Bridge mass flow and volume flow with fluid density, work 600 lb/hr of water at an approximate room-temperature density, then contrast a lighter fluid and reverse-check units.
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The calculation
US gal/min = (lb/hr) ÷ [density in lb/US gal × 60 min/hr]
Worked example
For 600 lb/hr of water at an approximate 8.34 lb per US gallon, first divide by 60 to get 10 lb/min. Then 10 ÷ 8.34 ≈ 1.20 US gal/min. At hypothetical density 6 lb/US gal, the same mass flow is about 1.67 US gal/min.
Pounds per hour cannot convert to gallons per minute without density. Use the operating temperature and correct gallon convention; water density varies with temperature and dissolved material.
Video chapters
- 0:00 — Mass flow is not volume flow
- 0:16 — Choose and state a density
- 0:29 — Change hours to minutes
- 0:40 — Change pounds to gallons
- 0:52 — Write the one-line formula
- 1:09 — Check in reverse
- 1:22 — A lighter fluid occupies more space
- 1:37 — Density completes the conversion
Read the full transcript
Six hundred pounds per hour tells us how much liquid mass passes a point. Gallons per minute tells us how much space that liquid occupies. The missing bridge is density: how many pounds fit in one U.S. gallon. Different fluids can produce different GPM at the same mass flow.
For a water example near room temperature, use an approximate density of eight point three four pounds per U.S. gallon. Density varies with temperature and dissolved material, so this is a stated assumption, not a universal constant for every fluid.
First divide six hundred pounds per hour by sixty minutes per hour. That gives ten pounds per minute. The one-hour stream becomes sixty equal one-minute slices, each carrying ten pounds of water.
Each U.S. gallon of this water weighs about eight point three four pounds. Divide ten pounds per minute by eight point three four pounds per gallon. The pounds cancel, leaving about one point two zero gallons per minute.
Combine the steps: GPM equals pounds per hour divided by density in pounds per U.S. gallon, then divided by sixty. Whether you divide by density or sixty first, you reach the same result. Keep the density units beside the number to make the cancellation visible.
Multiply one point two gallons per minute by eight point three four pounds per gallon, then by sixty minutes per hour. You recover about six hundred pounds per hour. The tiny difference comes from rounding one point one nine nine to one point two zero.
Imagine a different liquid with a density of six pounds per U.S. gallon. At the same ten pounds per minute, divide ten by six. The result is about one point six seven gallons per minute. Less mass per gallon means more gallons are needed to carry the same mass flow.
There is no fixed pounds-per-hour-to-GPM factor for all fluids. Convert hours to minutes, then divide the mass flow by the fluid's density in pounds per U.S. gallon. For six hundred pounds per hour of water under our stated density assumption, the result is about one point two U.S. gallons per minute.