Unit-conversion drills

Reviewed by the Pool Operator Prep editorial team. Worked from public water-chemistry and hydraulics facts. Last verified: 2026-07-24.

What unit conversions does pool math depend on?

Seven factors carry almost every pool-math problem: cubic feet to gallons times 7.48, gallons to pounds times 8.34, hours to minutes times 60, pounds to ounces times 16, fluid ounces to gallons divided by 128, percent to decimal divided by 100, and parts per million equals milligrams per liter. One missed factor is the most common wrong answer.

The recurring factors: cubic feet to gallons x 7.48; gallons to pounds x 8.34; hours to minutes x 60; pounds to ounces x 16; fluid ounces to gallons / 128; percent to decimal / 100; ppm equals milligrams per liter. Most wrong answers are a correct method with one of these factors missed or inverted.

The formula, worked

Formula - the conversions that decide the answer

x 7.48 cu ft→gal · x 8.34 gal→lb · x 60 hr→min · x 16 lb→oz

Worked example

Cubic feet to gallons: 16,500 × 7.48 = 123,420 galHours to minutes: 6 × 60 = 360 minPounds to ounces: 0.77 × 16 = 12.3 oz

= right factor, right direction, every time

Common slip Using base-10 habits on non-decimal factors: 16 ounces to a pound, not 10; 60 minutes to an hour; 128 fluid ounces to a gallon. Multiplying when you should divide (or the reverse) flips the answer by the whole factor. Write the unit next to every number and cancel them as you go.

Three differently shaped measuring vessels connected in a loop by coral arrows, representing converting the same quantity between different measurement units. — The same quantity, moved between units (the coral loop): the value changes, the water does not.

Where this shows up

Every other pool-math domain is arithmetic wrapped around these conversions, so drilling them cuts the number-one error class across the whole exam. Check any converted figure against the pool chemistry calculator.

Pool chemistry calculator Knowledge practice test All pool-math domains

Problem set: 6 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 knowledge practice test.

Watch for Almost every wrong pool-math answer is a correct method with one conversion factor missed or inverted. The non-decimal factors cause the most trouble: 16 ounces to a pound, 60 minutes to an hour, 128 fluid ounces to a gallon. Write the unit next to every number and cancel the units as you go.

Convert 16,500 cubic feet to gallons.
- A 2,206 gallons
- B 137,610 gallons
- C 16,500 gallons
- D 123,420 gallons
Show the worked solution

Correct answer: D. 123,420 gallons

Multiply by 7.48 gallons per cubic foot: 16,500 x 7.48 = 123,420 gallons. Dividing by 7.48 gives 2,206; using 8.34 (pounds per gallon) by mistake gives 137,610.

Source: Fixed conversion: 7.48 gallons per cubic foot
Convert 0.77 pounds to ounces.
- A About 0.05 oz
- B About 123 oz
- C About 7.7 oz
- D About 12.3 oz
Show the worked solution

Correct answer: D. About 12.3 oz

Multiply by 16 ounces per pound: 0.77 x 16 = 12.3 oz. Using 10 instead of 16 gives 7.7 oz, the single most common ounce error; dividing by 16 gives 0.05.

Source: Fixed conversion: 16 ounces per pound
Convert 6 hours to minutes.
- A 366 minutes
- B 60 minutes
- C 0.1 minutes
- D 360 minutes
Show the worked solution

Correct answer: D. 360 minutes

Multiply by 60 minutes per hour: 6 x 60 = 360 minutes. This is the conversion turnover and flow-rate problems hinge on; dividing by 60 gives 0.1, and adding 60 gives 366.

Source: Fixed conversion: 60 minutes per hour
Convert 64 fluid ounces to gallons.
- A 2 gallons
- B 64 gallons
- C 8,192 gallons
- D 0.5 gallon
Show the worked solution

Correct answer: D. 0.5 gallon

Divide by 128 fluid ounces per gallon: 64 / 128 = 0.5 gallon. Multiplying by 128 gives 8,192; the 128 is easy to forget because it is neither 100 nor 16.

Source: Fixed conversion: 128 fluid ounces per gallon
Convert 5.5 percent to a decimal.
- A 0.55
- B 550
- C 5.5
- D 0.055
Show the worked solution

Correct answer: D. 0.055

Divide by 100: 5.5 / 100 = 0.055. Moving the decimal only one place gives 0.55, which double-counts the product strength in a dosing problem.

Source: Fixed conversion: percent to decimal is divide by 100
Which operation converts cubic feet into gallons?
- A Multiply by 8.34
- B Divide by 128
- C Multiply by 16
- D Multiply by 7.48
Show the worked solution

Correct answer: D. Multiply by 7.48

Cubic feet to gallons is multiply by 7.48. The 8.34 converts gallons of water to pounds, 16 converts pounds to ounces, and 128 converts fluid ounces to gallons; keeping the four straight prevents most pool-math slips.

Source: Fixed conversion: 7.48 gallons per cubic foot

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