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Flow conversions (MGD, GPM, cfs)
How do you convert MGD to GPM and cfs?
Multiply million gallons per day by 694.4 to get gallons per minute, or by 1.547 to get cubic feet per second. The 694.4 is 1,000,000 gallons divided by the 1,440 minutes in a day; 448.8 gallons per minute equals one cubic foot per second. Keep the units written down and cancel them.
One million gallons per day (MGD) equals 694.4 gallons per minute (GPM) and 1.547 cubic feet per second (cfs). The conversions come from the day: 1,000,000 gallons over 1,440 minutes is 694.4 GPM, and one cubic foot is 7.48 gallons.
The formula, worked
Common slip Dividing when you should multiply, or mixing up 694.4 (MGD to GPM) with 448.8 (cfs to GPM). Write the unit beside every number and cancel them. If a plant-scale flow comes out in single digits of GPM, a factor was inverted.
Where this shows up
Flow is the first number every dose, detention time, and loading rate needs, so a unit slip here cascades into every later answer. Check a conversion in the process calculator, then drill the pounds formula, which reads flow in MGD.
Problem set: 5 worked problems
Each problem is original, authored from the underlying non-copyrightable relationship, with the full worked solution and the exact slip that produces each wrong answer. Work the problem before opening the solution. For a timed, randomized run with a per-topic score, use the free practice test.
Watch for Almost every wrong flow answer is the right method with one factor inverted. The three anchors are 1 MGD = 694.4 GPM, 1 MGD = 1.547 cfs, and 1 cfs = 448.8 GPM. Write the unit next to every number and cancel as you go.
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A plant runs at 2.5 million gallons per day (MGD). What is that flow in gallons per minute (GPM)?
- A 1,042 GPM
- B 174 GPM
- C 3,600 GPM
- D 1,736 GPM
Show the worked solution
Correct answer: D. 1,736 GPM
1 MGD = 1,000,000 gallons / 1,440 minutes = 694.4 GPM. So 2.5 x 694.4 = 1,736 GPM. Dividing by 1,440 instead of using 694.4 gives 1,042; multiplying MGD by 1,440 directly is a common slip.
Source: Public unit relationship: 1,000,000 gallons per day divided by 1,440 minutes per day = 694.4 GPM
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A pump station moves 900 gallons per minute. What is that in million gallons per day (MGD)?
- A 1.44 MGD
- B 0.63 MGD
- C 0.90 MGD
- D 1.30 MGD
Show the worked solution
Correct answer: D. 1.30 MGD
Divide GPM by 694.4 to reach MGD: 900 / 694.4 = 1.30 MGD. Equivalently 900 x 1,440 = 1,296,000 gallons per day = 1.30 MGD. Reading 0.90 forgets the conversion; 1.44 uses 625 by mistake.
Source: Public unit relationship: GPM / 694.4 = MGD
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A channel carries 3.0 cubic feet per second (cfs). What is that in gallons per minute?
- A 2,083 GPM
- B 673 GPM
- C 21 GPM
- D 1,346 GPM
Show the worked solution
Correct answer: D. 1,346 GPM
1 cfs = 448.8 GPM (one cubic foot is 7.48 gallons, times 60 seconds). So 3.0 x 448.8 = 1,346 GPM. Using 694.4 (the MGD-to-GPM factor) by mistake gives 2,083.
Source: Public unit relationship: 7.48 gallons per cubic foot x 60 seconds = 448.8 GPM per cfs
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Convert 1.5 MGD to cubic feet per second.
- A 0.97 cfs
- B 1,042 cfs
- C 3.87 cfs
- D 2.32 cfs
Show the worked solution
Correct answer: D. 2.32 cfs
1 MGD = 1.547 cfs, so 1.5 x 1.547 = 2.32 cfs. Dividing by 1.547 instead of multiplying gives 0.97; 3.87 would be 2.5 MGD, not 1.5.
Source: Public unit relationship: 1 MGD = 1.547 cfs
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A well produces 1,250 gallons per minute. Expressed in MGD, that is closest to:
- A 0.87 MGD
- B 0.69 MGD
- C 1.25 MGD
- D 1.80 MGD
Show the worked solution
Correct answer: D. 1.80 MGD
1,250 x 1,440 minutes = 1,800,000 gallons per day = 1.80 MGD. Or 1,250 / 694.4 = 1.80 MGD. Dividing by 1,440 rather than multiplying gives 0.87.
Source: Public unit relationship: GPM x 1,440 minutes = gallons per day
Drill the whole exam
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