How the pool chemical calculator works
Chlorine products carry different strengths. Liquid bleach at 12.5 percent, cal hypo at 65 percent and dichlor at 56 percent all need different amounts to raise the same pool by the same ppm.
The tool works from available chlorine, so a weak jug of old liquid will always underperform the number shown. Fresh product gives the closest match.
The formula, step by step
Ounces = Gallons x ppm Increase x 0.0000834 / Product Strength x 16
- Find the ppm gap between the current reading and the target.
- Multiply gallons by 8.34 and by the gap, then divide by one million. That is pounds of pure chlorine.
- Divide by the product strength as a decimal, for example 0.65 for cal hypo.
- Multiply by 16 to see the answer in ounces.
Worked example
A 15,000 gallon pool reads 1 ppm free chlorine and you want 3 ppm.
Gap = 2 ppm. Pure chlorine needed = 15,000 x 8.34 x 2 / 1,000,000 = 0.25 pounds.
Using 65 percent cal hypo: 0.25 / 0.65 = 0.385 pounds, which is about 6.2 ounces.
Watch it done
This short video walks through the same job in real life, so you can match what you see on screen to what you do at the pool.
Tips before you start
Add liquid chlorine in the evening. Sunlight burns off unstabilized chlorine fast, so a morning dose can be half gone by dinner without enough cyanuric acid in the water.
Never mix products in a bucket. Cal hypo and trichlor react with each other, and the reaction can catch fire. Add one product, let the pump run a full cycle, then test again before adding anything else.
Pool Chemical Calculator FAQs
How much chlorine do I add to my pool?
It depends on volume, the current free chlorine reading and the product strength. Most pools need 2 to 4 ppm of free chlorine held steady.
What is the difference between liquid chlorine and shock?
Shock is a large dose, not a product type. You can shock with liquid chlorine, cal hypo or dichlor. The goal is to push free chlorine to about 10 times your combined chlorine reading.
Why does my chlorine disappear overnight?
Low cyanuric acid, heavy swimmer load or algae feeding on the chlorine. Test stabilizer first, since a level near zero lets sunlight strip chlorine in hours.
How long until I can swim?
Wait until free chlorine falls back under 4 ppm and the pump has run at least one full turnover. That is usually four to eight hours after a shock dose.