Everyday Life๐
15 April 2025โฑ 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.
JW
James WhitfieldPersonal Finance & Maths WriterJames has written about personal finance, health metrics, and everyday mathematics for over six years. He holds a BSc in Mathematics from the University of Leeds.
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