Pool Chlorine Calculator

Pool chlorine calculator: the dosing formula

A pool chlorine calculator works out how much chlorine product raises your pool by a set number of parts per million. The pounds of pure chlorine are gallons × ppm rise × 0.00000834, and you divide by the product’s strength to get the product weight. A 20,000 gallon pool raised 3 ppm with a 10% product needs 5.00 lb.

This is a chemical estimate. Your test kit reading and the product label control the actual dose.

The formula in plain words

One part per million in a million gallons of water is 8.34 pounds, since a gallon of water weighs 8.34 pounds. That is where 0.00000834 comes from. Multiply it by your gallons and the ppm you want to add.

That gives pure chlorine. Real products are weaker. A 10% product is 0.10, so you divide by 0.10, which multiplies the weight by 10. Multiply pounds by 16 for ounces.

Worked example, routine maintenance

A 20,000 gallon pool reads low, and you want to add 3 ppm using a 10% product.

• Pure chlorine: 20,000 × 3 × 0.00000834 = 0.5004 lb
• Product: 0.5004 ÷ 0.10 = 5.00 lb
• In ounces: 5.00 × 16 = 80.1 oz

Worked example, a shock treatment

After a pool party and a storm, you decide to shock the same pool with a 10 ppm rise, using the same 10% product.

• Pure chlorine: 20,000 × 10 × 0.00000834 = 1.668 lb
• Product: 1.668 ÷ 0.10 = 16.68 lb
• In ounces: 266.9 oz

Shock is more than three times the maintenance dose because the ppm rise is 10 instead of 3.

Product weight by pool size and product

All doses below raise chlorine by 3 ppm.

Pool size	10% product	12.5% product	65% product
10,000 gal	2.50 lb	2.00 lb	0.38 lb
15,000 gal	3.75 lb	3.00 lb	0.58 lb
20,000 gal	5.00 lb	4.00 lb	0.77 lb
30,000 gal	7.51 lb	6.00 lb	1.15 lb

Check the strength on your label, since the figures here are only examples.

How to find your numbers

• Pool volume. Use the pool gallons calculator for a rough number, or the builder’s figure if you have it.
• Current reading. Test the water with a kit before you add anything. The ppm rise is the target level minus the current reading.
• Product strength. Read the percent available chlorine on the label.
• The result is a weight, so weigh liquid by checking the label’s weight per gallon, or use the label’s own dosing chart.

Common mistakes

• Dosing without testing first. You may not need any chlorine, or you may need more than you think.
• Using the wrong strength. A 65% product at the 10% rate would put in far too much.
• Adding the chemical in strong sun, which wastes some of it.
• Mixing products, or adding chlorine and acid close together. Add one chemical at a time, with the pump running, and never mix them in a bucket.
• Treating the result as exact. Water chemistry changes with sunlight, swimmers and temperature.

Pool chemicals can be hazardous. Follow the product label, wear the protective gear it lists, and ask a pool professional if you are unsure.

Adding it safely

Test first, calculate second, dose third. Run the pump while you add the product so it mixes through the whole pool. Add it slowly in the deep end, away from the skimmer and any fittings. Do not pour it into a bucket with other chemicals.

Retest a few hours later and again the next day. If the reading is still short, you can add more, but small steps are safer than one big dose. Chlorine falls faster in sunny weather and with many swimmers. A stabilizer in the water slows the loss from sunlight, and your test kit and the product label give the range that suits your pool. For the 20,000 gallon example, a follow-up rise of 2 ppm needs 20,000 × 2 × 0.00000834 ÷ 0.10 = 3.34 lb of 10% product, so a second small dose is easy to work out.

Related pages

Balance the water with the muriatic acid pool calculator. For a saltwater system, use the pool salt calculator, and for your volume figure use the pool gallons calculator.