Converting between bleach strengths
Every liquid chlorine product — pool chlorine, household bleach, "shock in a jug" — is sodium hypochlorite at a labeled strength. Doses convert by simple ratio: multiply by the strength you have, divide by the strength you want. The anchors worth memorizing:
| This much… | equals | equals |
|---|---|---|
| 1 gal of 12.5% | 1.25 gal of 10% | 2.1 gal of 6% |
| 10 fl oz of 12.5% | 12.5 fl oz of 10% | 20.8 fl oz of 6% |
| 1 gal of 10% | 0.8 gal of 12.5% | 1.7 gal of 6% |
Same chemical, same effect on your water. Catch one: it must be plain — "splashless" bleach carries thickeners that foam, scented versions carry fragrance oils, and CloroMax-type additives have no business in a pool. The label should say sodium hypochlorite and nothing interesting. Catch two: strength — household jugs run 6–8.25%, so you'll pour roughly twice the volume of a 12.5% pool jug for the same chlorine.
Which jug is actually cheaper?
Price-per-jug is a distraction; the number that matters is price per ppm of chlorine delivered. At example prices — a $6 gallon of 12.5%, a $5 gallon of 10%, a $4 household jug of 6% — the math lands like this for a 10,000-gallon pool:
| Product | Chlorine per jug | Cost per ppm |
|---|---|---|
| 12.5% × 1 gal @ $6 | 12.5 ppm | $0.48 |
| 10% × 1 gal @ $5 | 10 ppm | $0.50 |
| 6% × 121 oz @ $4 | 5.7 ppm | $0.71 |
Example prices for illustration — enter your local ones in the tool above and let it rank them. The strongest jug usually wins, but not always: a sale on 10% or a warehouse-club bleach price can flip the order, which is exactly why the comparison is worth thirty seconds.
Sodium hypochlorite decays — faster with heat, sunlight, and higher starting strength. A jug of 12.5% that spent a summer month in a hot store lot (or your garage) can test below fresh 10%. Buy from sellers with turnover, check the date code if the jug has one, buy only a few weeks of supply at a time, and store it cool and dark. This is also the honest case for grocery-store bleach in cooler months: it may be weaker on the label but fresher in the jug.
When the weaker jug makes sense
Small water changes the math: dosing a 400-gallon spa with 12.5% means measuring tablespoons, where 6% gives you pourable quantities and finer control. Availability matters too — in the off-season, grocery bleach is on a shelf ten minutes away when the pool store isn't. And if you're hauling jugs any distance, remember you're mostly carrying water: double-strength halves the trips. Whatever you buy, the chlorine calculator takes every strength — pick the jug, not the habit.