Free Chlorine
The active, available chlorine actually disinfecting your pool, the single most important number for swimmer safety.
- Safe range
- 1 – 4 mg/L
- Measured in
- mg/L
What it is
Free chlorine is the portion of chlorine still available to do work: to attack bacteria, viruses and algae. It is deliberately distinct from total chlorine, which also counts the chlorine that has already reacted with sweat, sunscreen and organic debris and is largely spent. A pool can show plenty of total chlorine while having almost no free chlorine left, which is exactly the situation that feels wrong (strong chlorine smell, stinging eyes) and is usually misread as too much chlorine rather than too little.
Why it matters
This is the parameter that decides whether the water is safe to be in. Pathogens that cause recreational water illness are controlled by free chlorine and essentially nothing else in a typical pool. Every other number here exists to help chlorine do its job or to stop the water damaging something.
How to measure it
This is the one number worth owning a proper test for. A strip will tell you the pool is not completely out of chlorine; it will not reliably tell you where in the safe range you are, and that difference is the whole parameter.
Liquid reagent kit, colour match
- How trustworthy
- Dependable
- Cost
- Low cost
- Effort
- A couple of minutes, at a sink or poolside
A measured sample in a plastic comparator block, a few drops of reagent, and a colour you match against printed standards beside the tube. The two-tube kit sold everywhere for pools is this.
The standard DPD test: a measured sample, one scoop or a few drops of reagent, and the pink that appears is matched against printed standards. This is what the chlorine side of the two-tube kit in every pool shop does.
Watch out: At very high chlorine the pink flashes and then bleaches back to clear, which reads as zero when the truth is the opposite. If the colour appears and fades, dilute the sample half and half with tap water, retest, and double the result.
Where to find it: Pool shops and online. Reagents expire, so buy replacement bottles rather than a whole new kit each season.
Drop-count titration kit
- How trustworthy
- Dependable
- Cost
- Low cost
- Effort
- A few minutes, and it rewards being unhurried
Add reagent one drop at a time until the sample changes colour, and count the drops. The count is the answer, so there is no colour to judge by eye and no lighting to get wrong.
FAS-DPD is the same chemistry turned into a drop count: the sample goes pink, then you add drops until the pink disappears. It resolves small steps, it does not bleach out at shock levels, and it is what people who have got tired of arguing with a colour chart end up buying.
Watch out: It is the kit whose reagents are most worth replacing on a schedule. The powder takes up moisture and the titrant weakens with age, and both failures push the count the same way, low.
Where to find it: Pool shops and online, usually as part of a multi-test kit. Aquarium shops sell the same chemistry in smaller bottles.
Test strips
- How trustworthy
- Ballpark
- Cost
- Low cost
- Effort
- Under a minute, wherever you drew the sample
A plastic strip with reagent pads on it. Dip it, hold it level, and match each pad against the chart on the bottle at the time the label tells you to. Most strips read several parameters at once.
The quickest way to check that nothing has gone badly wrong between real tests, and the right tool for a glance before somebody swims.
Watch out: Hold the strip level and do not shake the water off, or the chlorine pad runs into its neighbours. A bottle left open in a hot pool shed is degraded within weeks, so write the date on the lid when you open it.
Where to find it: Pool shops, hardware stores, supermarkets and online, sold both per parameter and as multi-packs. Buy strips made for your kind of water: pool strips and drinking water strips read very different scales for the same chemistry.
Digital photometer
- How trustworthy
- Dependable
- Cost
- Most expensive
- Effort
- A few minutes per parameter
The same reagents as a drop kit, read by an instrument instead of your eye. It removes the colour judgement and gives a repeatable number, which matters most for people who are colour blind or testing in poor light.
Reads DPD electronically, which matters if you are colour blind or testing at dusk, and gives numbers comparable week to week without a judgement call in between.
Where to find it: Pool trade suppliers and water-testing suppliers online. Reagent tablets or sachets are an ongoing cost.
Free test at a pool shop
- How trustworthy
- Dependable
- Cost
- Free
- Effort
- A trip, and a sample carried carefully
Take a sample in and they will run it, usually on a photometer, usually free, and print you a full panel. The catch is that the printout ends in a list of things to buy.
A full panel free, and a good way to find out whether your own kit is telling the truth. Take the sample the same morning: chlorine that has spent an afternoon in a warm car is not the chlorine in your pool.
Where to find it: Most pool and spa retailers. Ask which parameters they cover before driving over.
Taking the sample
For several of these the sample is where the error is, and no instrument recovers from a bad one.
- • Take the sample at elbow depth, away from the returns and the skimmer. The top inch of water is the most sun bleached in the pool and reads low.
- • Run the pump first. A pool that has been still overnight is not mixed, and testing it measures one corner of it.
- • Test at roughly the same time of day. Chlorine falls through an afternoon of sun, so a morning reading and an evening reading are answers to two different questions.
Further reading: CDC: Healthy Swimming
If it is too low
The pool isn't being adequately disinfected, raising the risk of bacteria and algae growth. Common causes: heavy use, hot or sunny weather (chlorine breaks down fast in UV light, especially without stabiliser), or heavy rain diluting the water.
- • Add chlorine and retest after it has circulated, usually a few hours with the pump running.
- • Check your stabiliser (CYA) level. Without it, sunlight can destroy most of your free chlorine in a single day.
- • If chlorine keeps vanishing, suspect a high organic load or early algae. That demand has to be broken with a shock, not topped up gradually.
If it is too high
Can irritate skin, eyes and airways, and bleach swimwear. Common right after shocking the pool; wait for levels to drop back into range before swimming.
- • Simply wait. Chlorine dissipates on its own, faster in sunlight with the cover off.
- • Do not swim until it is back inside the safe range printed above.
- • If you need the pool sooner, a chlorine neutraliser will bring it down quickly, but it is easy to overshoot.
Common questions
Why does my pool smell strongly of chlorine when the level is low?
That smell is chloramines: chlorine that has already combined with sweat, urine and body oils and is no longer able to sanitise. A strong chemical smell almost always means the pool needs more chlorine, not less. Clean, properly chlorinated water has very little odour.
What free chlorine level is safe to swim in?
The safe range is printed at the top of this page, from the same figures the app judges your readings against. Below it the water is not reliably disinfecting; above it chlorine starts to irritate skin and eyes. Aim for the middle rather than either edge, so that a normal day's drift does not take the pool out of range.
How is free chlorine different from total chlorine?
Free chlorine is what is still available to disinfect. Total chlorine is free chlorine plus combined chlorine (chloramines), which is spent. The gap between the two tells you how much of your chlorine has already been used up; a large gap is a sign the pool needs shocking.
WiseWaterGauge helps you record and understand your own readings. It is not a substitute for a certified laboratory test, and nothing here is medical or regulatory advice. If a health parameter is out of range, confirm it with an accredited lab and speak to your water provider.