Independent comparisons. We may earn from shop links. More info

Swimming pool winteriser: what to check before you buy

Updated: 21 October 2026

You are closing an outdoor pool in Britain, and the bottles do not agree. One swimming pool winteriser quotes gallons, another quotes cubic metres, and a third never says whether it contains copper. Check the water you will leave in, the sanitiser, then copper or copper free, and only then the price. Live rows stay on the [pool winteriser comparison](/hub/pool-care/pool-winteriser). Prices are amazon.co.uk figures from 19 October 2026. Peter F. compared the nine listings with public trade sheets. We did not lab-test a bottle. A qualifying purchase can earn a commission at no extra cost to you.

You are closing an outdoor pool in Britain, and the bottles do not agree. One swimming pool winteriser quotes gallons, another quotes cubic metres, and a third never says whether it contains copper.

Our top picks

Kleen Pool Kleen Pool algaecide, 1 L

Best overall

Kleen Pool Kleen Pool algaecide, 1 L
7.5/10about £17.95
View offer
HTH HTH Super Winterprotect, 3 L

Alternative

HTH HTH Super Winterprotect, 3 L
7.7/10about £43.95
View offer
Blue Horizons Blue Horizons winter time algaecide, 5 L

Rank 3

Blue Horizons Blue Horizons winter time algaecide, 5 L
8.6/10about £29.20
View offer
On this page
  1. 1.What you are buying
  2. 2.Volume before brand
  3. 3.Sanitiser, copper, and scale
  4. 4.A sensible order of checks

What you are buying

A swimming pool winteriser is the chemical you add when an outdoor pool stays full through the cold months. In this amazon.co.uk set it is a liquid, a granule, or a drop-in closing kit. It is not a cover pillow and not a pipe plug. Hold a liquid against a printed rate. HTH prints 300 ml per 10 m³. Blue Horizons winter time says up to 15,000 gallons and copper free. Relax says 14,500 liters. CATAGEN PS is 2 litres. Blu Line is a granule with no volume. The kits state gallon ceilings, and POOLHOUR does not state a pod count. Kleen Pool treats 28,000 litres for 6 months and says all year. The title does not say winteriser. A Water Technics sheet lists a KleenPool copper option. That sheet is not this Amazon title.

Volume before brand

Start with the water you will leave in, not the star rating. A title that says 15,000 gallons is not 15,000 litres. Do not paste a conversion onto a bullet that does not print it. HTH is the clearest rate: 300 ml equals 10 m³, so 3 litres is 100 m³. CATAGEN asks for 3.5 litres or 5 litres per 50 m³ and sells 2 litres. Ultimate and Blu Line print no dose on Amazon. A brand PDF is not the bullet you are paying for.
Kleen Pool Kleen Pool algaecide, 1 L

Testsieger

Kleen Pool Kleen Pool algaecide, 1 L
7.5/10about £17.95
View offer

Sanitiser, copper, and scale

Copy only the sanitiser the label names. Only Blue Horizons winter time names chlorine and bromine. HTH names active and passive wintering. CATAGEN says treatment without filtration. Kleen Pool says all year round. Copper is printed on two listings. Relax says organo-copper, spelled Cooper. Blue Horizons winter time says copper free. A brand page can say copper-free while the Amazon bullet stays silent. Stains, blonde hair, or a biguanide pool belong on the copper-free or copper winteriser page.

A sensible order of checks

Work down the label. Does one pack cover the water you will leave in, in the units that listing prints? Does it name your sanitiser, or a wintering mode you can follow? Does it say copper or copper free if that matters? Is there a stain or scale line if the water is hard? Price comes after those checks. About £12.70 is not a full 50 m³ dose, and about £43.95 is a rate you can multiply. A 4.6 can be UK texts, or buyers in Belgium, France, and Germany. When the bottle is chosen, the close-down order is how to winterise a swimming pool.

Common questions

How many litres of winteriser do I need?
You need the rate printed for the water you will leave in, not a guess from the title. HTH prints 300 ml per 10 m³, so a 3 litre pack is 100 m³ at that rate. CATAGEN prints 3.5 litres or 5 litres per 50 m³ and sells a 2 litre pack. Blue Horizons winter time says up to 15,000 gallons and does not print litres beside those gallons. Blu Line and Ultimate print no rate on Amazon.
Is a winteriser the same as a summer algaecide?
Not on every label. Blue Horizons winter time says it can also be used in summer, and Kleen Pool's title says all year round and does not say winteriser. HTH, CATAGEN, Relax, Blu Line, AquaJoyer, and POOLHOUR are worded as winter or closing products. A summer algaecide you already own is only a winter dose if that bottle says so.
Do I need a copper-free winteriser?
You need one when stains, blonde hair, a copper fitting, or a biguanide pool are already a worry. In this set only Blue Horizons winter time says copper free on the Amazon bullet, and only Relax says copper. The other seven listings do not print a copper line. A brand PDF that says copper-free does not change an Amazon bullet that is silent.
What if the bottle is smaller than the dose on the same page?
Treat it as a partial pack. CATAGEN is the worked example: about £12.70 for 2 litres, against a bullet that asks for 3.5 litres or 5 litres per 50 m³. One bottle does not meet that seller's own 50 m³ line. Buy enough litres to meet the printed rate, or choose a pack that is the treatment it claims.
Are US closing-kit reviews useful in the UK?
They tell you how those reviewers described a North American close, not a British winter. AquaJoyer's saved sample is the United States and Canada, including one report of black water at opening. POOLHOUR's 16 saved texts are labelled United States. HTH's saved texts are Belgium, France, and Germany. Do not read a 4.4 from 479 as 479 UK winters.
Does 15,000 gallons mean 15,000 litres?
No. Gallons and litres are different, and imperial gallons and US gallons are different again. Blue Horizons winter time, AquaJoyer, and POOLHOUR print gallon ceilings and do not print a litre conversion on the saved Amazon bullets. HTH and CATAGEN speak in cubic metres. Kleen Pool and Relax speak in litres, with Relax using the spelling liters.

Related pages

Best pool winteriser

Pool winteriser

How to winterise an above ground pool safely in the UK

Copper free winter algaecide or a copper winteriser

How to winterise a swimming pool before hard frost

Sources

Public UK sources used for this page in October 2026. Shelf prices and star lines are the amazon.co.uk figures for these nine listings, dated 19 October 2026.

  1. HSE: Biocidal product authorisation (retrieved 21 October 2026)
  2. HSE: Managing health and safety in swimming pools (HSG179) (retrieved 21 October 2026)
  3. HSE: Swimming pools (retrieved 21 October 2026)
  4. PWTAG: Microbiological testing TN53 (retrieved 21 October 2026)
  5. Lo-Chlor: Winteriser algaecide tech sheet (retrieved 21 October 2026)
  6. Blue Horizons Ultimate winteriser sheet (retrieved 21 October 2026)
  7. Water Technics: Swimming pool winterising guide (retrieved 21 October 2026)
  8. HTH: Super Winterprotect 3 L (retrieved 21 October 2026)