Chlorine gas is 100% chlorine. Nothing else on the pallet is, and forgetting that is the most expensive habit in this chapter. Sodium hypochlorite liquid is typically 12.5% available chlorine; calcium hypochlorite granules — HTH — are typically 65%; household bleach is around 5%. The rest is water, salt, lime and stabiliser, and none of it disinfects anything.

Going from the chlorine you need to the product you weigh: $m_{\text{prod}} = \dfrac{m_{\mathrm{Cl}} \times 100}{p}$ — read aloud m-prod equals m-C-l times one hundred, over p. $m_{\text{prod}}$ is the mass of product to weigh out; $m_{\mathrm{Cl}}$ is the mass of available chlorine the job actually requires; $p$ is the available chlorine strength as a percentage. Both masses must be in the same unit — grams with grams, kilograms with kilograms. You are solving for the product weight, and the answer is always larger than the chlorine weight. If it comes out smaller, you multiplied where you should have divided, and that single slip under-doses a 12.5% job by a factor of sixty-four.

Going the other way — what dose does this much product reach? $C = \dfrac{m\,p}{V}$, where $C$ is the chlorine concentration reached in mg/L, $m$ the mass of product used, $p$ its strength and $V$ the volume of water being dosed. Keep the bookkeeping in plain steps and it never goes wrong: grams of product times the strength gives grams of chlorine; a gram spread through a thousand litres is 1 mg/L.

And the third face of the same relation, $p = \dfrac{m_{\mathrm{Cl}}}{m_{\text{prod}}} \times 100$, is how a lab reads a drum with no label. It carries its own alarm: an answer above 100% is impossible, so it is telling you the ratio is upside down.

One nugget from the yard. Hypochlorite decays in storage — faster when it is warm, faster when it is strong, faster in the light. A 12.5% delivery can be 10% by the end of a hot month, and the plant that never re-tests its stock quietly under-doses all summer while every calculation on the sheet says it is fine.