Swimming Pool Chlorine Levels: UK Target Ranges for Service Engineers

The correct chlorine level for a UK swimming pool depends on the pool type, water temperature, pH, and bather load. Get it right and the water is safe, clear, and comfortable. Get it wrong and you face either a health risk (too low) or customer complaints, corrosion damage, and discomfort (too high).

This guide covers the UK target ranges from PWTAG guidance, explains what free and combined chlorine actually mean, and walks through how to interpret and correct common readings.

Free Chlorine, Combined Chlorine, and Total Chlorine

Chlorine in pool water exists in multiple forms. Understanding which form you're measuring matters.

Free chlorine (FC) is the active sanitiser — hypochlorous acid (HOCl) and hypochlorite ion (OCl⁻) in equilibrium. This is what kills pathogens, oxidises organics, and keeps the water safe. Every test method you use at poolside measures free chlorine as the primary output.

Combined chlorine (CC), sometimes called chloramines, forms when free chlorine reacts with nitrogen-containing compounds from swimmers — urine, sweat, skin cells. Combined chlorine is a much weaker sanitiser than free chlorine. At elevated levels it causes the "pool smell," eye and skin irritation, and respiratory discomfort that people often wrongly blame on "too much chlorine." High combined chlorine almost always means the pool needs superchlorination (shock treatment), not less chlorine.

Total chlorine (TC) = free chlorine + combined chlorine. Some test strips measure total chlorine only, which is why professional pool engineers use a photometer or DPD drop test that separates FC from TC.

UK PWTAG Target Ranges

The Pool Water Treatment Advisory Group (PWTAG) publishes the standards that UK commercial pools are expected to follow. It's worth being precise about what PWTAG itself specifies versus the wider operating band commonly cited in the industry.

For hypochlorite pools, the PWTAG Code of Practice (SWCOP19, section 9.5) states that "the free chlorine levels should be maintained at 1mg/l or below, to an absolute minimum of 0.5mg/l" (assuming pH around 7.2 and satisfactory microbiological results), and that "free chlorine levels above 3mg/l should not be necessary in any pool using hypochlorite." So PWTAG's operative aim is roughly 0.5–1.0 mg/L with a 3.0 mg/L upper limit — not a flat 1.0–3.0 mg/L target band. (PWTAG sets higher residuals for chloroisocyanurate/stabilised pools — 2.5–5.0 mg/L — and for spa pools.)

The wider 1.0–3.0 mg/L band you'll often see quoted comes from the general HSG179-era operating practice and is a reasonable practical working range for many commercial pools, but attribute it to that general guidance rather than to PWTAG's operative figure.

Free Chlorine Working Ranges

Pool type	Free chlorine (practical working range)
Standard indoor pool (25°C or below)	1.0–3.0 mg/L
Indoor pool at elevated temperature (>25°C)	1.5–3.0 mg/L
Outdoor pools	1.0–3.0 mg/L (upper end during high UV/bather load periods)
Spa pools and hot tubs	3.0–5.0 mg/L
Paddling pools / learner pools	1.5–3.0 mg/L

These are commonly-used working ranges; PWTAG's lower operative aim (0.5–1.0 mg/L, cap 3.0) above applies specifically to hypochlorite-dosed pools. The key principle either way: higher water temperatures and higher bather loads reduce the effective life of free chlorine, so target the upper end of the range in those conditions. Outdoor pools also lose chlorine to UV degradation faster than indoor pools — a pool at 60% capacity in direct summer sun degrades chlorine at several times the rate of the same pool on an overcast day.

Combined Chlorine Limit

Maximum combined chlorine: 1.0 mg/L. PWTAG's Code of Practice (SWCOP19) states combined chlorine "should always be less than half the free chlorine, and no more than 1mg/l no matter what the level of free chlorine."

In practice operators run spa pools to a tighter combined-chlorine figure than swimming pools because the aerosolised water at hot tub temperatures carries chloramine vapour that irritates airways — a working target around 0.2 mg/L is commonly applied to spas, though this tighter spa-specific figure is industry practice / HSG282-related guidance rather than a number stated in PWTAG's main Code of Practice combined-chlorine rule. If you're seeing combined chlorine readings above 0.5 mg/L in a pool, investigate the nitrogen load (often a bather load / ventilation issue in indoor pools) and plan a superchlorination. If it's above 1.0 mg/L, the pool should be considered for closure until the level drops.

A rule of thumb that follows directly from the PWTAG rule: combined chlorine should be less than half of free chlorine. If FC is 2.0 mg/L and CC is 1.2 mg/L, you have a problem even though FC looks fine.

pH: Why It Controls Chlorine Effectiveness

The pH of pool water directly determines what fraction of free chlorine is in the active hypochlorous acid (HOCl) form:

At pH 7.0: approximately 70% of free chlorine is HOCl (highly effective)
At pH 7.5: approximately 50% HOCl
At pH 8.0: approximately 20% HOCl

PWTAG target pH range: 7.2–7.4. At pH 7.4, free chlorine is roughly 45–50% active. At pH 8.0, the same free chlorine concentration is only 20% active — meaning a pool testing at 2.0 mg/L FC but with a pH of 8.0 has the effective sanitising power of a 0.4 mg/L pool at neutral pH.

