Flow rate equals the cross-sectional area of the water times its average velocity. In pressurised pipes the pipe is full; in drains, sewers and channels the pipe may only be partly full, and the wetted area is a circular segment. Typical design velocities are 1–2.5 m/s for water supply and 0.6–3 m/s for drains.
How to use the Liters per Minute Calculator
- Inner diameter
- Flow velocity
- Pipe
- Fill depth
- Every result field is editable: type into Flow rate and Flow velocity plus every other field update to match.
Results update as you type, and every field accepts the units shown in its menu.
How it works
Enter the pipe inner diameter and the flow velocity, and choose full or partly full with the fill percentage. Type a flow to work back to the velocity.
Tips for an accurate estimate
- Measure inside dimensions of tanks and pools – wall thickness can change the volume noticeably.
- Use the average depth for pools with a sloping floor.
- Remember the weight: water weighs about 8.34 lb per US gallon (1 kg per litre), so large volumes need solid support.
- Check whether a figure is in US gallons, imperial gallons or litres before ordering chemicals or equipment.
Common mistakes to avoid
- Using wall height instead of water depth for above-ground pools.
- Assuming a horizontal tank’s volume is proportional to the liquid level – it is not.
- Mixing up flow units such as GPM, GPH and litres per minute.
Inputs explained
| Field | What to enter |
|---|---|
| Inner diameter | Units: mm, cm, in, m |
| Flow velocity | Units: m/s, ft/s, km/h, mph |
| Pipe | Choose one: Full, Partly full |
| Fill depth | In % |
Example values
How the main result changes when Flow velocity changes and everything else stays at the example values above:
| Flow velocity (m/s) | Flow rate (L/min) |
|---|---|
| 0.75 | 88.36 |
| 1.13 | 132.54 |
| 1.5 | 176.71 |
| 1.88 | 220.89 |
| 2.25 | 265.07 |
| 3 | 353.43 |
Formula
Flow = area × velocityFull pipe: area = π D² ÷ 4Partly full (fill depth h): area = r² acos((r − h)/r) − (r − h)√(2rh − h²)Worked example
Inputs — Inner diameter: 50 mm; Flow velocity: 1.5 m/s; Pipe: Full; Fill depth: 50 %.
Results — Flow area = 19.63 cm²; Flow rate = 176.71 L/min.
Frequently asked questions
How do I convert flow velocity to litres per minute?
Multiply the pipe area (m²) by the velocity (m/s) and by 60,000.
What is a normal water velocity in pipes?
About 1–2 m/s (3–7 ft/s) in domestic supply pipes; higher speeds cause noise and water hammer.
How much water flows through a 50 mm pipe?
At 1.5 m/s, about 177 L/min (47 gpm).
What formula does the Liters per Minute Calculator use?
Flow = area × velocity.