Free tool No. 6
What to add to a spa, and how much
Keep free chlorine at 4 to 6 ppm (never below 2) or bromine at 4 to 6 ppm, pH 7.2 to 7.8, and the water no hotter than 40 °C. For example, a 1,500-litre spa with free chlorine at 1 ppm needs 10 g of dichlor (56%) to reach 5 ppm. Enter your own size and readings for your plan, in order.
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Worked example
A 1,500-litre spa with dichlor (56%), free chlorine 1 ppm and pH 7.8.
- 1
Find the chlorine target
Free chlorine at CYA 30 (assumed): 4 to 6 ppm, aim for 5, never below 2
- 2
Check pH
pH 7.8: within 7.2 to 7.8, so nothing to add
- 3
Work out the increase
5 ppm goal − 1 ppm now = 4 ppm
- 4
Convert the volume
1,500 L ÷ 3.785 = 396 US gal
- 5
Work out the dose
FC 1 → 5 ppm: 4 ppm × 396 gal ÷ 4149 = 0.4 oz
- 6
Round to what you can measure
0.38 oz = 10.8 g, rounded to 10 g
Add 10 g of dichlor (56%), then retest chlorine after 30 minutes.
Sprinkle it across the water (or pre-dissolve it in a bucket) with the pump running. It also adds about 3.64 ppm of stabiliser (CYA).
Spa target ranges
| Reading | Chlorine spa | Bromine spa |
|---|---|---|
| Sanitiser | Free chlorine 4 to 6, aim for 5 (never below 2) | Bromine 4 to 6, aim for 5 |
| pH | 7.2 to 7.8, aim for 7.5 | 7.2 to 7.8, aim for 7.5 |
| Total alkalinity | 50 to 90, aim for 60 | 50 to 90, aim for 60 |
| Calcium hardness | 100 to 250, aim for 150 | 100 to 250, aim for 150 |
| Stabiliser (CYA) | 20 to 40, aim for 30 | Not used |
| Water temperature | 40 °C or cooler | 40 °C or cooler |
Stabiliser doesn't protect bromine, so a bromine spa carries none. This calculator doesn't ask for calcium or stabiliser, so it plans no dose for them: test them now and then and compare. If stabiliser climbs above 50, replace water to bring it down.
Doses by spa size
| Product | Raises | 1,000 L | 1,500 L | 2,000 L |
|---|---|---|---|---|
| Dichlor (56%) | Free chlorine 1 ppm | 1.8 g | 2.7 g | 3.6 g |
| 12.5% liquid chlorine | Free chlorine 1 ppm | 8.0 mL | 12 mL | 16 mL |
| Buffer (sodium bicarbonate) | Alkalinity 10 ppm | 17 g | 25 g | 34 g |
The sizes are examples. Put your own spa's size in the calculator.
How to balance a spa
- 1
Check the temperature
Keep the water at 40 °C or cooler. If it's hotter, turn the heater down and leave the cover off until it cools, and don't soak until it's back in range.
- 2
Test the water
Test free chlorine or bromine and pH, and alkalinity if your kit reads it. A small tub changes fast, so test before you add anything.
- 3
Fix alkalinity, then pH
If alkalinity is below 50 ppm, add buffer (bicarb) first and wait an hour. Then bring pH back to 7.5: acid lowers it, and soda ash or borax raises it. Wait 30 minutes after each.
- 4
Raise chlorine or bromine
Bring free chlorine to 5 ppm (or bromine to 5 ppm) with the amount the calculator gives, added separately, with the pump running. For bromine, follow your product's label: the calculator gives the target, not the amount.
- 5
Retest
Retest chlorine 30 minutes after adding it, or bromine after an hour. Keep everyone out while chlorine or bromine is below 1 ppm.
How we calculate
The calculator runs the same chemistry engine as the app, for a spa with an acrylic shell, and gives its plan in order: keep everyone out if chlorine or bromine is below 1 ppm (too little to kill germs), raise alkalinity, fix pH, then raise chlorine or bromine and retest. When chlorine is that low, raising it comes first.
Free chlorine follows the published chart of chlorine against stabiliser (CYA) that pool-chemistry communities use, at CYA 30: 4 to 6 ppm, never below 2. Doses are ppm × US gallons (your litres, converted) ÷ a factor for each product; the pH dose also depends on alkalinity, and assumes 60 ppm when you leave it blank. Each amount is then rounded to what you can measure.
Its limits: it doesn't work out bromine amounts, which depend on the product, so follow its label. It doesn't ask for stabiliser (CYA), calcium or water temperature, so it plans nothing for them, and the chlorine target assumes CYA of 30. It doesn't plan a shock either.
Questions
How much chlorine do I put in a spa?
Enough to bring free chlorine to 4 to 6 ppm, aiming for 5. In a 1,500-litre spa, about 12 mL of 12.5% liquid chlorine or 2.7 g of dichlor raises it by 1 ppm, so going from 1 to 5 ppm takes 10 g of dichlor (56%). Enter your reading for your amount.
Should I use chlorine or bromine in my spa?
Both work. Bromine is held at 4 to 6 ppm and needs no stabiliser (CYA), because stabiliser doesn't protect bromine. Chlorine is held at 4 to 6 ppm with stabiliser around 30, built up with dichlor. Pick one and stay with it: don't mix the two in the same water.
When do I switch from dichlor to liquid chlorine?
Dichlor adds about 0.9 ppm of stabiliser (CYA) for every 1 ppm of chlorine. Use it until stabiliser reaches 20 to 40 ppm, then switch to liquid chlorine so it stops climbing. If it goes above 50, replace water to bring it down.
How hot should a spa be?
No hotter than 40 °C. If it reads hotter, turn the heater down and leave the cover off until it cools, and don't soak until it's back in range.
How soon can we get in after adding chemicals?
If chlorine or bromine is below 1 ppm, keep everyone out until chlorine is back above 2 ppm (or bromine is back to 4 to 6 ppm). Otherwise, with the pump running, retest chlorine 30 minutes after adding it (bromine after an hour), and keep the water at 40 °C or cooler.