Tradefox Tools

Pool chemicals. The dose, worked out.

Work out how much product to add to move a reading from where it is to where you want it, then check the water balance with the Langelier Saturation Index.

Safety first.

Never mix pool chemicals together, dry or in solution. Chlorine products and acids release toxic chlorine gas if they meet. Calcium hypochlorite and trichlor can react violently with each other and with organic matter.
Always add chemical to water, never water to chemical. Pre-dissolve or dilute only as the product label allows, in a clean bucket kept for that one chemical.
Wear the PPE the label and safety data sheet specify: as a minimum chemical splash goggles or face shield, chemical-resistant gloves, and protect skin and clothing.
Follow the product label and safety data sheet. They override any figure here. Under COSHH the employer must have a risk assessment, safe storage and trained staff for each chemical used.
Dose with the circulation running and no bathers in the water. Retest before reopening.

Pool chemical dose calculator

Pool volume

Use the pool volume calculator if you do not know it.

Free chlorine

Current free chlorine

mg/L

Target free chlorine

mg/L

Product

Available chlorine (from the label)

How the label states strength

Trade % (w/v) means grams of available chlorine per 100 mL. A w/w figure is converted using an approximate density.

–

Enter the volume, both readings and the product strength.

The equation

m = \frac{(C_{t} - C_{c}) \times V}{10 \times s}

where

$m$: product needed (mL for a liquid, since strength is per volume)
$C_{t}$: target free chlorine (mg/L)
$C_{c}$: current free chlorine (mg/L)
$V$: pool volume (litres)
$s$: available chlorine in the product (%)

Explain this equation

Raising free chlorine by 1 mg/L in 1 litre needs 1 mg of available chlorine, so the pool needs the rise times the volume in milligrams. Divide by 1,000 to get grams.

The product is only partly available chlorine. A product at s% contains s grams of available chlorine per 100 g (solids) or per 100 mL (liquid stated as w/v). Dividing grams needed by s/100 gives grams or millilitres of product. The two factors of 1,000 and 100 combine into the 10 on the bottom line.

Available chlorine falls as liquid chlorine ages, especially when warm. Old stock doses weaker than its label, so retest after circulating.

Sources and limits

Doses are worked out from the chemistry of each product (molar mass and active content). They agree with the treatment tables published by test-kit makers such as Taylor Technologies and with manufacturer labels, to within rounding.
Target ranges, test frequencies and operating practice for UK pools: Pool Water Treatment Advisory Group (PWTAG), Swimming Pool Water: Treatment and quality standards for pools and spas, and its technical notes.
Health and safety management for pools: HSE HSG179, Health and safety in swimming pools. Chemical hazards: HSE COSHH guidance.
Chemical handling and water quality in the US model code: CDC Model Aquatic Health Code (MAHC).
The pH dose uses the carbonate equilibrium at 25 °C with standard constants. Borates, temperature and ionic strength shift the real demand, so it is an estimate: dose in stages and retest.
The LSI uses the widely published Langelier formula with a cyanurate correction to alkalinity. Some references band LSI slightly differently.

Results are arithmetic only. Always follow the product label and your pool operating procedures.

Worked examples

Each example runs the calculator above for a 50,000 litre pool. Open one to see the equation, the reasoning behind it and every step of the arithmetic.

Raise free chlorine from 1 to 3 mg/L with liquid chlorine

A 50,000 litre pool at 1 mg/L free chlorine, raised to 3 mg/L with sodium hypochlorite at 12.5% available chlorine (trade w/v). The equation for chlorine is shown with the calculator above.

Working

$Δ C = 3.00 - 1.00 = 2.00 mg/L$
$m = \frac{2.00 \times 50,000}{10 \times 12.50} = 800.0 mL$

Result

Add 800 mL of sodium hypochlorite at 12.5% w/v to raise free chlorine by 2.00 mg/L.
Sodium hypochlorite is alkaline and pushes pH up slightly. Recheck pH after it has circulated.

Lower pH from 7.8 to 7.4 with hydrochloric acid

A 50,000 litre pool at pH 7.8 with total alkalinity 100 mg/L and no stabiliser, brought down to pH 7.4 with 32% hydrochloric acid.

C_{T} = \frac{TA - {A_{cy}}_{1}}{{(α_{1} + 2 α_{2})}_{1}}

n = V \times [(C_{T} β_{1} + {A_{cy}}_{1}) - (C_{T} β_{2} + {A_{cy}}_{2})]

β = α_{1} + 2 α_{2} = \frac{[H^{+}] K_{1} + 2 K_{1} K_{2}}{{[H^{+}]}^{2} + [H^{+}] K_{1} + K_{1} K_{2}}

where

$C_{T}$: total carbonate in the water (mol/L), assumed unchanged by acid in the short term
$TA$: total alkalinity (equivalents per litre; mg/L as CaCO₃ ÷ 50,044)
$A_{cy}$: cyanurate alkalinity (eq/L), from cyanuric acid with pKa 6.88
$β$: carbonate alkalinity per mole of carbonate at a given pH
$K_{1}, K_{2}$: carbonic acid constants at 25 °C (pK 6.35 and 10.33)
$n$: acid needed (mol of H⁺), or for soda ash the base dose found the same way
$V$: pool volume (litres)
$1, 2$: subscripts for the current and target pH

pH does not move in a straight line with dose. How much acid it takes depends mostly on total alkalinity, which is the buffer that soaks up acid. So the calculator models the buffer rather than using a single factor.

