Pool Shock Calculator

A pool that smells strongly of chlorine has too little of it, not too much, and understanding why explains most of what shocking is for.

Free chlorine is nearly odourless. What people smell is chloramine — chlorine that has already reacted with nitrogen compounds from sweat, urine, skin oils and cosmetics. Chloramine is a poor sanitiser, it is volatile, and it is the specific compound that irritates eyes and lungs and produces the smell associated with public pools.

So the smell is a symptom of chlorine having been consumed by contamination rather than of an excess. Adding more chlorine — enough more — is the correct response, which is exactly the opposite of the instinct.

The reading that matters is combined chlorine: total chlorine minus free chlorine. A well-run pool holds it at or below 0.2 ppm. Anything above 0.5 ppm is worth acting on, and above 1.0 the pool will smell and sting.

Test strips cannot resolve this. They report total chlorine or free chlorine but not both reliably enough for a difference of a few tenths to mean anything, and the difference is the entire measurement. A DPD drop kit — ideally FAS-DPD, which reads in 0.2 ppm steps — is what this calculation needs.

Breakpoint chlorination is a threshold process, and the behaviour below the threshold is what makes it worth getting right.

As free chlorine rises, it first reacts with ammonia and nitrogen compounds to form more chloramine. Combined chlorine goes up. Only once enough chlorine is present does the reaction shift to destroying those chloramines, oxidising them to nitrogen gas which leaves the water entirely. That crossover is the breakpoint, and it sits at roughly ten times the combined chlorine concentration.

A dose that stops short therefore leaves the pool worse than it started: more chloramine, a stronger smell, and more chlorine demand. The owner smells the result, concludes the pool needs shocking again, adds another partial dose, and the cycle repeats.

Pools do end up beyond practical recovery this way, at which point the answer is replacing a substantial fraction of the water. It is entirely avoidable, and the avoidance is simply to calculate the dose and add all of it.

The corollary is that there is no cost to overshooting beyond the price of the product. Where the test is uncertain, or the pool has had heavy use, going to fifteen or twenty times combined chlorine is the safe error.

Chloramine is destroyed by a threshold reached once. Algae is killed by a concentration maintained over time, and confusing the two is why so many green pools clear briefly and then return.

The level required scales with cyanuric acid, because stabiliser holds most of the chlorine in reserve and slows the active fraction down. A pool at 30 ppm of cyanuric acid needs far less than one at 100 to achieve the same killing rate — which is another reason over-stabilisation is expensive, since it raises both the level needed and the amount of product to reach it.

The method is: raise to the target, then test several times a day and top back up each time. Chlorine is consumed rapidly while it is oxidising algae, and every hour spent below the target is an hour the algae is recovering. A single large dose left to decay gives a pool that looks better on Tuesday and is green again by Friday.

Brushing matters as much as the chemistry. Algae attached to plaster sits under a biofilm that shields it from the water; brushing every surface daily is what exposes it to the chlorine. And the filter has to run continuously, because dead algae is what clouds the water and only the filter removes it — expect to clean or backwash it repeatedly as the pressure climbs.

Three conditions define finished: free chlorine holds overnight with a loss of 1 ppm or less, combined chlorine is at or below 0.5 ppm, and the water is clear. Clear water alone is not enough — a pool that still consumes chlorine overnight still has something living in it.

For routine chlorination the side effects of a product accumulate slowly enough to manage. A shock delivers several times a normal dose at once, so whatever the product carries arrives at once too.