How our calculators work
Every tool, its method, and where the method comes from.
Last updated October 2, 2026
Principles
- Exact where the math is exact. Unit conversions use exact definitions (1 US gallon = 231 cubic inches = 3.785411784 liters). ppm is treated as milligrams per liter.
- Models where the chemistry needs one. pH depends on your water's buffering, so the pH and alkalinity tools use a carbonate-equilibrium model rather than a single lookup table.
- Estimates are labeled. Results that depend on assumptions carry an "Estimate" badge and tell you how to verify them.
- Tested. The calculation code is separate from the page design and is covered by automated tests against reference values. The tests must pass before any change is deployed.
Methods by tool
| Tool | Method | Main sources |
|---|---|---|
| Pool volume | Surface area × average depth × 7.4805 gal/ft³. Kidney shapes use the industry approximation 0.45 × (A + B) × length. | NIST SP 811 |
| Chlorine | Grams of available chlorine = Δppm × liters ÷ 1,000, divided by product strength. FC targets scale with CYA (minimum ≈ 7.5% of CYA; CDC floors of 1 ppm for pools, 3 ppm for hot tubs). | CDC; CDC Model Aquatic Health Code; Trouble Free Pool |
| Shock | Shock level ≈ 40% of CYA (minimum 10 ppm) for algae; breakpoint dose ≈ 10 × combined chlorine for chloramines. | Trouble Free Pool; operator-course breakpoint rule |
| pH and alkalinity | Carbonate system (pK₁ 6.35, pK₂ 10.33 at 25 °C) with Davies activity correction from estimated dissolved solids; cyanurate alkalinity subtracted using pKa 6.88. Acid/base doses from stoichiometry (31.45% HCl ≈ 10.0 mol/L). | Stumm & Morgan, Aquatic Chemistry; CDC |
| CYA and salt | Mass = Δppm × liters ÷ 1,000 ÷ purity. Lowering by dilution: fraction = (current − target) ÷ (current − fill water). | CDC MAHC (CYA limit); manufacturer manuals (salt ranges) |
| Pump runtime and energy cost | Turnover = gallons ÷ (GPM × 60). Cost = W × h ÷ 1,000 × rate. Variable-speed comparison uses affinity laws (power ∝ speed³). | ENERGY STAR; EIA |
| Heater sizing | BTU/h = area × temperature rise × 12, with DOE wind factors 1.25 and 2.0. Heat-up BTU = gallons × 8.33 × ΔT. | US DOE Energy Saver |
| Hot tub electricity | Tested standby watts (or California's limit 3.75 × V^(2/3) + 40 W) scaled by (water − air temperature) ÷ 42 °F, plus jet-pump use. | California Energy Commission Title 20 |
| Closing and opening planners | Ordered checklists; chemical amounts reuse the chlorine and carbonate models above. | CDC; Trouble Free Pool; manufacturer instructions |
Sources we rely on
These are the references behind PoolOwl's calculators and guides. Each tool and guide also lists the specific sources it uses at the bottom of the page.
Government agencies and regulators
- CDC: Home pool and hot tub water treatment and testing. Free chlorine and pH recommendations.
- CDC: Model Aquatic Health Code. Public-pool code, including stabilizer (CYA) limits.
- US Department of Energy: Gas pool heaters and heat pump pool heaters. Heater sizing rule and heat pump efficiency.
- ENERGY STAR: Pool pumps. Pump runtime and variable-speed savings.
- US Energy Information Administration: Short-Term Energy Outlook and average electricity prices by state. Default electricity rate.
- California Energy Commission: Title 20 appliance standards and its appliance database. Hot tub standby-power limits and tested values.
- US Consumer Product Safety Commission: Pool Safely. Drowning-prevention guidance.
- NIST Special Publication 811. Exact unit conversions.
Chemistry references
- Stumm & Morgan, Aquatic Chemistry (3rd ed.). Carbonate equilibria used in the pH and alkalinity model.
- Standard molar masses and reaction stoichiometry for dosing and side effects (for example, CYA added by dichlor and trichlor).
Homeowner and public-health guidance
- Trouble Free Pool: Pool School. Free chlorine targets relative to CYA and the shock process. We cite it as homeowner guidance, not regulation.
- Oregon Health Authority: Cyanuric acid fact sheet.
- Equipment and chemical manufacturers' manuals and labels, which take precedence for their own products (for example, salt cell operating ranges and cover installation).
Rounding
We round results to amounts you can actually measure, with two or three significant figures. Showing "37.84 fl oz" would suggest precision that home test kits and product labels can't support.
Known limitations
- Product strengths fade over time, especially liquid chlorine in heat.
- Home test kits read in steps, often 0.2 pH or 0.5 ppm, so inputs carry uncertainty.
- The pH model does not include borates or the effect of chlorine bound to CYA on alkalinity.
- Energy estimates depend on weather, covers, and how you use the equipment.
Your data
Calculators run entirely in your browser. Your inputs go in the page address so you can bookmark or share a result, and your last pool volume is remembered in your browser so other tools can fill it in. Nothing you type is sent to us. See our privacy policy. The help page answers common questions about units, sharing results and what to have ready.