lb/hr to GPM Converter
A liquid given in pounds an hour becomes gallons a minute when you divide by its weight per gallon and by 60.
Quick answer: divide the lb/hr by 60, by 8.3454, and by the specific gravity. For example, 5,000 lb/hr, SG 1 is 9.986 GPM, 5,000 lb/hr, SG 0.85 is 11.748 GPM and 20,000 lb/hr, SG 1 is 39.942 GPM.
The lb/hr to GPM formula
Mass flow rate equals the volume flow times the density (Wikipedia: Mass flow rate).
Fresh water weighs about 8.3454 lb per US gallon, so GPM equals lb/hr, divided by 60 times 8.3454 times the specific gravity.
Take 5,000 lb/hr, SG 1. 5,000 ÷ (60 × 8.3454 × 1) gives 9.986 GPM.
Take 5,000 lb/hr, SG 0.85. 5,000 ÷ (60 × 8.3454 × 0.85) gives 11.748 GPM.
Weight to volume
A liquid has a weight for each gallon that depends on its density.
Specific gravity compares that density with water, so water is 1.
A US gallon is 3.785 litres (Wikipedia: Gallon), and a litre of water is about 1 kg, which is 2.2046 lb (Wikipedia: Pound (mass)).
How to convert lb/hr to GPM step by step
- Write down the mass flow in lb/hr.
- Write down the specific gravity of the liquid. Water is 1.
- Multiply 60 by 8.3454 and by the specific gravity.
- Divide the lb/hr by that number. The answer is in GPM.
Example 1: 5,000 lb/hr, SG 1
- Start with 5,000 lb/hr, SG 1.
- 5,000 ÷ (60 × 8.3454 × 1) = 9.986.
- The flow is 9.986 GPM.
5,000 lb/hr, SG 1 = 9.986 GPM
Example 2: 5,000 lb/hr, SG 0.85
- Start with 5,000 lb/hr, SG 0.85.
- 5,000 ÷ (60 × 8.3454 × 0.85) = 11.748.
- The flow is 11.748 GPM.
5,000 lb/hr, SG 0.85 = 11.748 GPM
Example 3: 20,000 lb/hr, SG 1
- Start with 20,000 lb/hr, SG 1.
- 20,000 ÷ (60 × 8.3454 × 1) = 39.942.
- The flow is 39.942 GPM.
20,000 lb/hr, SG 1 = 39.942 GPM
A pound is exactly 0.45359237 kg (Wikipedia: Pound (mass)), and a US gallon is 3.785411784 L (Wikipedia: Gallon). With water near 1 kg per litre, that gives about 8.345 lb per gallon.
lb/hr to GPM conversion table
| Mass flow and gravity | Flow (GPM) | Flow (L/min) |
|---|---|---|
| 5,000 lb/hr, SG 1 | 9.986 GPM | 37.799 L/min |
| 10,000 lb/hr, SG 1 | 19.971 GPM | 75.599 L/min |
| 1,000 lb/hr, SG 1 | 1.997 GPM | 7.56 L/min |
| 5,000 lb/hr, SG 0.85 | 11.748 GPM | 44.47 L/min |
| 2,000 lb/hr, SG 1.2 | 3.329 GPM | 12.6 L/min |
| 500 lb/hr, SG 0.8 | 1.248 GPM | 4.725 L/min |
| 20,000 lb/hr, SG 1 | 39.942 GPM | 151.2 L/min |
| 100 lb/hr, SG 1 | 0.1997 GPM | 0.756 L/min |
The highlighted row is the default of the calculator. The last column gives L/min.
A quick mental shortcut for lb/hr to GPM
For water, divide lb/hr by 500. So 5,000 lb/hr is about 10 GPM.
For other liquids, divide by 500 times the specific gravity.
What the sum finds
It finds how many gallons a minute a mass flow of liquid fills.
Boiler and pump staff use it.
What specific gravity is
Specific gravity compares a liquid with water. Water is 1, and oils are often below 1.
A salty liquid is above 1.
Where 8.3454 comes from
A gallon of water is 3.785 L, and a litre weighs about 1 kg, which is 2.2046 lb.
So a gallon weighs 3.785 × 2.2046 = 8.345 lb.
The 500 rule
For water, 60 × 8.3454 is about 500.7. So divide by 500 for a quick answer.
It is close enough for a first check.
Water temperature
Hot water is a little lighter than cold water, so the true weight per gallon is a bit lower.
The sum here uses the usual value for fresh water.
A boiler feed
A boiler makes 10,000 lb/hr of steam. Its feed water is about 19.97 GPM.
That is 75.6 L/min.
Going the other way
To find lb/hr from GPM, multiply by 60, by 8.3454 and by the specific gravity.
So 9.99 GPM of water is 5,000 lb/hr.
Kilograms
One pound is 0.45359 kg (Wikipedia: Pound (mass)). So 5,000 lb/hr is 2,268 kg/hr.
See the lb/hr to kg/hr page.
Check your answer
A heavier liquid should give a smaller GPM for the same lb/hr.
Multiply back and you should get your lb/hr.
A worked example
5,000 lb/hr of water is about 10.0 GPM. 2,000 lb/hr of a liquid with a specific gravity of 0.8 is about 5.0 GPM.
Quick conversion answers
| Conversion | Result |
|---|---|
| 100 lb/hr, SG 1 | 0.1997 GPM |
| 500 lb/hr, SG 0.8 | 1.248 GPM |
| 1,000 lb/hr, SG 1 | 1.997 GPM |
| 2,000 lb/hr, SG 1.2 | 3.329 GPM |
| 5,000 lb/hr, SG 1 | 9.986 GPM |
| 20,000 lb/hr, SG 1 | 39.942 GPM |
Where this conversion is used
Boiler staff size feed pumps.
Process engineers convert flows.
Students practise density maths.
Common mistakes to avoid
- Forgetting the specific gravity. Only water is 1.
- Mixing minutes and hours. lb/hr is per hour, GPM is per minute.
- Mixing US and imperial gallons. They differ by about 20 percent.
- Rounding too early. The answer for 500 lb/hr, SG 0.8 is 1.248 GPM. Cut it to 1.2 GPM and you are off by about 3.8 percent, and the gap grows if you reuse it.
Related converters
A fuel oil line
A line of 2,000 lb/hr of oil at SG 0.85 is 4.699 GPM.
That is 17.79 L/min.
A brine line
A line of 2,000 lb/hr of brine at SG 1.2 is 3.328 GPM.
That is 12.6 L/min.
Frequently asked questions
How do I convert lb/hr to GPM?
Divide lb/hr by 60, by 8.3454, and by the specific gravity.
How many GPM is 5,000 lb/hr of water?
About 9.99 GPM.
How many pounds in a gallon of water?
About 8.3454 lb.
What is specific gravity?
The density of a liquid compared with water.
Is there a quick way for water?
Yes. Divide lb/hr by 500.
Does it use the US gallon?
Yes.
Sources
- Wikipedia: Mass flow rate (en.wikipedia.org)
- Wikipedia: Pound (mass) (en.wikipedia.org)
- Wikipedia: Gallon (en.wikipedia.org)
