the same diameter and Cd.
Weirs are useful when flow is too low for inline meters. Rectangular weirs follow Q = Cw × b × H^(3/2), so a 25% increase in head raises flow by about 40%. V-notch weirs use H^(5/2), making them more sensitive at low heads; doubling head increases flow by about 5.7× when Cd and angle remain constant.
Operations teams may report m³/h, while pump curves and plumbing schedules often use gpm. The calculator converts from SI base flow (m³/s) using fixed factors, reducing rounding drift. Use the PDF export to capture method, inputs, and derived values for audit trails and commissioning records.
Measure diameters at multiple points, confirm whether values are internal or external, and avoid estimating velocity from turbulent surface appearance. For weirs, ensure a stable upstream pool and measure head at the specified distance from the crest. When uncertain, use conservative coefficients and verify results against a timed volume test. If your computed flow feeds a critical process, repeat the calculation with high and low assumptions to create a range. This supports risk-aware decisions on pump selection, valve sizing, and overflow capacity.
Use Pipe when you know internal diameter and average velocity. Use Orifice or Weir methods when you measure head over an opening or crest in a controlled setup.
Pipe area is proportional to diameter squared. Small diameter errors create amplified area changes, so flow from Q = A × V can shift noticeably even when velocity stays the same.
Sharp-edged orifices often use Cd around 0.60–0.65. If you have calibration data for your plate and installation, use that value for better accuracy.
They are empirical and depend on approach conditions, crest geometry, and measurement technique. For compliance or billing, use a calibrated structure and follow published installation requirements.
PDF captures the selected method, inputs, derived values, and conversions in a single page. This supports commissioning reports, maintenance logs, and peer review of assumptions.
Compare the calculated flow against a timed volume test, tank drawdown, or meter reading. If results differ, revisit coefficients, diameter basis, and head measurement location.
Important Note: All the Calculators listed in this site are for educational purpose only and we do not guarentee the accuracy of results. Please do consult with other sources as well.