What this calculator does
A GPM to LPM calculator converts flow rate between gallons per minute and litres per minute, the two units most pump, tap, shower and irrigation specifications are written in. Pump datasheets from the United States tend to quote GPM, while most of the rest of the world quotes LPM, so anyone comparing two products, or matching a pump to a manufacturer's specified flow requirement, ends up converting between the two.
The one detail that trips people up is which gallon is meant. A US gallon is 3.78541 litres, but the imperial (UK) gallon is a different, larger unit at 4.54609 litres, so the same GPM figure means a noticeably different flow rate depending on which one a manufacturer used. This tool lets you pick the gallon definition explicitly rather than guessing, and converts in either direction.
The formula
Multiply the flow rate in gallons per minute by the litres-per-gallon factor for the gallon type you select, US or imperial, to get litres per minute. To go the other way, divide the litres-per-minute figure by that same factor. Both factors are exact, fixed unit definitions, not estimates, so the conversion carries no rounding error beyond the display precision.
| Term | Meaning |
|---|---|
| GPM | Gallons per minute, a flow-rate unit common on US-market pump, tap and irrigation specifications. |
| LPM | Litres per minute, the flow-rate unit used on most metric specifications outside the US. |
| US gallon | Exactly 3.78541 litres, the gallon used throughout the United States. |
| Imperial gallon | Exactly 4.54609 litres, the gallon used in the UK and historically across the Commonwealth, about 20% larger than the US gallon. |
The inputs explained
| Field | What to enter |
|---|---|
| Value | The flow rate you want to convert, in whichever unit matches the direction you have chosen below. |
| Gallon type | Which gallon definition applies: US gallon for American specifications, imperial for UK/Commonwealth ones. |
| Convert | Whether you are converting gallons per minute to litres per minute, or the reverse. |
When to use it
Matching a pump to a metric spec sheet
A pump rated in GPM needs to be checked against a system requirement written in LPM, or vice versa, before it is ordered or installed.
Comparing shower or tap flow rates
Water-efficiency ratings and plumbing fixtures are labelled in different units depending on the country of manufacture, and putting both figures in the same unit makes the comparison meaningful.
Sizing irrigation or dosing equipment
Drip irrigation controllers and chemical dosing pumps are often specified in one unit while the rest of a system's documentation uses the other, so a quick conversion avoids under- or over-sizing a component.
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.
What is a given GPM figure in litres per minute?
Common pump flow rates converted from US gallons per minute to litres per minute.
| Flow rate (US GPM) | Flow rate in litres per minute | Per hour |
|---|---|---|
| 1 | 3.79 L/min | 227 L/h |
| 5 | 18.93 L/min | 1,136 L/h |
| 10 | 37.85 L/min | 2,271 L/h |
| 15 | 56.78 L/min | 3,407 L/h |
| 20 | 75.71 L/min | 4,542 L/h |
| 25 | 94.64 L/min | 5,678 L/h |
How much does the gallon definition change the result?
The same 20 GPM figure converted under each gallon definition, showing why it matters which one a spec sheet used.
Questions
Is a US gallon the same as a UK gallon?
No. A US gallon is 3.78541 litres and a UK (imperial) gallon is 4.54609 litres, about 20% more. A flow rate given in GPM without specifying which gallon it means is ambiguous, which is why this calculator lets you choose.
How do I convert GPM to LPM?
Multiply the gallons-per-minute figure by 3.78541 if it is a US gallon, or by 4.54609 if it is an imperial gallon, to get litres per minute.
Which gallon do US pump manufacturers use?
Almost always the US gallon. Manufacturers based in the UK, Ireland, or older Commonwealth documentation are more likely to mean the imperial gallon, so check the datasheet's country of origin if it is not stated explicitly.
Why does litres per hour matter alongside litres per minute?
Some system documentation, particularly for irrigation and water treatment, is written in litres or gallons per hour rather than per minute, so having both figures avoids a second conversion step.
For a one-off volume conversion rather than a flow rate, see the gallons to litres calculator. For flow rate worked out from pipe area, velocity and fluid density instead of a unit conversion, see the flow rate calculator.