Pool Calculator

Pool volume is the foundation of every chemistry dose — get it wrong by 20% and your chlorine, shock, and salt amounts are wrong by the same factor. The calculator uses four geometry formulas: rectangular pools = L × W × average_depth × 7.48; round pools = π × radius² × depth × 7.48; oval pools = π × (length/2) × (width/2) × depth × 7.48; kidney pools ≈ 0.45 × (L + W) × average_depth × 7.48. All inputs are in feet; the 7.48 factor converts cubic feet to US gallons. Results are shown in both US gallons (used by most US/CA chemical labels) and liters (UK/AU). Multiply gallons × 3.785 to convert to liters, or × 0.832 to convert to UK Imperial gallons.

For a rectangular pool with variable depth (shallow end to deep end), use average_depth = (shallow_depth + deep_depth) / 2. A 16 × 32 ft pool with a 3 ft shallow end and an 8 ft deep end has an average depth of 5.5 ft, so volume = 16 × 32 × 5.5 × 7.48 ≈ 21,082 US gallons (79,795 L). A 20 × 40 ft pool with 4 ft average depth holds 23,936 gallons. A constant-depth 12 × 24 lap pool at 4 ft holds 8,617 gallons. For pools with a hopper or shallow ledge, take measurements at three or four points along the bottom and average them — small errors here scale into the chemical dose.

Round above-ground pools dominate the budget end of the market — Intex, Bestway, Coleman 12-, 15-, 18-, 21-, 24-, and 30-foot rounds. Volume = π × (diameter/2)² × depth × 7.48. A 24-foot round with 52-inch sidewalls (4.33 ft) filled to 4 ft of water holds 13,562 US gallons (51,338 L). An 18-foot round at 4 ft water depth holds 7,646 gallons. A 30-foot round at 4 ft holds 21,243 gallons. Manufacturer specs sometimes use full sidewall height — always measure actual water depth, which is typically 6 inches below the top rail because of the skimmer cutout.

Oval pools use volume = π × (length/2) × (width/2) × depth × 7.48 — for a 16 × 32 ft oval at 4.5 ft depth: π × 8 × 16 × 4.5 × 7.48 ≈ 13,535 gallons. Kidney pools are harder; the approximation volume_gal ≈ 0.45 × (L + W) × average_depth × 7.48 treats the kidney as roughly 85% of the enclosing rectangle. For more precise estimates, break the shape into a primary oval plus two end semi-circles and sum the volumes, then add a small correction for the inward curve (typically −5% to −10%). Most pool stores accept the approximation for chemistry purposes since chlorine and shock have wide tolerance bands.

To raise free chlorine (FC) by N ppm in a pool of V gallons using a product with chlorine purity P (as a decimal): dose_ounces = N × V × 0.00013 / P. The constant 0.00013 comes from converting ppm (mg/L) to ounces per gallon. Product purities: calcium hypochlorite (cal-hypo) = 0.65; sodium dichloroisocyanurate (dichlor) = 0.56; trichloroisocyanuric acid (trichlor) = 0.90; sodium hypochlorite (liquid pool chlorine) = 0.125; household bleach = 0.06 (6%) or 0.0825 (8.25%). Example: raise FC by 2 ppm in 20,000 gallons with cal-hypo: 2 × 20,000 × 0.00013 / 0.65 ≈ 8 oz. Always pre-dissolve cal-hypo in a bucket and add slowly with the pump running.

Shock dose targets 10 ppm of free chlorine above the current level — enough to oxidize combined chlorine (chloramines) and kill algae. Using cal-hypo: 1 lb (16 oz) of 65% cal-hypo per 10,000 gallons raises FC by about 7.5 ppm. So a 20,000-gal pool needs about 2.7 lb of cal-hypo for a standard 10 ppm shock. Dichlor (56%) needs 1.2 lb per 10,000 gal for the same 10 ppm. After an algae bloom, double or triple the dose (20–30 ppm shock). Shock at dusk so UV doesn't burn off the chlorine before it sanitizes. Don't swim until FC drops below 5 ppm — usually overnight with the pump running.

