How to Lower Pool Hardness: The Partial-Drain Calculation

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To lower pool hardness, replace part of the water with softer tap water; NSW and Queensland Health say no chemical takes calcium out. Fraction to replace = (reading − target) ÷ (reading − tap-water hardness). A 50,000 L concrete pool at 600 mg/L on Sydney water (calcium about 35 mg/L) needs 17.7 kL replaced to reach 400.

The short answer

Only water exchange lowers calcium. NSW Health’s 2026 guidelines and Queensland Health both say to drain some water and refill with lower-calcium make-up water; there is no remover to buy.
The tap-water rule: drain fraction = (reading − target) ÷ (reading − tap-water hardness). The common shortcut, 1 − target ÷ reading, assumes the new water has no calcium and under-drains.
Tap water hardness varies more than twentyfold. Sydney Water reported calcium hardness of 30–35 mg/L (April–June 2026); Yarra Valley Water area means were 10–43 mg/L total hardness (2025–26); Perth localities 29–228 mg/L (2024–25).
Same pool, different city: a 50,000 L concrete pool from 600 to 400 mg/L needs 17.7 kL replaced on Sydney water, 19.8 kL in South Perth and 26.9 kL in Two Rocks.
Fibreglass in a hard-water area may never reach 200 mg/L (the top of NSW Health’s fibreglass band) by dilution: Two Rocks tap water averages 228. NSW Health’s fallback is lower pH or alkalinity.

The tap-water drain table: share of a 50,000 L pool to drain and refill once, and kilolitres, to lower calcium hardness, by the hardness of the refill water (utility figures read 18 October 2026; bands from NSW Health 2026: 200–400 mg/L masonry, 80–200 mg/L fibreglass)
Pool and target	Sydney Water (calcium hardness ≤ 35)	Yarra Valley Water, hardest area (43)	Perth, South Perth/Kewdale (94)	Perth, Two Rocks (228)
Concrete, 600 → 400 mg/L	35.4% = 17.7 kL	35.9% = 18.0 kL	39.5% = 19.8 kL	53.8% = 26.9 kL
Fibreglass, 350 → 200 mg/L	47.6% = 23.8 kL	48.9% = 24.4 kL	58.6% = 29.3 kL	Not reachable: the tap water itself is 228

How do I lower the calcium hardness in my pool? Drain and refill, counting the tap water

There is one method, and both state health departments that publish public-pool guidance give it. NSW Health’s 2026 guidelines: if calcium hardness needs to be reduced, exchange water with make-up water containing lower levels of calcium hardness. Queensland Health: drain some water and introduce make-up water with lower calcium.

Test calcium hardness, not total hardness. Pool kits read calcium hardness as mg/L of calcium carbonate; that is the number the bands use.
Find your tap water’s hardness in your utility’s water-quality report (the table above uses the latest published figures from three utilities). If only total hardness is published, use it: calcium is part of it, so the answer errs towards draining slightly more.
Set the target at the top of your band, not the middle. NSW Health’s 2026 bands are 200–400 mg/L for a masonry pool or spa and 80–200 mg/L for vinyl or fibreglass; SA Health’s home-pool range is 150–400.
Work out the fraction: (reading − target) ÷ (reading − tap-water hardness). Multiply by the pool’s litres.
Drain once, refill once, then rebalance pH, alkalinity, chlorine and, in a salt pool, salt, because the new water dilutes all of them.

Worked example. A 50,000 L concrete pool reads 600 mg/L; target 400; Sydney Water calcium hardness at most 35. (600 − 400) ÷ (600 − 35) = 200 ÷ 565 = 35.4%, so drain and refill 17.7 kL. At Sydney Water’s 2026–27 usage price of $3.41 a kilolitre, the refill water costs about $60. The shortcut 1 − 400 ÷ 600 gives 33.3% and would leave the pool about 12 mg/L above target.

The tap-water rule: how much you drain depends as much on what comes out of your hose as on what is in your pool. If the reading came from a scaling or cloudy pool, why a pool goes cloudy and how to clean calcium off pool tiles cover the symptoms; this page covers the number.

How hard is the tap water I would refill with?

Each utility publishes it, in different formats:

Sydney Water reports by delivery system each quarter. For April to June 2026, Potts Hill showed total hardness 44–58 mg/L and calcium hardness 31–34 mg/L as calcium carbonate; Prospect East 48–57 total and 30–35 calcium. Sydney is one of the few utilities that separates calcium from total hardness, which is the figure a pool needs.
Yarra Valley Water (Melbourne) reports total hardness by sampling area for 2025–26: area means from 10 mg/L (Yarra Glen) to 43 mg/L (Whittlesea and Yarrambat), most areas 13–30.
Water Corporation (Perth) reports total hardness by locality for 2024–25: means from 29 mg/L (Dwellingup) to 228 mg/L (Two Rocks), with Yanchep 198, Neerabup 190, Mirrabooka 124 and South Perth/Kewdale 94. It footnotes Two Rocks: elevated hardness is characteristic of the source supplying this locality.