First it works out how much carbonate is in the water from the alkalinity and pH you measured, after taking off the part of the alkalinity that comes from cyanuric acid. Then it works out what the alkalinity would be at the target pH with the same carbonate. The difference is the acid needed, in moles, per litre.

Hydrochloric acid supplies one mole of acid per 36.46 g of HCl. Sodium bisulfate supplies about one mole per 120.06 g. Soda ash adds carbonate as well as raising pH, so its dose is solved with both effects included, at 105.99 g per mole.

Real pools also contain borates, phosphates and other buffers, and carbon dioxide gassing off changes things over the following hours. Treat the figure as approximate. Add about two-thirds, circulate, retest and then top up.

Working

$TA = \frac{100}{50044} = 1.998e-3 eq/L$
${A_{cy}}_{1} = 0.000e+0 eq/L, β_{1} = 0.9687, β_{2} = 0.9193$
$C_{T} = 2.062e-3 mol/L$
$n = 50,000 \times 1.022e-4 = 5.11 mol H⁺$
$m = 5.11 \times 36.46 = 186 g HCl$
$Vol = \frac{186}{0.3200 \times 1.159} = 502 mL of acid$

Result

Add about 502 mL of 32.0% hydrochloric acid (density about 1.16 g/mL) to lower pH from 7.8 to 7.4.
Dose in stages. Add about 335 mL first, diluted or poured slowly into the deep end or a return flow as the label directs, circulate for at least an hour (or one turnover), retest, then add the rest if needed. Acid demand is approximate.
Total alkalinity will fall to about 95 mg/L.

Raise total alkalinity from 80 to 100 mg/L

A 50,000 litre pool with total alkalinity of 80 mg/L as CaCO₃, raised to 100 mg/L with sodium bicarbonate.

m = \frac{({TA}_{t} - {TA}_{c}) \times V}{1000 \times p} \times \frac{84.01}{50.04}

where

$m$: sodium bicarbonate needed (g)
${TA}_{t}, {TA}_{c}$: target and current total alkalinity (mg/L as CaCO₃)
$V$: pool volume (litres)
$p$: product purity as a fraction
$84.01$: molar mass of sodium bicarbonate, NaHCO₃ (g/mol), one equivalent per mole
$50.04$: equivalent weight of CaCO₃, the unit alkalinity is reported in

Alkalinity is reported as if it were calcium carbonate, so a rise of 1 mg/L means one-fiftieth of a milliequivalent per litre (50.04 mg is one equivalent of CaCO₃).

Each mole of sodium bicarbonate, 84.01 g, supplies one equivalent of alkalinity. So every 1 mg/L rise needs 84.01 ÷ 50.04 = 1.68 mg of bicarbonate per litre of water.

Bicarbonate barely changes pH, which is why it is the product of choice for alkalinity. Soda ash raises alkalinity too but pushes pH up hard.

Working

$m = \frac{20 \times 50,000}{1000 \times 1.000} \times 1.679 = 1,679 g$

Result

Add 1.68 kg of sodium bicarbonate (≈ 3.70 lb) to raise total alkalinity by 20 mg/L.

Raise calcium hardness from 200 to 250 mg/L

A 50,000 litre pool at 200 mg/L calcium hardness, raised to 250 mg/L with 94% anhydrous calcium chloride.

m = \frac{({CH}_{t} - {CH}_{c}) \times V}{1000 \times p} \times \frac{110.98}{100.09}

where

$m$: calcium chloride product needed (g)
${CH}_{t}, {CH}_{c}$: target and current calcium hardness (mg/L as CaCO₃)
$V$: pool volume (litres)
$p$: CaCl₂ content of the product as a fraction
$110.98$: molar mass of calcium chloride, CaCl₂ (g/mol)
$100.09$: molar mass of calcium carbonate, the unit hardness is reported in

Hardness is reported as calcium carbonate. One mole of calcium shows up as 100.09 mg/L of hardness, and one mole of calcium chloride (110.98 g) supplies one mole of calcium.

So each 1 mg/L rise needs 110.98 ÷ 100.09 = 1.109 mg of pure CaCl₂ per litre. Divide by the CaCl₂ content of the product: flake calcium chloride is the dihydrate and is only about 77% CaCl₂, so you need more of it than of anhydrous pellets.

Calcium chloride gets hot as it dissolves. Pre-dissolve in a large bucket of water, adding the chemical slowly to the water.

Working

$m = \frac{50 \times 50,000}{1000 \times 0.940} \times 1.109 = 2,949 g$

Result

Add 2.95 kg of calcium chloride at 94% CaCl₂ (≈ 6.50 lb) to raise calcium hardness by 50 mg/L.
Pre-dissolve and add slowly around the pool with the pump running. Large doses can cloud the water briefly.