Saltwater pools use a chlorine generator (SWG) that electrolyzes dissolved salt into chlorine — typical salinity target is 3,000–3,400 ppm depending on the unit (Hayward AquaRite, Pentair IntelliChlor, Jandy AquaPure, CircuPool). To raise salinity by 1,000 ppm in 10,000 gallons of water, add 83 lb of pool salt (sodium chloride, 99%+ purity). For a 20,000-gallon pool starting at 0 ppm and targeting 3,200 ppm: 3.2 × 20,000 × 8.34 / 1,000,000 × 1,000 ≈ 533 lb of salt (12 standard 40-lb bags). Salt is only lost through splash-out, backwash, and leaks — not consumed by the generator. Always test salinity before adding more.

Pool pH should stay between 7.2 and 7.8 (ideal 7.4–7.6); total alkalinity 80–120 ppm. To lower pH: 1 quart of muriatic acid (31.45% HCl) per 10,000 gallons lowers pH by approximately 0.2 units and total alkalinity by about 10 ppm. Dry acid (sodium bisulfate) at 1.5 lb per 10,000 gallons does similar. To raise pH only (without changing alkalinity much): use soda ash (sodium carbonate) at 6 oz per 10,000 gallons to raise pH by about 0.2. To raise alkalinity 10 ppm in 10,000 gallons: 1.5 lb of baking soda (sodium bicarbonate). Always retest after 6 hours of circulation and adjust pH and alkalinity in separate sessions, never simultaneously.

Cyanuric acid (CYA, stabilizer, conditioner) acts as a UV shield for free chlorine in outdoor pools. CDC MAHC recommends 30–50 ppm for manually-dosed pools and 60–80 ppm for SWG-generated pools (because lower-output generators benefit from longer chlorine residence time). Below 30 ppm, free chlorine burns off in 1–2 hours of summer sun. Above 80 ppm, the chlorine-CYA equilibrium reduces hypochlorous acid (the active killer) and bacteria killtimes balloon. To raise CYA 10 ppm in 10,000 gallons, add 13 oz of granular stabilizer (broadcast in front of return jets or place in skimmer sock — never dump granular CYA directly on liner). CYA only decreases through dilution (rain overflow, splash-out, backwash, drain-and-refill).

Target ranges for residential pools (CDC MAHC + PWTAG + Health Canada consensus): Free chlorine 1–3 ppm (2–4 ppm during heavy use); combined chlorine < 0.4 ppm (shock if higher); pH 7.4–7.6 (acceptable 7.2–7.8); total alkalinity 80–120 ppm; calcium hardness 200–400 ppm; CYA 30–50 ppm (60–80 for SWG); salinity 3,000–3,400 ppm (SWG only). Test free chlorine + pH 2–3 times per week, full panel (alkalinity, CYA, calcium, salinity) weekly during summer and biweekly off-season. DPD-based test kits and Taylor K-2006/K-2005 are the standard; test strips are convenient but less accurate above 5 ppm chlorine.

Weekly: test FC + pH and dose to target; skim surface; vacuum or robot-clean; brush walls and tile line; empty pump and skimmer baskets; check filter pressure (clean when 8–10 psi over baseline). Monthly: full chemistry panel (alkalinity, CYA, calcium, salinity if SWG); inspect O-rings and gaskets; backwash sand or DE filter; check pool light, ladder, and handrail bolts. Seasonal: spring opening (uncover, reconnect plumbing, shock, balance over 3–5 days before swimming); fall closing (drop pH to 7.2, super-shock, add winterizing algaecide, blow out lines, lower water below skimmer, cover). Save chemistry logs each season — pattern recognition saves chemicals and identifies leaks early.

Pool chemistry labels around the English-speaking world use different units, and using the wrong one over-doses or under-doses your pool. US gallons (3.785 L) are standard for US and Canadian residential pool products. UK (Imperial) gallons (4.546 L) appear on some UK pool chemistry containers — 1 UK gallon = 1.201 US gallons, so a 20,000-US-gal pool is only 16,652 UK gallons. Australia and most of the EU/UK commercial sector use liters directly; 20,000 US gal = 75,708 L. Always check whether the product label says 'gallons' (US), 'Imp gal' (UK Imperial), or 'L' (metric) before computing dose. This calculator displays both US gallons and liters so you can apply the right unit to your product.