Pool Chlorine Calculator
Exact dose for liquid, trichlor, dichlor or cal-hypo — with the free-chlorine target your CYA actually demands, and the side-effect ledger for every product.
Why most chlorine calculators get the target wrong
Two pools, both at 3 ppm chlorine. One is crystal clear and safe. The other is growing algae right now.
The difference is a number most chlorine calculators never ask for: your stabilizer level. Chlorine targets aren't fixed — they scale with CYA, and a 3 ppm reading that's fine at CYA 30 is dangerously low at CYA 80.
Here's the deal: tell this calculator your volume, your current and target free chlorine, and which product you're holding — liquid chlorine, trichlor tabs, dichlor, or cal-hypo — and you'll get the exact dose in gallons, ounces, or grams. You'll also get the honest extras: what your target should be for your CYA, and what else each product quietly adds to your water. That last part is where most pool problems are born.
What you'll give us
Four numbers and a product. The diagram below shows where each input fits — and the safety strip on the bottom is the rule that overrides everything else.
The calculator
Pick your product, type your numbers, hit calculate. Defaults match worked example 1. The CYA field is optional but pre-fills your real FC target the moment you enter it.
Calculate your chlorine dose
Four numbers in, exact dose out — plus what else just went in the water and what your CYA-adjusted target actually is.
Don't know your gallons? Pool volume calculator — two minutes, any shape, deep-links straight back here.
What your FC target should actually be
Cyanuric acid grabs most of your chlorine and holds it in reserve — that's why a stabilized pool survives a sunny afternoon. But held-in-reserve chlorine isn't killing anything.
The active fraction (hypochlorous acid) depends on the ratioof FC to CYA. That's why fixed targets like "1–3 ppm" fail: at CYA 30, 3 ppm is comfortable; at CYA 80, the same 3 ppm leaves your active sanitizer starving.
Practical rule, derived from the published cyanurate equilibria: keep FC at about 11.5% of your CYA, and never let it fall below 7.5%. The calculator above pre-fills the target the moment you type a CYA — leave it blank and we default to the EPA label range (1–4 ppm), which is fine for indoor or unstabilized pools and a coin-toss for anyone else.
If your CYA is already too high to support a sane FC target, the only honest fix is dilution. The cyanuric acid calculator does the swap math — exact gallons to drain, two-rounds-with-retest plan, plus the season-long "trichlor staircase" that put you there in the first place.
The liquid-chlorine identity rule
Trade percent means grams of available chlorine per 100 millilitres. Run the units and a lovely coincidence falls out: one gallon of 12.5% liquid chlorine in a 10,000-gallon pool raises FC by exactly 12.5 ppm. One gallon of 10% raises it 10 ppm.
The number on the jug IS the ppm per gallon per 10,000 gallons — no chart, no app, no arithmetic. Scale linearly with pool size: 1 gal in 5,000 gal pool ⇒ 25 ppm; 1 gal in 20,000 gal ⇒ 6.25 ppm.
Proof: 12.5 g / 100 mL = 125 g/L; one US gallon = 3.785 L = 473 g of available chlorine; 473,000 mg ÷ 37,854 L = 12.5 mg/L = 12.5 ppm. The same arithmetic for the dry products lives in the side-effect ledger below.
Every bucket has a second ingredient
Pure chlorine is the goal; pure chlorine is not what's in your jug. Each product carries something else along — and the "something else" is permanent (or close to it) for everything except liquid's pH cycle.
Per 10 ppm of free chlorine delivered: trichlor deposits 6.1 ppm cyanuric acid and is acidic; dichlor deposits 9.1 ppm cyanuric acid and is mildly acidic; cal-hypo deposits 7.1 ppm calcium hardness (as CaCO₃) and is mildly basic; liquid chlorine deposits 16.5 ppm residual salt and is pH-neutral over the full sanitize-and-revert cycle.
Nothing is free.The four numbers above are mole-ratio arithmetic from the molecular weights, and they live as a single set of constants in our dosing engine — change any one and every chemistry page on the site updates. The liquid-salt residual is the same constant the pool salt calculator uses for "why your current salt is never zero" — one number, one source, full chain consistency.
lib/dosing/chemicals.ts — change the constant once, every page on the chemistry chain updates.Worked examples — eight common scenarios
Run any of these through the calculator above and the math will land in the same place. Every side-effect figure derives from the same dosing-engine constants, so the ledger here, the chart, and the calculator panel never disagree.
Example 1
How much chlorine to add to a pool: 15,000 gallons, FC 1 → 3
15,000 gal, Δ2 ppm, 12.5% liquid.
