# Liters per Minute Calculator

URL: https://speedycalculator.com/calculators/construction/lpm/
Last updated: 2026-10-25

> The litres per minute calculator finds the flow rate in a pipe from its inner diameter and the flow velocity, for full or partly full pipes, in L/min, GPM and m³/h.

## Inputs

- Inner diameter (mm, cm, in, m)
- Flow velocity (m/s, ft/s, km/h, mph)
- Pipe
- Fill depth (%)

## Results

- Flow area
- Flow rate

## Formula

```
Flow = area × velocity
Full pipe: area = π D² ÷ 4
Partly full (fill depth h): area = r² acos((r − h)/r) − (r − h)√(2rh − h²)
```

## Explanation

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

FieldWhat to enterInner diameterUnits: mm, cm, in, mFlow velocityUnits: m/s, ft/s, km/h, mphPipeChoose one: Full, Partly fullFill depthIn %

### 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)0.7588.361.13132.541.5176.711.88220.892.25265.073353.43

## 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**.

## FAQ

**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.