How much trichlor for a 20 × 40 ft rectangular pool to raise chlorine 2 ppm
| Dose0.38 kg | 13.5oz |
|---|---|
| Pool volume40 × 20 ft rectangular, 5–10 ft deep (avg 7.5 ft) | 44,883gallons |
| Volume in ft³× 7.48052 gal per ft³ | 6,000ft³ |
| Target risefree chlorine | 2 ppm |
| What else it adds | adds about 0.6 ppm cyanuric acid per ppm, and is strongly acidic |
| — Same job, other products — | |
| Cal-hypo (65%)adds about 0.8 ppm calcium hardness per ppm of chlorine | 18oz |
| Dichlor (56%)adds about 0.9 ppm cyanuric acid per ppm of chlorine | 20.6oz |
| Liquid chlorine (12.5%)adds nothing but salt — raises pH slightly | 96fl oz |
| Household bleach (8.25%)same chemistry as liquid chlorine, weaker | 143.6fl oz |
- Pool size
- 20 × 40 ft rectangular
- Shallow end
- 5 ft
- Deep end
- 10 ft
- Raise by
- 2 ppm
- Product
- trichlor
Volume first — everything else is volume × a constant
A 20 × 40 ft rectangular pool running 5 to 10 ft deep holds 44,883 gallons. Average depth is 7.5 ft, and 6,000 cubic feet × 7.48 gives the gallons.
Get this wrong by a fifth and every chemical you add for the life of the pool is wrong by a fifth. Measure the pool. The brochure figure is for the model, not for what got built.
The dose
13.5 oz of trichlor raises free chlorine by 2 ppm.
What else it adds — and this is the part that matters
trichlor adds about 0.6 ppm cyanuric acid per ppm, and is strongly acidic.
That is not a footnote. Cal-hypo builds calcium hardness until it scales the tile line and the heater. Dichlor and trichlor build cyanuric acid, and above about 50 ppm CYA holds chlorine so tightly that the chlorine stops killing anything — and the only cure is draining and refilling. Liquid chlorine adds nothing but salt and a little pH.
Rotating products across a season, rather than using one, is how you avoid ending up with water that has to be replaced to be fixed.
How to add it
Pump running. Slowly, at the deep end or through the skimmer. Undissolved granules resting on a vinyl liner bleach it permanently and that damage does not come out.
Test after four hours of circulation. A reading taken straight after dosing tells you about the water beside the return jet, not about the pool.
Notes
- **Volume first, and everything else follows.** A 40 × 20 ft rectangular pool at 7.5 ft average depth holds 44,883 gallons. Every dose on this page and every dose you will ever add is that number × a constant — get it wrong by a fifth and every chemical for the life of the pool is wrong by a fifth. Measure the pool; do not trust the brochure.
- Average depth here is (5 + 10) ÷ 2 = 7.5 ft. That is exact for a pool with a straight slope. Pools with a bench, a spa step or a flat shallow section that drops off need the sections worked out separately and added.
- **Trichlor (90%) adds more than chlorine.** It adds about 0.6 ppm cyanuric acid per ppm, and is strongly acidic. Tablets are convenient and they quietly drive both CYA up and pH down over a season. Rotating products, rather than using one all season, is how you avoid ending up with water that needs draining to fix.
- Cyanuric acid is the trap. It protects chlorine from sunlight, which is why stabilized products exist — but above about 50 ppm it holds chlorine so tightly that the chlorine stops killing anything, and the only way down is draining and refilling. Test CYA monthly if you use these.
- Add it with the pump running, and add it slowly at the deep end or through the skimmer. Undissolved granules sitting on a vinyl liner bleach it permanently.
- Test after four hours of circulation, not immediately. A reading taken straight after dosing tells you about the water beside the return, not about the pool.
Nearby sizes
- How much trichlor for a 18 × 36 ft rectangular pool to raise chlorine 2 ppm
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- How much trichlor for a 20 × 40 ft rectangular pool to raise chlorine 2 ppm
- How much trichlor for a 20 × 40 ft rectangular pool to raise chlorine 2 ppm
- How much trichlor for a 20 × 40 ft rectangular pool to raise chlorine 2 ppm
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- How much dichlor for a 20 × 40 ft rectangular pool to raise chlorine 2 ppm
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Read more
- How Much Chlorine to Add: Dose per 10,000 Gallons, and What Each Product Also AddsThe dose is the easy half. What the product also adds to the water is the half that decides whether the pool still works in September.
- Cyanuric Acid Too High: Why the Pool Goes Green With Plenty of ChlorineA pool that reads 5 ppm chlorine and is still green almost always has one problem, and adding more chlorine is not the fix.
- Pool Water Balance: Alkalinity First, Then pH, Then HardnessAdjust pH while alkalinity is out of range and the alkalinity pulls pH straight back. That is a fortnight of chasing your own tail with a test kit.
- How to Calculate Pool Volume in Gallons — the Number Every Dose MultipliesGet the gallons wrong by a fifth and every dose for the life of the pool is wrong by a fifth, in the same direction, forever.