Pool Volume & Chlorine Dose Calculator

Understanding the pool volume and chlorine calculator — concept and principle

This calculator estimates the water volume of a rectangular pool with a sloping floor and the amount of chlorine product needed to raise the free chlorine level by a chosen amount. Pool owners use it when opening a pool, after heavy use or rain, and whenever a test shows chlorine below the target range.

44.8 m³ of water needs 89.6 g of chlorine for +2 ppm — about 138 g of 65% product.

How it is calculated

Volume (m³) = length × width × (shallow depth + deep depth) ÷ 2. The average of the two depths stands in for a floor that slopes evenly.
Litres = m³ × 1,000.
Product to add (g) = volume (m³) × ppm rise ÷ (strength % ÷ 100).

Strength is the available chlorine stated on the product, for example 65% for many calcium hypochlorite granules.

The principle

One part per million (ppm) by mass in water is the same as one milligram per litre. Since 1 m³ of water is 1,000 L, raising 1 m³ by 1 ppm needs 1,000 mg, which is exactly 1 g of pure available chlorine. So the grams of chlorine needed equal m³ × ppm. A product that is only 65% available chlorine must be used in proportionally larger amounts, which is why the result is divided by the strength. The volume formula treats the side profile as a trapezium.

Worked example

Inputs: 8 m × 4 m pool, depth from 1.0 m to 1.8 m, target rise 2 ppm, product strength 65%.
Average depth: (1.0 + 1.8) ÷ 2 = 1.4 m.
Volume: 8 × 4 × 1.4 = 44.8 m³, or 44,800 L.
Pure chlorine needed: 44.8 × 2 = 89.6 g.
Product: 89.6 ÷ 0.65 ≈ 137.8 g.

Each point is the calculator re-run with only “Length (m)” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Length (m)	Volume (m³)
2	11.2
4	22.4
6	33.6
8	44.8
10	56
12	67.2
16	89.6

How to read the result

The result is the dose for a rise, not for the final level, so test first and enter the difference between your reading and your target. Residential pools are commonly kept at around 1–3 ppm free chlorine; sunlight, warm water, swimmers and organic debris use it up, so the dose needed each day varies. A weaker liquid product needs far more: at 12.5% strength the same rise would need about 717 g of product. Add chlorine with the pump running and retest after it has circulated.

Common mistakes

Using the deep-end depth for the whole pool, which overstates volume.
Ignoring the product's actual strength, or treating a liquid's grams as millilitres.
Dosing without testing pH; chlorine works much less effectively at high pH.

Keep in mind: Pool chemicals are hazardous. Never mix different chlorine products or add water to chemicals, store them dry and separate, and follow the label and local health rules. This estimate does not replace water testing.

Frequently asked questions

How do I find the volume of a round pool?

Use π × radius² × average depth. A 4 m diameter pool 1.2 m deep holds about 15.1 m³.

What is the difference between free and total chlorine?

Free chlorine is the active sanitiser; combined chlorine (total minus free) has already reacted with contaminants and causes the "chlorine smell".

Can I swim right after adding chlorine?

Wait until the water has circulated and a test shows levels back in the normal range; the product label gives specific times.

Sources: World Health Organization, Guidelines for Safe Recreational Water Environments, Volume 2: Swimming Pools and Similar Environments (2006) · U.S. Centers for Disease Control and Prevention, Model Aquatic Health Code

Checked 8 Nov 2026. Typical coverage rates and standard formulas. Always check the product label and local building rules; buy 5–10% extra for waste.

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