0.24 gal = 30.7 fl oz
Identity rule: gallons = (Δ × pool/10,000) ÷ trade % = (2 × 1.5) ÷ 12.5 = 0.24 gal ≈ a quart. Side effect: 3.3 ppm residual salt.
The identity rule at work — most top-ups are smaller than people think.
Example 2
Free chlorine calculator with CYA: what should my level actually be?
CYA 50 → target = 11.5% × 50 ≈ 5.8 ppm (round to 6), minimum 3.75 ppm. Currently 3 ppm in 20,000 gal; raise to 6 with 65% cal-hypo.
0.77 lb = 12.3 oz
(3 × 20,000 × 8.345×10⁻⁶) ÷ 0.65 = 0.77 lb. Side effect: +2.1 ppm CH (3 × 0.71 = 2.13 ppm CH, rounds to 2.1).
"3 ppm" wasn't fine — at CYA 50 it's sitting below minimum-adjacent territory. The fixed-target charts would never have told you.
Example 3
Chlorine ppm calculator: 10,000 gallons with 10% liquid
Δ4 ppm, 10% trade liquid.
0.4 gal = 51.2 fl oz (1.51 L)
Identity rule: 4 ÷ 10 = 0.4 gal. Side effect: +6.6 ppm residual salt.
With 10% liquid and a 10k pool the math is mental arithmetic — ppm needed ÷ 10 = gallons.
Example 4
How much FC does one trichlor tablet add? (12,000 gallon example)
One 8 oz (0.5 lb) tab at 90% available chlorine, in 12,000 gal.
+4.5 ppm FC
FC = (0.5 × 0.90) ÷ (12,000 × 8.345×10⁻⁶) = 4.5 ppm. CYA side effect: +2.7 ppm CYA.
Tabs are convenient and they keep a tax ledger — every tab is also a CYA dose. Season-long math lives on the tablets page (shipping in a later phase).
Example 5
Dichlor dosage calculator: raising 10,000 gallons by 4 ppm
Δ4 ppm, dichlor 55.5%.
0.60 lb = 9.6 oz
(4 × 10,000 × 8.345×10⁻⁶) ÷ 0.555 = 0.60 lb. CYA side effect: +3.6 ppm CYA.
Dichlor stabilizes almost twice as fast as trichlor per ppm of chlorine — fine for a fresh fill, ruinous as a daily habit.
Example 6
Chlorine calculator in litres: 40 m³ metric example
40 m³ (40,000 L), Δ3 ppm, 12.5% liquid (125 g available Cl₂ per litre).
0.96 L of liquid
Need 3 mg/L × 40,000 L = 120 g of available Cl. 120 g ÷ 125 g/L = 0.96 L of 12.5% liquid.
Metric again collapses to one line — grams needed = ppm × m³.
Example 7
How much chlorine per day does a pool need?
15,000 gal, measured loss 1.5 ppm/day (user-measured: dusk-to-dusk test), 12.5% liquid.
0.18 gal/day = 23 fl oz/day ≈ 1.26 gal/week
Daily = (1.5 × 1.5) ÷ 12.5 = 0.18 gal. Weekly = 7 × 0.18 ≈ 1.26 gal.
Demand is YOUR pool's number, not a chart's — measure it once at midsummer and dosing becomes a 30-second ritual. Don't trust a one-size-fits-all loss rate.
Example 8
Why is my chlorine not holding? (the high-CYA trap)
CYA 120 ppm → target 11.5% ≈ 13.8 ppm (which the quick FC/CYA table below rounds to 14.0). Pool 25,000 gal, currently FC 5 → Δ9 ppm with 12.5% liquid.
1.8 gallons of liquid — every time
(9 × 2.5) ÷ 12.5 = 1.8 gal. That's the dose to reach 13.8; you'll be back at it tomorrow.
Chlorine IS holding — it's bound up by overgrown CYA. At 120 ppm CYA you're paying triple for sanitation. Lower your CYA with a partial drain before chasing FC — the cyanuric acid calculator gives you the exact swap; a 50% swap usually beats a summer of heroic dosing.
Free, combined, and total chlorine — only one of them sanitizes
Your test kit gives you up to three numbers. Free chlorine (FC) is the active sanitizer. Combined chlorine (CC) is what's already reacted with sweat, urine, and skin proteins — chloramines. Total chlorine (TC) is just the sum.
The strong chlorine smell at a public pool isn't too much chlorine — it's the chloramines, the CC, signaling that the FC has been running too low to break them down.
