Convert a pump flow step by step and see why US and imperial gallons give different counts for the same water.
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The calculation explained
US GPM ≈ m³/h × 4.402868; imperial GPM ≈ m³/h × 3.666154
Identify the gallon system first. A US liquid gallon is 3.785411784 liters, while an imperial gallon is 4.54609 liters. They describe different volumes. A larger gallon gives a smaller number of gallons for a fixed amount of water.
The example starts with 6 m³/h. Multiplying by 1,000 gives 6,000 L/h; dividing by 60 gives 100 L/min. Divide that by the liters per gallon: the result is approximately 26.42 US GPM or 22.00 imperial GPM. Both answers describe the same flow. The pump has not changed speed simply because its units changed.
For a reverse check, multiply an unrounded gallon-per-minute result by its matching liters-per-gallon factor, multiply by 60, and divide by 1,000. The example returns 6 m³/h. Keep intermediate precision and label the final answer with US or imperial so another reader can identify the intended gallon definition.
In this video
- 0:00 Which gallon do you mean?
- 0:14 1 m³ = 1,000 liters
- 0:30 6,000 L/h ÷ 60 = 100 L/min
- 0:42 Divide by the size of a US gallon
- 0:56 US GPM ≈ m³/h × 4.402868
- 1:10 A larger gallon means fewer gallons
- 1:26 26.42 US GPM ≈ 22.00 imperial GPM
- 1:41 Check the units in reverse
- 1:55 Choose the gallon. Convert volume and time.
Read the full video transcript
A pump moves six cubic meters of water per hour. How many gallons per minute is that? Before calculating, check which gallon you mean. A U S liquid gallon and an imperial gallon are different sizes, so they produce different numerical answers for the same flow.
Start with a unit both systems can share: the liter. One cubic meter contains one thousand liters. Picture a one-meter cube divided into smaller containers. For our pump, six cubic meters per hour means six thousand liters passing through in one hour.
Now change the time interval. One hour has sixty minutes. Divide six thousand liters per hour by sixty to get one hundred liters per minute. We have changed how we describe the flow, while the pump still moves the same amount of water.
A U S liquid gallon holds about three point seven eight five four one two liters. Divide one hundred liters per minute by that number of liters per gallon. The liters cancel, and the answer is about twenty-six point four two U S gallons per minute.
You can combine all three steps into one factor: multiply by one thousand, divide by sixty, then divide by the liters in a U S gallon. This is approximately four point four zero two eight six eight. Multiply six by that factor to get the same answer.
Now keep the water flow fixed and switch to imperial gallons. Each imperial gallon holds four point five four six zero nine liters. The larger container means fewer gallons for the same water. One hundred divided by that size is about twenty-two imperial gallons per minute.
Place the answers side by side. About twenty-six point four two U S gallons per minute and twenty-two imperial gallons per minute describe the same six cubic meters per hour. Neither answer means the pump became faster. Only the gallon definition changed.
To check a U S result, multiply gallons per minute by liters per gallon, then by sixty minutes per hour, and divide by one thousand liters per cubic meter. With unrounded values, you return to six cubic meters per hour. Keep full precision until the final rounding.
The method is: identify the gallon system, convert cubic meters to liters, change hours to minutes, and divide by liters per gallon. Always write U S or imperial beside G P M. That small label prevents a large unit mix-up.
Sources and further reading
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