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Pool Chlorine Calculator

Find out how much liquid chlorine, cal-hypo, dichlor or trichlor to add to bring your pool's free chlorine up to the level you want.

Level: Grade 7

  • Common Core standard 7.RP.A.3Use proportional relationships to solve multistep ratio and percent problems. Examples: simple interest, tax, markups and markdowns, gratuities and commissions, fees, percent increase and decrease, percent error.
Printable worksheet

Pool Chlorine Calculator

Know the answer you need? Pick an input to work backward.

Inputs

Not sure? The pool volume calculator works it out from the pool's size.

From a test kit or strip.

Pool liquid chlorine is usually 10% or 12.5%. Household bleach is weaker, often 6% to 8.25%.

Result

Liquid chlorine to add

41 fl oz

Add about 41 fl oz of liquid chlorine (sodium hypochlorite) to raise free chlorine by 2 ppm. Follow the product label, add it to the water, and never mix chlorine products with each other or with acid.

In US gallons
0.32 US gal
Free chlorine increase
2 ppm
Available chlorine added
151.4 g

See the step-by-step solution ↓

Step-by-step solution

  1. Turn gallons into liters, because chlorine levels are measured per liter (1 US gal = 3.785411784 L)

    20,000 gal×3.785411784=75,708.24 L

  2. Subtract your current reading from the target to see how much to raise it

    3−1=2 ppm

  3. Find the grams of chlorine needed: 1 ppm is 1 mg in each liter, and 1,000 mg make a gram

    2×75,708.241,000=151.416 g

  4. Divide by the label strength to get how much liquid to pour (12.5% means 12.5 g of chlorine in each 100 mL)

    151.4160.125=1,211.33 mL

  5. Turn milliliters into US fluid ounces so you can measure it (1 fl oz = 29.5735 mL)

    1,211.3329.5735=40.96 fl oz

Test yourself

20 quick questions built from this calculator's formula: work out results, predict what happens when an input changes, and work backward from an answer. Every answer comes with the working.

What this calculator does

Find out how much chlorine to add to bring your pool up to the level you want. It all comes down to one fact. A part per million (ppm) in water is the same as a milligram per liter. So once you know how many liters your pool holds and how many ppm to add, you know the grams of chlorine. The strength on the product label then turns that into fluid ounces of liquid, or ounces by weight of granules or tablets. This calculator does that math for the four common pool chlorine products.

It also shows side effects that matter over a season. Stabilized products (dichlor and trichlor) add cyanuric acid along with the chlorine. Cyanuric acid, or stabilizer, shields chlorine from sunlight. Calcium hypochlorite (cal-hypo) adds calcium hardness. Both build up in the water, and the only way to lower them is to replace some of the water. That's why many pool owners keep track of what each dose adds.

Where it's used

Why this math matters outside the classroom:

  • Weekly pool care. A pool owner raises a 20,000-gallon pool from 1 to 3 ppm with about 41 fl oz of 12.5% liquid chlorine.
  • Buying in bulk. A homeowner works out that 1 gallon of 12.5% liquid raises 10,000 gallons by 12.5 ppm, so one jug covers several doses.
  • Comparing products. A pool store worker shows that 1 ppm in 10,000 gallons takes about 2.05 oz of 65% cal-hypo or 10.2 fl oz of 12.5% liquid.
  • Stabilizer tracking. An owner using trichlor notes that each 2.5 ppm of chlorine adds about 1.5 ppm of cyanuric acid to the water.
  • Concentration math. Chemistry students convert 2 ppm in 75,708 L into 151.4 g of chlorine, since 1 ppm in water is 1 mg per liter.

Grade level and standards: Grade 7; Common Core State Standards for Mathematics 7.RP.A.3. © Copyright 2010. National Governors Association Center for Best Practices and Council of Chief State School Officers. All rights reserved.

How to use it

  1. Enter your pool's volume in US gallons or liters. Not sure? The pool volume calculator can work it out from the pool's size.
  2. Enter the free chlorine from your latest test and the level you want to reach.
  3. Choose your product, and check that the available chlorine percent matches its label.
  4. Read the amount to add. Then follow the directions on the product label for adding it safely.

What the inputs mean

Pool volume
Not sure? The pool volume calculator works it out from the pool's size.
Current free chlorine
From a test kit or strip.
Target free chlorine
The CDC recommends at least 1 ppm of free chlorine in pools, and at least 2 ppm when you use cyanuric acid (stabilizer). For hot tubs, it recommends at least 3 ppm. In all cases, pH should be between 7.0 and 7.8. The right target for your pool also depends on its stabilizer level. Follow your product's label or a pool professional's advice.
Chlorine product
Dichlor and trichlor contain cyanuric acid. It protects chlorine from sunlight, but it builds up over time. The CDC recommends not using cyanuric acid, or products that contain it, in hot tubs.
Available chlorine on the label
Pool liquid chlorine is usually sold at 10% or 12.5%. The calculator reads the percent as grams of available chlorine in each 100 mL. Household bleach labeled by weight holds slightly more chlorine per cup than its percent suggests. So with bleach, the result leans toward a little extra.

