How Much Chlorine for a 20,000-gallon Pool?
A 20x40 ft in-ground pool at average depth.
Routine dose — raise free chlorine to 3 ppm
Starting from 0 ppm, here is how much of each product a 20,000-gallon pool needs to reach a day-to-day free-chlorine level of 3 ppm.
| Product | Amount for 20,000 gallons |
|---|---|
| Cal-hypo shock (Calcium hypochlorite, 65%) | 12.3 oz |
| Cal-hypo (Calcium hypochlorite, 73%) | 11 oz |
| Dichlor shock (Sodium dichloro-s-triazinetrione, 56%) | 14.3 oz |
| Trichlor tablet (90%) | 8.9 oz |
| Liquid chlorine (Sodium hypochlorite, 12.5%) | 52.6 fl oz |
| Liquid chlorine (Sodium hypochlorite, 10%) | 67.4 fl oz |
| Household bleach (Sodium hypochlorite, 6%) | 117.4 fl oz |
Shock dose — raise to 10 ppm
A routine shock (breakpoint chlorination) for a 20,000-gallon pool, from 0 ppm to 10 ppm. Clearing algae can take two to three times this much.
| Product | Amount for 20,000 gallons |
|---|---|
| Cal-hypo shock (Calcium hypochlorite, 65%) | 2.57 lb |
| Cal-hypo (Calcium hypochlorite, 73%) | 2.29 lb |
| Dichlor shock (Sodium dichloro-s-triazinetrione, 56%) | 2.98 lb |
| Trichlor tablet (90%) | 1.85 lb |
| Liquid chlorine (Sodium hypochlorite, 12.5%) | 1.37 gal |
| Liquid chlorine (Sodium hypochlorite, 10%) | 1.75 gal |
| Household bleach (Sodium hypochlorite, 6%) | 3.06 gal |
Adjust for your exact readings
The calculator below is preset to 20,000 gallons. Enter your current free-chlorine reading and target to get a dose for your specific water.
Chlorine & shock dose calculator
Enter your pool volume, your current free-chlorine reading, and your target. The result is the amount of product to add. Default target is 10 ppm (a routine shock); day-to-day free chlorine should sit in the 1–3 ppm range.
You need to add
Show the math
Raising 1 ppm in 10,000 gal takes 37.854 g of pure available chlorine (1 mg/L × 37,854 L).
Pure chlorine needed = 37.854 × 10 ppm × 20,000 / 10,000 = 757.1 g.
Cal-hypo shock (Calcium hypochlorite, 65%) is 65% available chlorine, so product needed = 757.1 g / 0.65 = 41.1 oz.
Most common bagged pool shock. Raises calcium hardness slightly.
Estimate only. Broadcast dissolved product with the pump running, re-test after ~4 hours, and never mix chlorine products together. Trichlor and dichlor also raise cyanuric acid over time.
Circulation for a 20,000-gallon pool
To turn 20,000 gallons over in 8 hours you need roughly 41.7 GPM of flow and a cartridge filter of at least 111.1 sq ft. Good circulation keeps chlorine evenly distributed — size it with the pump & filter sizing calculator.
Chlorine, shock & test supplies
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Granular cal-hypo shock for breakpoint chlorination. Pairs with the shock dosing calculator.
Shop on AmazonSlow-dissolve stabilized 3" tabs for a floater or inline feeder. Adds cyanuric acid over time.
Shop on AmazonFast strips for free chlorine, pH, total alkalinity, cyanuric acid and hardness — the inputs every calculator here needs.
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