Formula reference · Class D
Operator math formula reference
Every formula and constant the Texas Class D exams ask for, in one place. Keep it open while you practice, or print it. Each formula links to a free calculator that works it step by step. A non-programmable calculator is allowed on the exam.
Constants to know cold
- 7.48
- gallons in one cubic foot of water
- 8.34
- pounds in one gallon of water
- 0.785
- pi divided by four, for area from a diameter
- 1,440
- minutes in one day
- 60
- minutes in one hour
Conversions
Cubic feet to gallons
Open the calculatorgallons = cubic feet x 7.48
Gallons to cubic feet
Open the calculatorcubic feet = gallons / 7.48
Gallons of water to pounds
pounds = gallons x 8.34
Area and volume
Area of a circle
Open the calculatorarea (sq ft) = 0.785 x diameter x diameter
Rectangular tank volume
Open the calculatorgallons = length x width x depth x 7.48
Round tank volume
Open the calculatorgallons = 0.785 x diameter x diameter x depth x 7.48
Flow
Gallons per day from GPM
Open the calculatorGPD = GPM x 1,440
Million gallons per day
Open the calculatorMGD = GPD / 1,000,000
Gallons moved by a pump
Open the calculatorgallons = flow rate x time
Dosage and chemicals
Chlorine dose
Open the calculatordose = demand + residual (all in mg/L)
Pounds per day of chemical
lbs/day = flow (MGD) x dose (mg/L) x 8.34
Dose from pounds per day
dose (mg/L) = lbs/day / (flow (MGD) x 8.34)
Detention time
Detention time in hours
hours = tank volume (gal) / flow (gal per hour)
How long water stays in a tank. Match the volume and flow units.
Detention time in minutes
minutes = tank volume (gal) / flow (gal per minute)
Loading rates (wastewater)
Hydraulic loading
Wastewatergpd/sq ft = flow (gpd) / surface area (sq ft)
Filter loading
Wastewatergpm/sq ft = flow (gpm) / filter area (sq ft)
Common questions
- What formulas are on the Texas Class D operator exam?
- Conversions using 7.48 and 8.34, area and volume, flow, chemical dosage, and detention time. The wastewater exam adds loading rates. Every problem uses one of these with a handful of constants.
- What are the key operator-math constants?
- 7.48 gallons per cubic foot, 8.34 pounds per gallon of water, 0.785 for the area of a circle from its diameter, and 1,440 minutes in a day.
- Can I use a calculator on the exam?
- Yes, a non-programmable calculator. Knowing which formula to use, and setting it up, is what these problems test.