Filling a pool: what chemicals to add, and in what order
A freshly filled pool starts with whatever your fill water contains, which is usually no stabilizer, little or no chlorine, and alkalinity, hardness and pH set by your local water. Getting it balanced is straightforward if you go in the right order. This guide covers a pool refilled after draining, or a new vinyl, fiberglass or above-ground pool. For new plaster or pebble finishes, follow the startup procedure your builder or plasterer gives you, because fresh plaster needs special handling for its first few weeks.
Before you start
- Know your volume. Every dose depends on it. Use the pool volume calculator.
- Test the fill water for pH, alkalinity, calcium hardness and, if it's well water, metals (iron, copper, manganese).
- Have a reliable test kit. See how to test pool water.
The order
- Metals first, if any. If your fill water contains metals, add a metal sequestrant before any chlorine. Chlorine oxidizes dissolved metals, which can turn the water green or brown and stain the surface. See pool stains.
- Start circulation and filtration. Run the pump continuously for the first day or two.
- Stabilizer (CYA), for outdoor pools. It dissolves slowly, so add it early. Without it, sunlight destroys chlorine within hours. A common target is 30–50 ppm, or the range in your salt cell's manual. Use the stabilizer calculator.
- Total alkalinity into range (often 60–120 ppm, lower for salt pools). The alkalinity calculator gives the baking soda or acid dose.
- pH to about 7.4–7.6. The pH calculator uses your alkalinity to estimate the dose.
- Calcium hardness for your surface: higher for plaster, lower for vinyl and fiberglass. The calcium calculator gives the dose.
- Salt, if you have a salt chlorine generator, with the cell turned off. Use the salt calculator, wait 24 hours, retest, then turn the cell on.
- Chlorine last, to the level your CYA calls for. With a fresh fill, many owners take it a little above the daily target for the first day. The chlorine calculator gives the dose.
Add one chemical at a time, with the pump running, and allow time to circulate (an hour or more) between them. Never mix chemicals together. Read pool chemical safety first.
Worked example: a refilled 15,000-gallon pool
Say the fill water tests pH 7.8, alkalinity 70 ppm, calcium hardness 120 ppm, no CYA and no metals. For a vinyl pool:
| Step | Target | Approximate dose |
|---|---|---|
| Stabilizer | 0 → 40 ppm | about 5 lb granular cyanuric acid |
| Alkalinity | 70 → 90 ppm | about 4.2 lb baking soda |
| Calcium | 120 → 180 ppm | about 10.8 lb of 77% calcium chloride |
| Chlorine | 0 → 4 ppm | about 61 fl oz of 12.5% liquid chlorine |
Retest pH after the baking soda and calcium have mixed in. Then use the pH calculator if it needs moving. The doses come from the calculators on this site. Your own readings will give different numbers.
The first week
- Test daily. New water often drifts as stabilizer finishes dissolving and as the water warms.
- Retest CYA after about a week. Granular stabilizer can take days to show fully on a test.
- Brush daily, especially for a new surface, and keep the filter clean.
- Check the saturation index with the LSI calculator once everything has settled. Very soft fill water can be aggressive to plaster and grout.
Frequently asked questions
How soon can I swim after filling the pool?
Once chemicals have dissolved and mixed, chlorine and pH are in range, and the water is clear. That's often within a day for chemistry. A new plaster finish has its own timeline set by the builder.
Do I need to shock a new pool?
Not if the fill water is clean and you bring chlorine up properly. Some owners raise chlorine a little higher for the first day as a precaution. If the water arrives cloudy or green, or you used a well, follow the metals and shock steps.
Why is my new pool water green but clear?
Usually metals in the fill water, often copper or iron, oxidized by chlorine. Add a sequestrant and see pool stains.