Everyday Lifeโฑ 4 min read
How to Calculate Swimming Pool Chemical Doses
A well-balanced pool is safe, clear, and doesn't irritate eyes. Here is how to calculate chlorine dose, pH adjustment, and alkalinity correction for any pool volume.
Pool chemistry comes down to four parameters: free chlorine, pH, total alkalinity, and calcium hardness. Getting all four right requires knowing your pool volume and calculating doses correctly.
Calculate Pool Volume First
Rectangular pool:
Volume (litres) = Length x Width x Average Depth x 1,000
Circular pool:
Volume = ฯ x radius^2 x depth x 1,000
Irregular pool: divide into sections, calculate each.
Example: 8m x 4m pool, 1.4m average depth:
Volume = 8 x 4 x 1.4 x 1,000 = 44,800 litres = 44.8 m3
Always know your pool volume -- all chemical dosing is relative to it.
Chlorine Dosing
Target free chlorine: 1-3 ppm (mg/litre)
Shock treatment (algae, after heavy use): 5-10 ppm temporarily
Granular chlorine (e.g. trichlor 90% active):
1 ppm increase in 44,800 litre pool:
= 44,800 litres x 0.001 g/litre / 0.9 = 49.8g granules
To raise from 0.5 ppm to 2 ppm (increase by 1.5 ppm):
Dose = 1.5 x 49.8 = 74.7g = approximately 75g
Liquid chlorine (12% sodium hypochlorite):
1 ppm increase in 44,800 litres:
= 44,800 x 0.001 / 0.12 = 373ml
For 1.5 ppm increase: 560ml
Always add chemicals to water (not water to chemicals).
Add chlorine in the evening to prevent UV degradation.
pH Adjustment
Target pH: 7.2-7.6 (ideal: 7.4)
Below 7.2: corrosive, irritates eyes and skin
Above 7.6: chlorine becomes ineffective (>pH 8.0: 20% effective vs 7.2)
To RAISE pH (pH increaser / sodium carbonate):
Raise 44,800 litres by 0.2 pH units:
Approximately 250g of sodium carbonate per 50,000 litres per 0.2 pH
Adjusted for 44,800L: 250 x (44,800/50,000) = 224g
To LOWER pH (pH minus / sodium bisulphate):
Lower by 0.2 pH units in 44,800 litres:
Approximately 400g sodium bisulphate per 50,000 litres per 0.2 pH
Adjusted: 400 x (44,800/50,000) = 358g
Always test again 4 hours after adding pH chemicals before re-dosing.
Total Alkalinity and Calcium Hardness
Total Alkalinity (TA):
Target: 80-120 ppm
TA buffers pH changes -- too low and pH fluctuates wildly
To raise TA by 10 ppm in 44,800 litres:
Add sodium bicarbonate (bicarb): 1.4g per 1,000 litres per 10 ppm
Dose = 1.4 x 44.8 = 62.7g bicarb
Calcium Hardness:
Target: 200-400 ppm
Too low: plaster and grout erode (corrosive water)
Too high: calcium deposits, cloudy water
Raise calcium by 10 ppm: calcium chloride at 1.3g/1,000L per 10 ppm
44,800L: 1.3 x 44.8 = 58.2g calcium chloride