Skip to main content

Methodology & Formula Sources

Last reviewed: April 2026

How We Derive Formulas

Every formula on Pool Calculator Hub traces to a specific, named standard. We do not use industry rules-of-thumb or unverifiable "common practice" figures. Each calculator documents its source in the per-calculator section below.

When standards conflict, we follow the more conservative value (lower maximum chemical concentration, tighter pH band). For public-pool standards applied to residential pools, we note where the standard goes beyond typical residential requirements so you can calibrate your own risk tolerance.

Primary Standards

The numeric values used by every calculator on this site trace to one of four source categories listed below. Each canonical constant in our codebase (src/lib/canonical-constants.ts) carries an inline // SOURCE: comment naming the standard or datasheet it derives from. Community-wiki references (e.g., TroubleFreePool) are not promoted to this list; they are noted in the body of guides where they inform an editorial protocol, not where they substitute for a citable source.

  • CDC Model Aquatic Health Code, 2023 edition
    cdc.gov/mahc
    Health-based minimum chlorine levels and FC (free chlorine) / CYA (cyanuric acid stabilizer) ratio requirements for public aquatic facilities. We apply the CDC minimums as a floor. Residential pools that meet these minimums are well within safe operating range.
  • ANSI/PHTA/ICC-11 (American National Standard for water quality in public pools and spas)
    phta.org/standards
    Published by the Pool and Hot Tub Alliance. Primary standard for chemistry target ranges and dosing formulas. Defines acceptable ranges for FC (free chlorine), pH, alkalinity (TA, the pH buffer), calcium hardness, and CYA. Cited at ANSI/PHTA-11 §5.3 for chlorine-species pH equilibrium.
  • DOE 10 CFR Part 431 + ENERGY STAR Pool Pumps v2.0
    energystar.gov/products/pool_pumps
    DOE 10 CFR 431 (effective July 19, 2021) sets the Weighted Energy Factor (WEF) threshold of 0.711 hydraulic horsepower (HHP) that effectively requires variable-speed motors on most residential pool pumps ≥1 HP. ENERGY STAR Pool Pumps v2.0 supplies efficiency ratings and energy-savings calculations.
  • Manufacturer performance bulletins (Pentair, Hayward)
    Pentair SuperFlo VS, IntelliFlo VSF; Hayward MaxFlo VS spec sheets
    Pump GPM-at-TDH performance curves used by the pump-size calculator come from the SuperFlo VS (EC-342001) and MaxFlo VS (W3SP2303VSP) specification sheets, taken at 3,450 RPM and intentionally biased conservative versus published curves to allow for filter loading and aging plumbing. Comparison- page spec rows derive from the same datasheets.

Additional primary research is cited in body where applied: O'Brien et al. (1974) for the CYA/FC sliding-scale relationship, NSPF Pool & Spa Operator Handbook (4th & 9th eds.) for hydraulics methodology, ANSI/APSP/ICC-5 (2025) for static and pipe-friction TDH values.