Formula and method

liters = US gallons × 3.785411784
ppm increase = target free chlorine − current free chlorine
chlorine needed (g) = ppm increase × liters ÷ 1,000
dry product (g) = chlorine (g) ÷ (available chlorine % ÷ 100);  oz = g ÷ 28.3495
liquid product (mL) = chlorine (g) ÷ (percent ÷ 100);  fl oz = mL ÷ 29.5735
CYA added = ppm × 0.910 (dichlor) or × 0.607 (trichlor);  calcium hardness added = ppm × 0.706 (cal-hypo)

In the formulas, g is grams, mL is milliliters, oz is ounces by weight and fl oz is fluid ounces. CYA is cyanuric acid, the stabilizer. Free chlorine is the chlorine still free to clean the water.

One ppm in water is one milligram per liter. So the rise in ppm times the liters gives milligrams. Dividing by 1,000 turns that into grams of available chlorine. The product's available chlorine percent says how much of each gram, or each 100 mL of liquid, is useful chlorine. Dividing by it gives the amount of product to add.

The side-effect numbers come from the chemistry of each product. Dichlor releases two units of chlorine for each unit of cyanuric acid, and trichlor releases three. So trichlor adds less stabilizer per ppm of chlorine. Calcium hypochlorite releases two units of chlorine per unit of calcium. The ratios are worked out from standard molar masses (the weight of each chemical's basic unit). Hardness is counted as calcium carbonate, the way test kits report it.

Worked example

A 20,000-gallon pool tests at 1 ppm free chlorine and the owner wants 3 ppm, using 12.5% liquid chlorine.

  1. Volume in liters: 20,000 × 3.785411784 = 75,708.24 L.
  2. Increase: 3 − 1 = 2 ppm.
  3. Chlorine needed: 2 × 75,708.24 ÷ 1,000 = 151.416 g.
  4. Liquid product: 151.416 ÷ 0.125 = 1,211.33 mL.
  5. In fluid ounces: 1,211.33 ÷ 29.5735 = 40.96 fl oz.

Result: Add about 41.0 fl oz (roughly a third of a gallon) of 12.5% liquid chlorine.

Understanding the result

Free chlorine drops through the day as sunlight and organic matter use it up. So a dose that hits the target in the evening may be lower by the next afternoon. Test again after the pump has mixed the water. Then adjust a little at a time, rather than adding a big extra amount at once.

Keep an eye on the cyanuric acid and calcium hardness numbers over the season. If stabilizer keeps climbing, switch some doses to an unstabilized product such as liquid chlorine. That slows the build-up.

Assumptions and limitations

  • The chlorine mixes evenly through the whole pool.
  • The label strength is accurate. Liquid chlorine loses strength in storage, and faster when warm, so older product can fall short.
  • It doesn't include chlorine used up by sunlight, debris and swimmers while the dose mixes in.
  • This works out the amount of product. It is not a water-safety check. Always follow the product label and local health rules.
  • Safety first: never mix chlorine products with each other, with acid or with other pool chemicals. Never add water to the chemical. Add each product to the pool water on its own, as its label directs.
  • Combined chlorine (chlorine that has already reacted with contaminants), pH, alkalinity and stabilizer all affect how well chlorine works. Test them too.

Frequently asked questions

How much liquid chlorine raises 10,000 gallons by 1 ppm?

About 10.2 fl oz of 12.5% liquid chlorine, or about 12.8 fl oz of 10%. One ppm in 10,000 gallons is 37.85 g of chlorine. At 12.5 g per 100 mL, that's 302.8 mL.

How do I work out a shock dose?

Shocking means raising free chlorine to a much higher level for a while, often to clear algae or break down combined chlorine. Enter your current reading. For the target, enter the shock level on your product label or the one your pool professional recommends. The calculator gives you the amount.

Which chlorine product should I use?

Liquid chlorine adds nothing but chlorine and a little salt. Cal-hypo adds calcium, which suits pools with soft water. Dichlor and trichlor add stabilizer, which helps outdoor pools that have little. But they raise it with every dose. Many owners use a mix, guided by their test results.

Why does my test show less chlorine than the calculator predicted?

Sunlight, leaves, algae and swimmers use up chlorine. Liquid chlorine also weakens as it sits in storage. Wait for the pump to mix the water before you test. Then base the next dose on the new reading.

Sources

How this calculator is checked

Method: Mass balance: 1 ppm = 1 mg/L, chlorine mass = ppm × volume; product amount = chlorine ÷ available chlorine fraction. By-product ratios from standard molar masses (Cl₂ 70.906, cyanuric acid 129.075, CaCO₃ 100.086). Target guidance from the CDC's home pool and hot tub page. The calculation is covered by automated tests with known reference values and edge cases. Last updated 2026-10-18. Found a problem? Tell us.

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