Water operator math tutorial
Weir Overflow Rate
Divide daily flow by weir length for gallons per day per foot.
Weir overflow rate shows how much flow spills over each foot of a clarifier's weir. High rates can pull solids over the weir into the effluent.
The flow is given in MGD but the rate is in gallons per day, so convert MGD to gallons first.
The formula
weir overflow rate (gpd/ft) = flow (gpd) ÷ weir length (ft), with flow (gpd) = MGD × 1,000,000
| Variable | Unit |
|---|---|
| Flow | MGD |
| Weir length | ft |
| Result: Weir overflow rate | gpd/ft |
Conversion factors used: 1 MGD = 1,000,000 gpd.
Applies to: Water Treatment, Wastewater Treatment.
Common exam tip: The answer is large because it is gallons per day, not millions. That is normal.
Calculator
Weir overflow rate
16,000 gpd/ft
- 1. 2.4 MGD × 1,000,000 = 2,400,000 gpd
- 2. 2,400,000 gpd ÷ 150 ft = 16,000 gpd/ft
For instructional use. Results assume the standard conversion factors shown on this page. Actual field calculations may need chemical strength, density, temperature, and other adjustments, so check your plant’s procedures.
Worked examples
Example 1
A clarifier handles 2.4 MGD over 150 ft of weir.
- 2.4 MGD × 1,000,000 = 2,400,000 gpd
- 2,400,000 gpd ÷ 150 ft = 16,000 gpd/ft
Answer: 16,000 gpd/ft
Example 2
1.5 MGD flows over 125 ft of weir.
- 1.5 MGD × 1,000,000 = 1,500,000 gpd
- 1,500,000 gpd ÷ 125 ft = 12,000 gpd/ft
Answer: 12,000 gpd/ft
Common mistakes
Wrong result: 0 gpd/ft
using MGD without converting to gpd
Wrong result: 8,000 gpd/ft
using the wrong weir length
Practice questions
1. 3.0 MGD over 200 ft of weir. Rate in gpd/ft?
- A7,500 gpd/ft
- B15,000 gpd/ft
- C150,000 gpd/ft
- D1,500 gpd/ft
Show answer and explanation
Correct answer: B. 3 MGD × 1,000,000 = 3,000,000 gpd 3,000,000 gpd ÷ 200 ft = 15,000 gpd/ft Answer: 15,000 gpd/ft. A common slip is using the wrong weir length, which gives 7,500 gpd/ft.
2. 1.8 MGD over 135 ft of weir. Rate in gpd/ft?
- A6,667 gpd/ft
- B133,330 gpd/ft
- C13,333 gpd/ft
- D1,333 gpd/ft
Show answer and explanation
Correct answer: C. 1.8 MGD × 1,000,000 = 1,800,000 gpd 1,800,000 gpd ÷ 135 ft = 13,333 gpd/ft Answer: 13,333 gpd/ft. A common slip is using the wrong weir length, which gives 6,667 gpd/ft.
3. 5.0 MGD over 400 ft of weir. Rate in gpd/ft?
- A6,250 gpd/ft
- B125,000 gpd/ft
- C1,250 gpd/ft
- D12,500 gpd/ft
Show answer and explanation
Correct answer: D. 5 MGD × 1,000,000 = 5,000,000 gpd 5,000,000 gpd ÷ 400 ft = 12,500 gpd/ft Answer: 12,500 gpd/ft. A common slip is using the wrong weir length, which gives 6,250 gpd/ft.
Where this formula applies
This calculation can appear in the disciplines below. Exact exam coverage depends on your state, certification level, and official exam outline.
Water Treatment
Wastewater Treatment
Study guides
Keep going with the full guide
Water Operator Math Made Simple walks through operator math step by step and includes printable references plus a full practice exam.
Water Operator Math Made Simple
A Step-by-Step Guide to the Math Every Water Operator Needs to Know