If you're adding chlorine to a pool that isn't responding, check pH first. High pH and low sanitiser effectiveness often go together.

Reading Your Photometer Results

Most UK pool engineers use a DPD-based photometer for on-site testing. The test works by adding DPD1 reagent (measuring free chlorine) and then DPD3 reagent (which adds total chlorine — the difference is combined chlorine).

Typical photometer procedure:

Rinse the cell with sample water (not tap water)
Add the sample to the 10 ml fill line
Add DPD1 tablet or powder (crush if tablet) — read FC
Add DPD3 to the same cell — read TC
TC − FC = CC

Common errors:

Stale DPD reagents — DPD tablets and powder degrade with heat and light. Keep in a cool, dark place and check expiry dates quarterly. Stale reagent gives low or erratic readings.
Dirty photometer cell — calcium deposits, mineral film, or chemical residue on the cell walls scatter light and produce false readings. Clean the cell monthly with the manufacturer's recommended solution.
Sample temperature — test water should ideally be close to the pool temperature when tested. Very cold samples can shift readings slightly.
Reading delay — DPD1 and DPD3 reactions are time-sensitive. Read within the timeframe specified by the reagent manufacturer (typically 60–120 seconds after adding reagent).

When Chlorine Is Out of Range

Free Chlorine Too Low (<1.0 mg/L)

First, check pH. Low chlorine and high pH is a common pairing. Reduce pH toward 7.2–7.4 with carbon dioxide dosing or hydrochloric acid, then re-test FC — you may find the effective chlorine level is already adequate once pH is corrected.

If pH is in range and FC is still low, dose to raise FC to the upper end of the target range (2.0–2.5 mg/L), check your dosing system's output rate (flow probe, peristaltic pump rate), and review the pool's chlorine demand. High bather load, organic contamination (leaves, algae, sunscreen), or a UV system that's degrading chlorine faster than dosing can replace it are common causes.

Free Chlorine Too High (>3.0 mg/L for standard pools)

Allow the pool to off-gas naturally — avoid swimming until FC drops below 3.0 mg/L. If you need to lower FC faster (pool needs to open), sodium thiosulphate dechlorinator is the standard treatment. Apply carefully and re-test — overdosing will strip chlorine entirely.

High FC often indicates either an overdose (dosing pump malfunction, or manual dosing error) or a sudden drop in bather load in a pool that was dosed for high usage. Review dosing records.

Combined Chlorine High (>1.0 mg/L)

Superchlorinate to break-point: dose free chlorine to approximately 10× the combined chlorine level. At 1.5 mg/L CC, dose to approximately 15 mg/L FC (an aggressive shock treatment). This oxidises the chloramine compounds.

For indoor pools, improve ventilation during the shock treatment — chloramines off-gas from the water surface and accumulate in the air space above the pool. This is why indoor pool staff sometimes experience respiratory irritation: it's chloramine vapour, not chlorine gas.

Testing Frequency

PWTAG guidance on minimum testing frequencies for commercial pools:

Pool type	Minimum test frequency
Public pool, high bather load	Every 1–2 hours during operation
Hotel / leisure pool, moderate bather load	Every 2 hours during operation
Spa pool / hot tub	Every 1 hour during operation
Private residential pool on service engineer visit	At start, mid, and end of visit (3 tests minimum)

Records must be kept for each test — date, time, tester, FC, CC/TC, pH, temperature. These records are part of the pool's legal compliance evidence under HSG179 and are what PWTAG expects operators to maintain.

Calibrating Expectations with Your Client

Chlorine levels vary during the day, not just on your visit. A morning test on a commercial pool that recorded 2.5 mg/L FC might show 0.8 mg/L at 3 pm after a swimming lesson. This is normal — and exactly why automated dosing systems (that continuously monitor and adjust) produce more consistent water quality than manual dosing on a fixed schedule.

When reviewing records with a client, look at the trend across a week rather than the snapshot on your visit. Consistent FC between 1.0 and 3.0 mg/L across all readings is better evidence of effective management than a single on-visit reading of 2.0 mg/L with gaps in the record.

Summary: Key Numbers for UK Pools

Parameter	Standard pool target	Spa pool target
Free chlorine (FC)	1.0–3.0 mg/L	3.0–5.0 mg/L
Combined chlorine (CC)	<1.0 mg/L	<0.2 mg/L
pH	7.2–7.4	7.2–7.4
Test record keeping	Every 1–2 hrs (commercial)	Every 1 hr

For the full picture on pool water quality testing — including total alkalinity, calcium hardness, and LSI — see our swimming pool water testing guide. For hot tub and spa pool specifics, our hot tub water testing guide covers the PWTAG targets for spa pools.

PoolRound is building water chemistry logging that captures FC, CC, pH, temperature, and LSI in one entry — so every visit produces a PWTAG-compliant record without manual data entry. Join the waitlist to get early access.

This guide reflects PWTAG guidance as understood at the date of last review. PWTAG updates its standards periodically — always verify against the current PWTAG Code of Practice and technical notes for formal compliance purposes. Not safety or regulatory advice.

Sources

PWTAG Code of Practice — Pool Water Treatment Advisory Group water quality standards for UK pools and spas
HSE HSG179: Managing health and safety in swimming pools — commercial pool operator guidance (4th edition, updated February 2024)