Industry convention puts the "time to shock" threshold at roughly 0.2–0.5 ppm combined chlorine. When combined chlorine climbs past that range it's time to shock — the breakpoint math lives on the dedicated shock page (shipping in a later phase). The CDC Model Aquatic Health Code carries an authoritative number for public pools; the calculator stays in residential mode until that PDF is verified against the facts table.
Reference tables
Three tables you can cite directly. All released under CC BY 4.0 — reuse them with a link back.
T1 · Product master
MW, available chlorine percent (label and stoichiometric), the dose to raise 10,000 gallons by 1 ppm FC, per-product side effect, qualitative pH effect, and the verified-fact source.
| Product | Formula | MW | Available Cl (label) | Stoich. | Per 10k gal / +1 ppm | Per 10 ppm FC, also adds | pH | Sources |
|---|---|---|---|---|---|---|---|---|
| Liquid chlorine | NaOCl | 74.44 | 12.5% trade | — | 10.24 fl oz | +16.5 ppm salt | neutral | F1, F3 |
| Trichlor⚠ | C₃Cl₃N₃O₃ | 232.41 | 90% | 91.5% | 1.48 oz | +6.1 ppm CYA | acidic | F5, F7 |
| Dichlor⚠ | C₃Cl₂N₃NaO₃·2H₂O | 255.98 | 56% | — | 2.41 oz | +9.1 ppm CYA | mildly acidic | F6, F8 |
| Cal-hypo⚠ | Ca(OCl)₂ | 142.98 | 65% | 99.2% | 2.05 oz | +7.1 ppm CH (as CaCO₃) | mildly basic | F4, F9 |
CYA MW 129.07 g/mol underlies F7 and F8.
T2 · FC targets by CYA
Derived from the cyanurate–chlorine equilibria (F11). Both columns rounded to 0.5 ppm. Minimum = 7.5% of CYA; target = 11.5% of CYA.
| CYA (ppm) | Minimum FC | Target FC |
|---|---|---|
| 0 | 1–4 ppm | 1–4 ppm |
| 30 | 2.5 | 3.5 |
| 40 | 3.0 | 4.5 |
| 50 | 4.0 | 6.0 |
| 60 | 4.5 | 7.0 |
| 70 | 5.5 | 8.0 |
| 80 | 6.0 | 9.0 |
| 100 | 7.5 | 11.5 |
| 120 | 9.0 | 14.0 |
At CYA 0 the FC/CYA derived rule reduces to zero; on a real pool with no stabilizer the EPA-registered label range of 1–4 ppm applies instead (F12). For indoor pools, follow the operator-practice convention on the cyanuric-acid page.
At CYA 120, 11.5% gives 13.8 ppm; the table rounds to 14.0 (worked example 8 keeps the unrounded 13.8 explicit, so both numbers stay honest).
T3 · Liquid identity quick table
Fluid ounces of liquid chlorine to raise a pool by +1 ppm FC, across the four common trade strengths.
| Pool size | 6% trade | 8.25% trade | 10% trade | 12.5% trade |
|---|---|---|---|---|
| 10,000 gal | 21.3 fl oz | 15.5 fl oz | 12.8 fl oz | 10.2 fl oz |
| 15,000 gal | 32.0 fl oz | 23.3 fl oz | 19.2 fl oz | 15.4 fl oz |
| 20,000 gal | 42.7 fl oz | 31.0 fl oz | 25.6 fl oz | 20.5 fl oz |
| 25,000 gal | 53.3 fl oz | 38.8 fl oz | 32.0 fl oz | 25.6 fl oz |
Read across to your trade %. Cross-check against the identity rule: for any pool at any trade %, the fl oz scale linearly with both pool size and target ppm.
Methodology & sources
Every dose on this page is mole-ratio arithmetic from molecular weights in the CRC Handbook and PubChem. The available-chlorine percentages are the label conventions (90% trichlor, 55.5% dichlor, 65% cal-hypo), with the stoichiometric values shown alongside in T1 so you can see how close the labels run.
The free-chlorine-to-CYA targeting rule is the practical form of the cyanurate–chlorine equilibria published by O'Brien, Morris and Butler (1974) and analyzed in Wojtowicz's Journal of the Swimming Pool and Spa Industry papers. We present the 7.5% / 11.5% figures as derived practice, not regulation.
Label dosing ranges come from EPA-registered product labels (F12). Public-pool minimum free chlorine values come from the CDC Model Aquatic Health Code; the calculator does not display an MAHC number until the current PDF is verified against our facts table (F13). The never-mix safety warning ships unconditionally regardless of citation status (F14) — the hazard is well-documented in U.S. chemical-safety advisories; the specific CSB/CPSC citation lands when verified.