Add stabiliser to reach 30 mg/L cyanuric acid

A new 50,000 litre outdoor pool with no stabiliser, dosed to 30 mg/L cyanuric acid with 99% granular stabiliser.

m = \frac{({CYA}_{t} - {CYA}_{c}) \times V}{1000 \times p}

where

$m$: stabiliser product needed (g)
${CYA}_{t}, {CYA}_{c}$: target and current cyanuric acid (mg/L)
$V$: pool volume (litres)
$p$: product purity as a fraction

Cyanuric acid is reported as itself, so 1 mg/L in 1 litre is simply 1 mg of cyanuric acid. Multiply by the volume, convert milligrams to grams and allow for purity.

Granular stabiliser dissolves slowly, often over several days. Add it as the label directs (commonly in a sock in the skimmer or pre-dissolved) and do not retest for a few days or backwash in the meantime.

Stabiliser protects chlorine from sunlight but also slows how fast chlorine works, so more free chlorine is needed as cyanuric acid rises. Many UK indoor and public pools run unstabilised: follow PWTAG guidance and your operating procedures.

Working

$m = \frac{30 \times 50,000}{1000 \times 0.990} = 1,515 g$

Result

Add 1.52 kg of cyanuric acid at 99% (≈ 3.34 lb) to raise stabiliser by 30 mg/L.

Check water balance with the Langelier Saturation Index

Pool water at pH 7.5 and 27 °C with calcium hardness 300 mg/L, total alkalinity 100 mg/L, no cyanuric acid and 1,000 mg/L total dissolved solids.

LSI = pH - {pH}_{s}

{pH}_{s} = (9.3 + A + B) - (C + D)

A = \frac{\log_{10} TDS - 1}{10}

B = - 13.12 \log_{10} (T + 273) + 34.55

C = \log_{10} CH - 0.4

D = \log_{10} A_{c}, A_{c} = TA - \frac{CYA}{2.58 (1 + 10^{6.88 - pH})}

where

${pH}_{s}$: the pH at which the water would be exactly saturated with calcium carbonate
$TDS$: total dissolved solids (mg/L)
$T$: water temperature (°C)
$CH$: calcium hardness (mg/L as CaCO₃)
$A_{c}$: carbonate alkalinity (mg/L as CaCO₃)
$TA$: total alkalinity (mg/L as CaCO₃)
$CYA$: cyanuric acid (mg/L)

The Langelier Saturation Index compares the actual pH with the pH at which the water would be exactly saturated with calcium carbonate. Zero means balanced. A negative value means the water is under-saturated and will tend to dissolve calcium from grout, plaster and cement and corrode metals. A positive value means it is over-saturated and will tend to deposit scale on surfaces, heat exchangers and in filters.

Each term adjusts the saturation pH: A for dissolved solids, B for temperature (warmer water scales more readily), C for calcium and D for carbonate alkalinity. Higher calcium or alkalinity lowers the saturation pH, so the same pH sits nearer the scaling side.

Cyanuric acid shows up in the alkalinity test but does not help hold calcium carbonate in balance, so its share is subtracted first. At pH 7.5 that share is close to one-third of the cyanuric acid reading.

A range of about −0.3 to +0.3 is widely treated as balanced. Adjust pH first, as it moves the index most easily, then alkalinity or calcium.

Working

$A_{c} = 100 - 0.0 = 100.0$
$A = \frac{\log_{10} 1,000 - 1}{10} = 0.200$
$B = - 13.12 \log_{10} (27.0 + 273.15) + 34.55 = 2.047$
$C = \log_{10} 300 - 0.4 = 2.077$
$D = \log_{10} 100.0 = 2.000$
${pH}_{s} = (9.3 + 0.200 + 2.047) - (2.077 + 2.000) = 7.47$
$LSI = 7.50 - 7.47 = 0.03$

Result

Langelier Saturation Index: +0.03
Balanced. The water is close to saturation with calcium carbonate: neither aggressive nor scale-forming.
Saturation pH 7.47. Carbonate alkalinity 100 mg/L.

Related: find the water volume first with the pool volume calculator, and log readings with the pool maintenance checklist. For pool plant faults, use the troubleshooting guide and appliance manuals finder.

Pool chemicals. The dose, worked out.

Pool chemical dose calculator

Free chlorine

pH

Total alkalinity

Calcium hardness

Cyanuric acid (stabiliser)

Langelier Saturation Index

The equation

Sources and limits

Worked examples

Raise free chlorine from 1 to 3 mg/L with liquid chlorine

Result

Lower pH from 7.8 to 7.4 with hydrochloric acid

Result

Raise total alkalinity from 80 to 100 mg/L

Result

Raise calcium hardness from 200 to 250 mg/L

Result

Add stabiliser to reach 30 mg/L cyanuric acid

Result

Check water balance with the Langelier Saturation Index

Result

Pool chemical dosage calculator

Apps

Product

For Organisations

Campaigns

Support

About us

Social