Every calculator on this site shows its own formula. This page collects the constants and conventions they share, so you can check any answer for yourself.
Volume
All shapes work the same way: surface area × average depth = volume. One cubic foot of water is 7.48052 US gallons. Average depth on a sloping floor is the shallow end plus the deep end, divided by two.
| Shape | Surface area |
|---|---|
| Rectangle | length × width |
| Round | π × radius² |
| Oval | π × (length ÷ 2) × (width ÷ 2) |
| Kidney | (widest + narrowest) × length × 0.45 |
| L-shape | the two rectangles added together |
The 0.45 constant for kidney and freeform pools is the figure used across the pool industry. It lands within roughly 5% for most designs — comfortably inside the tolerance of any chemical dose.
Chemical dosing
One fact underpins every dose on the site: raising 10,000 US gallons by 1 ppm takes 37.85 grams — about 1.34 ounces — of active ingredient. That is 10,000 gallons expressed as 37,854 litres, and 1 ppm as 1 milligram per litre.
From there, the amount of product depends entirely on its strength. A solid takes 1.34 ÷ its purity; a liquid is worked out by volume from its concentration. This is why the product selector changes the answer, and why it matters that you pick what is actually in your shed.
| Adjustment | Rate per 10,000 US gallons |
|---|---|
| Total alkalinity +10 ppm | 1.5 lbs sodium bicarbonate |
| Calcium hardness +10 ppm | 1.25 lbs calcium chloride (77%) |
| Cyanuric acid +10 ppm | 13 oz cyanuric acid |
| Salt +100 ppm | 8.35 lbs pool salt |
| Borate +10 ppm | 4.8 lbs boric acid |
| pH +0.1 (at TA 100) | about 3 oz soda ash |
| pH −0.1 (at TA 100) | about 5 fl oz muriatic acid, 31.45% |
pH is the one adjustment that is not a simple dose. Alkalinity buffers pH, so the same acid moves pH further in a pool with low alkalinity than in one with high. Every pH figure on this site is scaled by your alkalinity reading for that reason.
Shock level
Two things set it. Cyanuric acid binds chlorine, so the shock level is taken as 40% of the CYA reading. Separately, clearing combined chlorine takes roughly ten times the combined chlorine. Whichever is higher is the target.
Hydraulics
Flow, velocity and pipe capacity all come from one relationship: GPM = velocity (ft/s) × area (sq ft) × 448.83, using Schedule 40 internal diameters rather than the nominal size printed on the pipe. The accepted ceilings are 6 ft/s on suction lines and 8 ft/s on returns.
Filter sizing uses the published design rates: sand 15 GPM per sq ft, DE 2 GPM per sq ft, cartridge 0.375 GPM per sq ft.
Heating
Water takes 1 BTU to raise one pound by one degree Fahrenheit, and a US gallon weighs 8.34 lbs. So BTU = gallons × 8.34 × temperature rise. On top of that, an uncovered pool is losing heat the whole time it is warming, so a 35% allowance is added — 5% with a cover.
Conversions used: 100,000 BTU per therm, 91,500 BTU per gallon of propane, 3,412 BTU per kWh.
Prices
No cost calculator on this site assumes a price. Electricity, gas and water rates are all entered by you, because they vary several-fold between regions and any national average would be wrong for most visitors.
Where these figures come from
The chemistry constants are derived from the molecular arithmetic above rather than copied from a chart, so they can be checked. The industry conventions — the 0.45 shape constant, the velocity ceilings, the filter design rates, the 40% shock level — are long-standing published figures used across pool service and equipment manuals.
Every calculator also lists its own assumptions and limitations on its page. If a calculation does not hold in your situation, that is where it will say so.