Exact definitions
The converters use exact definitions, not rounded rules of thumb: 1 U.S. gallon is 3.785411784 liters (the NIST Handbook 44 figure) and 231 cubic inches, and one cubic foot holds 7.480519 gallons. Converting between gallons, liters, cubic feet and cubic meters is multiplication by a fixed factor, which is why the results are precise even when your inputs are not.
ppm and mg/L
Pool chemistry is measured in parts per million, and water test results are often given in milligrams per liter. For water-based solutions with a density of about one kilogram per liter, which includes normal pool, spa and drinking water, the two are equal, so the ppm to mg/L converter treats them as the same number. That equivalence does not hold for very salty or very concentrated solutions.
Where converters help
Pool products and test kits mix units: a pool is sold in gallons, a pump may be rated in liters, and a treatment label may use cubic meters. Convert first, then use the volume and chemical calculators for doses. See how we calculate for the sources.