The 1.65 ppm residual salt per ppm of FC dosed (liquid chlorine) is the same constant the pool salt calculator uses — it lives once in lib/dosing/core.ts and is imported by every page that cites it.
Sources used on this page
- Standard Methods for the Examination of Water and Wastewater (APHA)— the DPD analytical basis for measuring free chlorine "as Cl₂", where 1 ppm = 1 mg/L of Cl₂-equivalent (F1).
- CRC Handbook of Chemistry and Physics— molecular weights for Cl₂ (70.906), trichlor (232.41), dichlor (255.98), cal-hypo (142.98), cyanuric acid (129.07), CaCO₃ (100.09); water density 8.345 lb/gal at 60 °F (F4–F9).
- PubChem— cross-reference for chemical identities and formulas of the four sanitizing products listed in T1.
- O'Brien, Morris & Butler (1974), cyanurate–chlorine equilibria — the primary literature underneath the FC/CYA targeting rule (F11). Wojtowicz's analyses in the Journal of the Swimming Pool and Spa Industry develop the practical % rule from those equilibria.
- EPA-registered product labels— the 1–4 ppm residential FC convention (F12) and the per-product available-chlorine label percentages used as calculator defaults.
- CDC Model Aquatic Health Code— public-pool minimum FC. The calculator ships in residential mode only; the exact MAHC value lands when the current PDF is verified (F13).
- U.S. chemical-safety advisories (CSB / CPSC)— the never-mix warning anchor. The hazard (violent reaction of trichlor + cal-hypo / in feeders / wet residues) is documented in agency advisories; the specific PDF citation is pending verification, but the safety warning itself is unconditional (F14).
Reference tables T1, T2 and T3 above are released under CC BY 4.0. Reuse them anywhere — just attribute PoolSolver and link back.
Frequently asked questions
How much liquid chlorine for a 10,000 gallon pool?
The identity rule: 1 gallon of X% trade liquid in 10,000 gallons raises FC by X ppm. So 1 gallon of 12.5% raises it 12.5 ppm; a 1 ppm bump needs 10.2 fl oz of 12.5%. Scale linearly with pool size and target ppm — see the T3 quick table above.
What should my free chlorine level be?
The EPA-registered product labels say 1–4 ppm (F12). That's a convention. The real answer is 11.5% of your CYA, minimum 7.5% (F11) — at CYA 30, 3 ppm is comfortable; at CYA 80, 3 ppm is dangerously low. Test your CYA and read the T2 table above.
What's the difference between free, combined, and total chlorine?
Free chlorine (FC) is the active sanitizer. Combined chlorine (CC) is what's already reacted with sweat, urine, and skin proteins — chloramines. Total chlorine = FC + CC. When CC climbs past convention range (about 0.2–0.5 ppm), it's time to shock; the breakpoint math has its own dedicated page (shipping in a later phase).
Why does my pool smell like chlorine?
That sharp pool-deck smell is chloramines (CC), not free chlorine. It signals too LITTLE active sanitizer, not too much — the FC has been running below the threshold needed to break down combined chlorine. Add chlorine, retest, and consider whether a shock is overdue.
Does sunlight really burn off chlorine?
Yes — unstabilized free chlorine photolysis is fast in direct summer sun (F15). Cyanuric acid binds chlorine and shields it from UV, but the same binding suppresses the active fraction (F11). Stabilization is a trade-off: more CYA preserves more chlorine, and demands more chlorine for the same sanitation.
Can I mix different chlorine types?
No — never.Trichlor contacting cal-hypo, in a feeder, in storage, or as wet residues, can react violently with fire and toxic-gas risk. Don't use one product in another's feeder, don't store them adjacent, don't combine. The hazard is well-documented in U.S. chemical-safety advisories (CSB / CPSC); the specific PDF citation is pending verification, but the warning ships unconditionally (F14). Empty a feeder completely before switching product types.
When can I swim after adding chlorine?
Follow the product label — we don't invent hour figures. General practice: circulate, retest, and swim when FC is back within your target range and CC is below the convention threshold.
Liquid vs tablets — which is better?
Trade-offs via the side-effect ledger above. Liquid is pH-neutral over the sanitize-and-revert cycle and deposits residual salt (matches the salt calculator's invariant). Trichlor tabs are convenient but each ppm of FC deposits 0.61 ppm of permanent CYA. Cal-hypo raises calcium hardness. Pick by your stabilizer history and your daily routine — deep-dive pages for each product ship in later phases.
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