How We Calculate

How the PoolCareCalcs tools work, where the rules come from and what the results can and cannot tell you.

Our approach

A calculator is only useful if you can see how it reached its answer. Each PoolCareCalcs page explains its method in plain language, names the formulas and published figures behind it and tells you which numbers are defaults you should replace. We use published formulas and public sources, we avoid hidden assumptions and we do not let advertising change a result.

Pool volume

Volume is the base of every dose. A rectangular pool is length × width × average depth × 7.5 gallons per cubic foot. Round and oval pools use the diameter squared, or length times width, times the average depth, times 5.9. Above-ground pools use the manufacturer’s typical wall height and fill depth. If your pool has a deep end, use the average of the shallow and deep ends for depth.

Chemical doses

Chemical tools use published rule-of-thumb rates and scale them by your volume and by the strength of the product. Examples used on the pages: about 10 fluid ounces of 12.5 percent liquid chlorine per 10,000 gallons raises free chlorine by 1 ppm; 1 pound of salt per 120,000 gallons changes salt by 1 ppm; routine shock is about 1 pound of calcium hypochlorite per 10,000 gallons, with 2 to 3 pounds for algae or a heavy shock treatment; about 6 ounces of soda ash raises pH by about 0.2 and about 8 fluid ounces of 31.45 percent muriatic acid lowers it by about 0.2 per 10,000 gallons; and about 1.5 pounds of baking soda raises total alkalinity by about 10 ppm per 10,000 gallons. Always test your water and follow the label on the product you actually have.

Pumps and heaters

Pump size comes from your turnover target: gallons per minute equals gallons divided by (turnover hours × 60), and a residential turnover target of 6 to 10 hours is common, with 8 hours a typical default. Heater size uses the physics of heating water: one gallon weighs 8.34 pounds and one BTU raises one pound of water by 1°F, so heating a pool by a set number of degrees in a set time gives a BTU rate. Manufacturer charts also account for surface heat loss, climate and covers.

Hot tubs and spas

Hot tub tools use the same physics and volume rules at spa scale: heat-up time from 8.34 BTU per gallon per degree and the conversion 1 watt = 3.412 BTU per hour, fill time as volume divided by flow rate (a garden hose delivers roughly 9 to 10 gallons per minute) and chemical doses scaled from pool rates. Pad load uses 8.34 pounds per gallon of water. The water change interval uses a commonly cited rule of thumb of gallons divided by users, times 3 days, capped at roughly 90 to 120 days. Municipal water prices and average adult weight are editable assumptions.

Planning

The pool cost calculator gives a planning-stage range using widely published construction cost ranges from industry guides and pool-builder associations, such as roughly $60 to $100 per square foot for gunite or concrete, $40 to $75 for vinyl liner and $45 to $85 for fiberglass, and about $1,500 to $6,000 for an above-ground pool, by size. Local costs vary widely, so a real budget needs quotes. Swimming calories use calories = MET × weight in kilograms × time in hours.

Converters

Converters use exact definitions: 1 U.S. gallon = 3.785411784 liters (NIST Handbook 44), 231 cubic inches and 7.480519 gallons per cubic foot. ppm and mg/L are treated as equal, which is accurate for water-based solutions of about 1 kilogram per liter density, including normal pool, spa and drinking water.

Finders

The seven finders ask a few questions and match you to a result, such as in-ground or above-ground, the cover, filter, heater or pump type that fits, whether salt or traditional chlorine suits you or what may be wrong with your water. The matches are rule-based and meant as a starting point, and each result points to the calculators that help you plan.

Limits

Pool chemistry changes with sunlight, bather load, temperature and the products you use, so a calculated dose is a starting point and a test kit gives the real answer. Test, dose, retest and follow the label.