Per-Calculator Sources

Salt Calculator
lbs = (target PPM − current PPM) × volume × 8.34 ÷ 1,000,000Source: ANSI pool-water standard. Volume conversion: 7.48 gal/ft³ (not 7.5; the rounding error is material at pool scales).
Chlorine Calculator
oz = fcChange × volume × 8.34 ÷ (product_pct × 1,000,000)Source: ANSI pool-water standard. CYA/FC minimum from O'Brien et al. (1974) via TFP.
Shock Calculator
lbs = fcChange × volume × 8.34 ÷ (0.65 × 1,000,000)Source: ANSI pool-water standard. Cal-hypo 65% active calcium hypochlorite is the most common residential strength.
pH Calculator
oz = delta_pH × volume × 0.83 ÷ 1,000 × (31.45 ÷ acid_pct) (muriatic acid, pH down)oz = delta_pH × volume × 2.0 ÷ 1,000 (soda ash, pH up)Source: ANSI pool-water standard. The 31.45% baseline is technical-grade muriatic acid; weaker pool-store dilutions (commonly 14.5% or 20%) need a proportionally larger dose, which the calculator applies via the strength-factor term above. Correction note: at pH 8.0, about 25% of chlorine is active as HOCl. The commonly cited "<3%" figure is a widely circulated error.
Alkalinity Calculator
lbs = (target − current) × volume × 8.34 ÷ (1,000,000 × 0.67)Source: ANSI pool-water standard. NaHCO₃ 67% purity factor.
CYA Calculator
oz = (target − current) × volume × 8.34 ÷ (0.98 × 1,000,000)Source: ANSI pool-water standard. The 0.98 factor reflects granular cyanuric acid being sold ~98% pure (the remainder is binders and anti-caking agents). CYA ≥ 80 ppm is flagged as a health risk per CDC pool-health code.
Calcium Hardness Calculator
lbs = (target − current) × volume × 8.34 ÷ (1,000,000 × 0.77)Source: ANSI pool-water standard. CaCl₂ dihydrate 77% purity.
Pool Volume Calculator
Shape-specific geometric formulas (rectangular, round, oval, kidney, L-shaped, freeform). All use 7.48 gallons/cubic foot, never 7.5.
Pump Run Time Calculator
daily_hrs = turnovers × (volume ÷ (gpm × 60))Source: DOE ENERGY STAR Pool Pumps v2.0. Variable-speed pumps use about 75% less energy than single-speed pumps at equivalent flow.
Pump Size Calculator Reviewed 2026-04-28

Step 1: Required GPM

required_gpm = max(volume ÷ (turnover_hours × 60), 36)

The 36 GPM floor reflects PHTA-5 / ISPSC minimum filtration flow rate; small pools that mathematically need less still require a pump capable of 36 GPM.

Step 2: Total Dynamic Head (TDH) estimate

tdh = static_head + filter_head + pipe_friction + fittings
  • Static head: 5 ft in-ground, 3 ft above-ground (PHTA typical range 3–8 ft)
  • Filter head: sand 20 ft, cartridge 15 ft, DE 25 ft (manufacturer mid-cycle specs)
  • Pipe friction: 1½" PVC at 150 ft equivalent ≈ 27 ft; 2" PVC ≈ 10 ft (Hazen-Williams C=150; ANSI/APSP/ICC-11 values for new PVC)
  • Fittings allowance: 8 ft (check valve, unions, elbows)

Step 3: HP class selection

Minimum HP class whose deliverable GPM at estimated TDH ≥ required GPM. Performance curves sourced from Pentair SuperFlo VS (EC-342001) and Hayward MaxFlo VS (W3SP2303VSP) specification sheets, taken at maximum (3,450 RPM) speed and intentionally biased conservative versus published curves to allow for filter loading and aging plumbing.

DOE variable-speed mandate:

DOE 10 CFR Part 431 (effective July 19, 2021) sets minimum Weighted Energy Factor (WEF) scores for dedicated-purpose pool pumps. The compliance threshold is 0.711 hydraulic horsepower (HHP). Most single-speed pumps with a nameplate rating ≥1 HP exceed this threshold and cannot meet the higher WEF score, so they have effectively been replaced by variable-speed equivalents in the residential market. We always recommend a variable-speed pump.

Sources: PHTA Pool & Spa Operator Handbook, 4th ed. & ANSI/PHTA/ICC-5 2025; NSPF Pool & Spa Operator Handbook, 9th ed., Ch. 4; Pentair SuperFlo VS & Hayward MaxFlo VS performance bulletins; DOE 10 CFR Part 431 (VS pump mandate, July 2021); Pentair "Why Hydraulic Horsepower Matters" white paper.

Audit note (2026-04-28): Methodology audited against PHTA-5, NSPF Pool & Spa Operator Handbook, Pentair / Hayward performance bulletins, and DOE 10 CFR 431 / Pentair HHP white paper. All findings PASS or ADJUST (36 GPM floor and DOE HHP threshold language applied inline). TDH estimation uses fixed residential averages. Actual TDH varies ±20 ft depending on plumbing layout. Verify with your installer's pressure-gauge measurement before final pump selection.

What We Do Not Do

  • We do not adjust formulas to favor higher-commission products. Formula sources are independent of affiliate relationships.
  • We do not use unattributed industry rules-of-thumb without tracing them to a primary source.
  • ANSI standards are commercially available for purchase and are not freely reproducible on this site. Citations describe how each standard is applied without copying its text.