What this calculator does
An LPH calculator converts a volume of liquid transferred over a measured time into a flow rate expressed in litres per hour, along with the equivalent rate in litres per minute and litres per second. LPH is a common way to rate pumps, drippers, fuel lines and small irrigation systems, particularly where the flow is slow enough that litres per minute would give an awkwardly small number.
The underlying calculation is simple: rate equals volume divided by time. The only real trick is keeping the time in consistent units, since hours, minutes and seconds are usually entered separately when someone times a real-world flow with a stopwatch.
The formula
The hours, minutes and seconds entered are added together as a single time in hours. Dividing the volume transferred by that total time gives the flow rate in litres per hour; dividing by 60 gives litres per minute, and dividing by 60 again gives litres per second.
| Term | Meaning |
|---|---|
| LPH | Litres per hour, a flow rate expressing how many litres pass a point in one hour. |
| Flow rate | The volume of liquid moving past a point per unit of time, however that time unit is expressed. |
| Volume transferred | The measured amount of liquid that moved during the timed period, in litres. |
The inputs explained
| Field | What to enter |
|---|---|
| Volume transferred (L) | The total volume that was measured moving during the timed period. |
| Time taken (hours) | The whole hours component of the time taken, if any. |
| Time taken (minutes) | The minutes component of the time taken. |
| Time taken (seconds) | The seconds component of the time taken. |
When to use it
Rating a slow drip or dosing pump
Timing how long a pump takes to move a small measured volume, then converting to litres per hour, gives a flow rate that is far easier to compare against a manufacturer's LPH specification than a fraction of a litre per minute.
Checking irrigation dripper output
Catching a dripper's output over a few minutes and converting to litres per hour lets you compare the measured rate against the rated flow printed on the fitting, to spot a partially blocked line.
Estimating tank fill or drain time
Once a flow rate in litres per hour is known, dividing a tank's volume by that rate gives an estimated fill or drain time, without needing to time the whole process.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How flow rate in L/h changes with time taken, at a fixed volume
The same 50 litre volume, timed over a range of durations.
| Time taken (minutes) | Flow rate | Rate in L/min |
|---|---|---|
| 5 min | 600.00 L/h | 10.000 L/min |
| 10 min | 300.00 L/h | 5.000 L/min |
| 20 min | 150.00 L/h | 2.500 L/min |
| 30 min | 100.00 L/h | 1.667 L/min |
| 60 min | 50.00 L/h | 0.833 L/min |
How flow rate in L/h changes with volume, at a fixed time
A range of volumes, all measured over the same 30 minute period.
| Volume transferred | Flow rate | Rate in L/min |
|---|---|---|
| 10 L | 20.00 L/h | 0.333 L/min |
| 25 L | 50.00 L/h | 0.833 L/min |
| 50 L | 100.00 L/h | 1.667 L/min |
| 100 L | 200.00 L/h | 3.333 L/min |
| 200 L | 400.00 L/h | 6.667 L/min |
Questions
What does LPH stand for?
LPH stands for litres per hour, a unit of volumetric flow rate showing how many litres pass a given point in one hour.
How is this different from the flow rate calculator elsewhere on the site?
The flow rate calculator works from pipe area, fluid velocity and density. This LPH calculator works from a directly measured volume and time, which suits a quick stopwatch measurement rather than a pipe specification.
Why does litres per hour matter alongside litres per minute?
Some equipment ratings and system documentation, particularly for dosing pumps and irrigation, are written in litres per hour rather than per minute, so having both figures avoids doing a second conversion by hand.
Can I use this for gallons instead of litres?
The formula is the same regardless of the volume unit; simply enter the volume in gallons and read the result as gallons per hour, minute or second instead of litres.
For flow rate calculated from pipe area and velocity instead of a timed volume, see the flow rate calculator. For converting between gallons and litres per minute directly, see the GPM to LPM calculator.