These are drinking-water figures measured for a 200 mg/L aesthetic guideline, not pool figures, and a utility’s average hides variation: Water Corporation’s Lexia locality ranged 76–160 mg/L across two samples. If your area is not listed, ask the utility, or test a glass of tap water with the same pool kit.

Hardness creeps up even if you never add calcium. Evaporation removes water but leaves calcium behind, and every top-up adds more. HealthyWA says an uncovered 8 m by 4 m pool can lose 15 mm a day. Over a 30-day month that is 14.4 kL of top-up water; in a pool of about 44.8 kL (1.4 m average depth, our assumption), Sydney water at 35 mg/L adds up to about 11 mg/L a month and Two Rocks water up to about 73. Splash-out, backwashing and rain overflow remove some of it, so treat those as ceilings. A pool cover slows the climb; what a pool adds to the water bill has the annual evaporation figure.

Lowering hardness in a fibreglass pool when the tap water is hard

Dilution can only take the pool down towards the tap water’s own hardness, never below it. A fibreglass pool at 350 mg/L can reach 200 on Sydney water by replacing 47.6% (23.8 kL of 50,000 L). On Two Rocks water at 228 it cannot reach 200 at all. NSW Health’s 2026 guidelines say as much: lowering calcium hardness is not always possible due to the composition of the fill water.

Their fallback is the saturation index. If calcium hardness is high, NSW Health says pH or total alkalinity should be lowered instead, so the water is less likely to scale even with the calcium where it is; it prefers pH at 7.2–7.6. The doses are on how to lower pH in a pool and how to lower alkalinity, and the index itself, with NSW Health’s factors, is worked through on how to raise pool hardness, which this page does not repeat.

The fill-water floor: no amount of draining takes a pool below the hardness of the water you refill it with. In that case, manage the index, not the number.

Before you drain: where the water goes and how much to take out at once

Draining a third or half of a pool is a bigger job than a top-up. Where the water may lawfully go is set by the water utility and the state: draining a pool to the sewer in NSW and the WA rules set them out. How low you can safely take the level, and why a high water table can lift an empty shell, is on what emptying and refilling a pool costs.

Drain once and refill once where you safely can. Topping up while the pool overflows mixes new water into old and needs more water for the same result; the stabiliser page, how to lower pool stabiliser, works that penalty through. Refill cost at other tariffs is on what filling a pool costs.

When high calcium is a surface problem, not a water problem

If the pool keeps scaling after the water is in range, or the interior is rough and etched from years of the wrong balance, the fix is the surface, not more draining. HealthyWA notes salt chlorinator electrodes scale faster where calcium is high; why a chlorinator stops making chlorine covers cleaning the cell. For a surface that needs work, pool resurfacing cost by interior compares the finishes; compare pool quotes on the same written scope.

Common questions

How do you lower calcium hardness in a pool?

Drain part of the water and refill with lower-calcium tap water. The NSW Health 2026 pool water quality guidelines (section 7.3) say to exchange water with make-up water containing lower calcium hardness; no chemical removes it.

How much water do I need to drain to lower calcium hardness?

(Reading − target) ÷ (reading − tap-water hardness), times the pool volume. A 50,000 L pool from 600 to 400 mg/L on Sydney water at 35 mg/L: 200 ÷ 565 = 35.4%, or 17.7 kL.

Can I lower pool hardness without draining?

Not the number itself. NSW Health’s 2026 guidelines say that when calcium hardness is high and cannot be lowered, pH or total alkalinity should be lowered instead, which reduces the tendency to scale.

What is the hardness of Sydney, Melbourne and Perth tap water?

Sydney Water reported calcium hardness of 30–35 mg/L in April–June 2026. Yarra Valley Water’s 2025–26 area means were 10–43 mg/L total hardness. Water Corporation’s Perth localities averaged 29–228 mg/L in 2024–25.

What should calcium hardness be in a fibreglass pool?

80–200 mg/L on NSW Health’s 2026 guidelines; 200–400 mg/L for concrete. SA Health gives 150–400 mg/L for all home pools.

Sources

General information only, current at 2026-10-18. Pool rules differ by state and by council, and penalty amounts are updated regularly. Confirm your own obligations with your council or a registered inspector before acting.

How to lower pool hardness