Liquid chlorine strength: trade-% or weight-%?
A jug of liquid pool chlorine says “12.5 %”. That number can describe two different strengths, and the difference changes your dose by about 14 %. This guide explains the two bases, shows how to convert between them and why our chlorine calculator makes you choose one.
Safety first
- NEVER MIX CHEMICALS. Never combine pool chemicals with each other, in a bucket, a scoop, a feeder or the skimmer. Add one product at a time, directly to the pool water as its label directs.
- FOLLOW THE PRODUCT LABEL. The label is the law for dose, protective equipment and how long to wait before swimming. If this calculator and the label disagree, follow the label.
- KEEP OUT OF REACH OF CHILDREN. Store pool chemicals in their original, closed containers, locked away from children and pets.
More safety rules
- Never mix chlorine and acid. Mixing chlorine products with muriatic (hydrochloric) acid or sodium bisulfate can release toxic chlorine gas. Add them on different occasions and let the pump circulate in between.
- Never mix different chlorine products. Calcium hypochlorite (cal-hypo shock) and trichlor or dichlor can start a fire or explosion, even in small amounts. Use separate, clean, dry scoops and never put cal-hypo in a tablet feeder.
- Always add chemical to water, never water to chemical. If the label tells you to pre-dissolve, fill a clean plastic bucket with pool water first, then add the product.
- ONE CHEMICAL AT A TIME. Open one container at a time, close it before opening the next, and retest before adding anything else.
- Protect yourself. Wear chemical-splash goggles and chemical-resistant gloves. Read the product label and Safety Data Sheet before handling.
- Wait before swimming. Do not swim until free chlorine has dropped to the level allowed on your product label (4 ppm or lower on the labels we checked). Measure free chlorine again with your test kit before anyone gets in the pool.
- Liquid chlorine loses strength. An old or warm jug can be much weaker than its label. Retest after dosing instead of adding more blindly.
- STORE SAFELY. Keep pool chemicals in their original, closed containers in a cool, dry, ventilated place, apart from each other and out of reach of children and pets.
Sources: CDC Pool Chemical Safety poster and toolkit; EPA-registered labels 550-198, 748-311 and 1258-1237; CCOHS.
Two ways to write the same jug
Weight-percent (wt-%) is the share of the liquid's weight that is sodium hypochlorite. The EPA-registered labels we checked list the active ingredient this way, for example “Sodium Hypochlorite 12.5%” on EPA label 550-198 (a 12.5 % product registered for institutional and industrial use). Because the solution is heavier than water, a litre of it weighs more than a kilogram.
Trade-percent (trade-%) is grams of available chlorine per litre, divided by 10, as defined in OxyChem's Sodium Hypochlorite Handbook. It is a volume measure: 12.5 trade-% means 125 g of available chlorine in every litre. We did not find trade-% printed on any EPA label we checked.
Some labels print two numbers. EPA label 55852-3 lists “Sodium Hypochlorite 12.5%” and “Available Chlorine 15%”, without saying which basis the 15 % uses.
Converting between them
Going from weight-% to grams per litre needs the density of the solution, which rises with strength. OxyChem's Sodium Hypochlorite Handbook tabulates both from 10 to 200 g/L. Interpolating in that table:
| Label says (weight-% NaOCl) | g/L available chlorine | Density (SG) | Same jug in trade-% | fl oz per 1 ppm in 10,000 US gal |
|---|---|---|---|---|
| 5.25 % | 53.8 | 1.075 | 5.38 % | 23.80 |
| 8.25 % | 88.3 | 1.124 | 8.83 % | 14.49 |
| 10 % | 109.9 | 1.154 | 10.99 % | 11.65 |
| 12.5 % | 142.9 | 1.200 | 14.29 % | 8.96 |
Read the table the other way to see why the two bases matter. A jug of 12.5 % trade strength holds 125 g/L and needs 10.24 fl oz per ppm in 10,000 gallons. A jug of 12.5 % by weight holds about 143 g/L and needs 8.96 fl oz. Same number on the label, a different dose.
What this means for a shock dose
Take a 20,000 US gallon pool and a shock dose of 10 ppm from the label. With 12.5 % trade-% you would add 1.60 US gallons. With 12.5 % weight-% you would add 1.40 US gallons. The gap is about 26 fl oz (758 mL) for one dose.
The label itself is not always consistent. Label 550-198 states weight-% in its ingredient box, yet its shock directions (52 fl oz for 5 ppm in 10,000 gallons) match trade-%. If you calculate with the weight-% it states, you get 44.8 fl oz. When the label and a calculator disagree, follow the label, then measure free chlorine again.
Strength fades over time
Liquid chlorine loses strength in storage. The liquid chlorine label we checked says to use the product within a month. An old or warm jug may be weaker than either number on its label, so retest after dosing instead of adding more blindly.
How our calculator handles it
- It has no default strength and no default basis. No dose appears until you choose weight-% or trade-%.
- It shows the grams per litre, the density and the fl oz per ppm, so you can check the conversion.
- It gives no dose outside the OxyChem table (10–200 g/L), instead of guessing.
Sources: EPA labels 550-198 and 55852-3; OxyChem Sodium Hypochlorite Handbook, Table 1. Full list on the methodology page.
Disclaimer: GoodToSwim.com gives estimates for home (residential) chlorine pools based on published primary sources. It does not cover spas, hot tubs or bromine, and it does not report water quality at beaches, lakes, rivers or other open water. It never tells you that your water is safe to swim in. Results depend on your test accuracy, your pool volume and the real strength of your product. They do not replace the product label, local health regulations or a qualified pool professional. Public and commercial pools must follow local code.