Calculator
Pool chlorine dose calculator
How much chlorine do you need to raise your pool’s FC by a given amount? Enter your gallons + current FC + target FC, and the calculator returns the exact dose for the four most common chlorine sources side-by-side, with side-effect warnings for CYA and pH.
| Chlorine source | Dose |
|---|---|
Cal-hypo (73%) Adds CH ~0.7 ppm per ppm FC raised — watch for 30K+ gal scale. | 7.8 oz |
Liquid chlorine (12.5%) Pour around perimeter with pump running; degrades 20-50%/month. | 7.8 oz |
Household bleach (6%) Use only fragrance-free unscented; expensive vs liquid pool chlorine. | 16 oz |
Dichlor (56%) ALSO RAISES CYA ~0.9 ppm per ppm FC — pool only if CYA < 30. | 10 oz |
Trichlor (90%) ALSO RAISES CYA ~0.6 ppm per ppm FC; LOWERS pH. Stop using when CYA > 50. | 7.8 oz |
Lithium hypochlorite (35%) | 16 oz |
Always pour chlorine slowly with the pump running, around the perimeter. Never combine acid and chlorine in the same skimmer or bucket — chlorine gas. Re-test FC 30 minutes after dosing.
Pro tip — let AI do this for you
Don’t want to type numbers? Photo your test strip and PoolAssist AI chat extracts FC + CYA + everything else, factors in your pool’s 30-day history + current season, and writes the dosing for you. Same math, less typing.
How chlorine dosing math works
Every chlorine source has a verified ppm-per-unit ratio at a given concentration — for example, 0.13 oz of cal-hypo 73% per 1,000 gallons raises FC by 1 ppm. To find your dose:
- Compute the FC delta you need (target FC − current FC).
- Multiply the per-1000-gal-per-ppm dose by your delta and your gallons / 1000.
- Round up slightly to account for chlorine demand from sun / swimmer load over the next few hours.
All formulas in this calculator are sourced from the Trouble Free Pool PoolMath calculator — used by ~500K pool owners and considered the de-facto residential standard.
Side-effects to watch for
Not all chlorine is interchangeable. Three pile on side products you may not want:
- Cal-hypo 73% raises calcium hardness ~0.7 ppm per ppm of FC — fine in soft-water pools, watch above CH 450.
- Dichlor 56% adds ~0.9 ppm of CYA per ppm of FC. Use only when your CYA is below 30 — otherwise you build CYA uncontrollably.
- Trichlor 90% (the 3″ tabs in the floating feeder) adds ~0.6 ppm CYA per ppm FC and lowers pH. Stop using when CYA crosses 50 — chlorine lock follows.
- Liquid chlorine 12.5% adds nothing extra — just chlorine and a tiny amount of salt. Cleanest day-to-day.
The calculator above flags these inline. For routine dosing on a healthy pool, liquid chlorine is almost always the right answer.
FAQ
How much chlorine should I add to my pool?
It depends on three things: pool gallons, your current free-chlorine (FC) reading, and the target FC for your CYA level. The Trouble Free Pool rule is FC ≥ 7.5% of CYA at all times. For a 20,000-gal outdoor pool with CYA 50, target FC is around 4 ppm. Use the calculator above to convert that target into ounces or pounds for your specific chlorine source.
Cal-hypo or liquid chlorine — which should I use?
Liquid chlorine (sodium hypochlorite 12.5%) is the cleanest day-to-day chlorine because it adds nothing else (no calcium, no CYA, no acid). Cal-hypo (calcium hypochlorite 73%) is cheaper per ppm but raises calcium hardness — safe under CH 350 ppm but watch for scale risk above 450. Avoid trichlor or dichlor for routine dosing once your CYA is above 50 ppm — they pile on more stabilizer with every dose and eventually create chlorine lock.
Does this calculator account for CYA?
Not in the math itself — the dose math is purely pool gallons × ppm-delta × chemical concentration. But the calculator flags when you'd be using a stabilizer-bearing chlorine (dichlor, trichlor) so you can pick a safer alternative if your CYA is already high. For full CYA-aware advice (e.g. 'your CYA is 80 so you need to slam at FC 32 not FC 12'), use the AI chat below.
Why is liquid chlorine cheaper than the cal-hypo bag price?
Cal-hypo bags often look cheaper per pound, but liquid chlorine is sold by gallon at much higher available chlorine per dollar at most pool stores. Compare cost-per-ppm-FC, not cost-per-pound. A gallon of 12.5% liquid chlorine at $4-7 typically beats a 1-lb bag of cal-hypo at $5-7 once you factor in actual ppm output.
How long after adding chlorine should I retest?
Wait at least 30 minutes for circulation, ideally 4 hours for outdoor pools to let the chlorine equilibrate. Retest before the next sunny period — UV consumes chlorine fast in unstabilized water. If you're slamming algae, retest every 2-3 hours throughout the day and re-dose to keep FC at SLAM target (40% of CYA).
Can the AI chat do this and more?
Yes — PoolAssist's AI chat reads photos of your test strip directly, factors in your pool's 30-day history, and writes multi-day SLAM protocols when one dose isn't enough. The calculator is a quick lookup; the chat is a consult. Both use the same verified formula table.
Related tools