What should my pool water hardness be?

What hardness means

Hardness is the amount of dissolved calcium and magnesium in the water. The 6-in-1 strip measures total hardness, and the label shows 100-500 ppm as acceptable. For most pools and hot tubs, 150-250 ppm is the sweet spot to aim for.

Water that is too soft tries to take calcium from wherever it can - grout, plaster and metal - and hot tubs with very soft water tend to foam. Water that is too hard leaves chalky scale on tiles, heaters and jets, and can turn cloudy, particularly when pH is high.

It depends a lot on where you live

UK tap water varies enormously. London, the South East and East Anglia typically have hard water, often over 250 ppm. Scotland, Wales, the North West and much of the South West usually have soft water, sometimes under 50 ppm. Your water supplier publishes hardness for your postcode.

How to raise low hardness

• Balance alkalinity first, then hardness, then fine-tune pH.
• Work out the dose with the app's plan or the label. As a guide, about 1.5 g of hardness increaser per 1,000 litres raises hardness by 1 ppm.
• With the pump running, add the hardness increaser to a bucket of pool water - never water to chemical. It gets hot as it dissolves, so use a plastic bucket, add it slowly and stir.
• Pour it slowly around the deep end or in front of a return. Do not add any other chemical at the same time.
• Wait 24 hours (a couple of hours for a hot tub) and retest before adding more.

How to live with hard water

There is no easy chemical fix to lower hardness. Instead:

• Keep pH at the lower end, 7.2-7.4, and alkalinity near 80-100 ppm. This greatly reduces scaling.
• Ask us about a scale and stain preventer, which keeps minerals dissolved.
• Replace some of the water with softer water where possible. Hot tubs are easiest - their regular drain and refill resets hardness.

When to call a professional

If a hot tub heater keeps tripping or you find heavy scale in pipework, get it serviced before the heater element is damaged. For pools with very hard water, a professional can advise on scale treatment and water balance.

Frequently asked questions