Pool Volume Calculator
Gallons and litres for rectangular or round pools.
Pool & Recreation
Pool Shock Calculator
How much shock to add to raise free chlorine by a set amount, using the strength of the product you actually have.
Your result
Sample values2.57
Pounds of product
41.1
Ounces of product
10.0
Chlorine added, ppm
20,000
Pool volume, gallons
20,000 gallons, 1.0 to 11.0 ppm, 65% cal-hypo.
Typical available chlorine
| Product | Available chlorine |
|---|---|
| Cal-hypo granular | about 65% |
| Dichlor granular | about 56% |
| Liquid chlorine | about 12.5% |
| Trichlor | about 90% |
| Always check | The label on your tub |
How to use this calculator
Test the water for free chlorine, decide what level you want to reach, and enter the strength printed on the product you are holding. The calculator works out how much of that product delivers the difference.
Do not assume products are interchangeable by weight. A pound of 65% cal-hypo delivers more than five times the chlorine of a pound of 12.5% liquid, so a dose worked out for one is badly wrong for the other. The strength field stays editable because labels vary between brands and even between batches.
Formula
Increase = Target − Current
Water weight = Volume × 8.345 lb/gal
Chlorine needed = Water weight × Increase ÷ 1,000,000
Product = Chlorine needed ÷ (Strength ÷ 100)
- Available chlorine
- the percentage of the product that is usable chlorine
- 8.345
- pounds in a US gallon of water
Liquid chlorine is converted to gallons at 9.6 lb per gallon, since it is sold by volume.
Worked example
A 20,000 gallon pool going from 1.0 to 11.0 ppm with 65% cal-hypo.
- Increase: 11.0 − 1.0 = 10.0 ppm
- Water weight: 20,000 × 8.345 = 166,900 lb
- Chlorine needed: 166,900 × 10 ÷ 1,000,000 = 1.669 lb
- Product: 1.669 ÷ 0.65 = 2.57 lb
- Or 41.1 ounces
2.57 lb of 65% cal-hypo
Understanding your results
This calculates the dose to reach a chlorine level, not the dose to solve a problem. Breakpoint chlorination — the shock that clears combined chloramines — needs roughly ten times the combined chlorine reading, which is a different calculation and needs a test that separates free from total chlorine.
Shock in the evening. Sunlight destroys unstabilised chlorine quickly, so a dose added at midday can be half gone before it has circulated. Run the pump overnight and retest in the morning.
Questions
About this calculator
No, and this is the most expensive mistake in pool chemistry. Cal-hypo is around 65% available chlorine, dichlor 56%, liquid only 12.5%. A dose calculated for granular shock will barely register if you use the same weight of liquid, which is why the strength field is a required input rather than an assumption.
About 7.8 ppm in 10,000 gallons at 65% strength. That figure is a consequence of the formula rather than a rule: it falls proportionally as pool volume rises and as product strength drops.
Cal-hypo yes — dissolve it in a bucket of pool water and pour it around the deep end, because undissolved granules sitting on a vinyl liner or coloured plaster will bleach a permanent mark. Dichlor dissolves quickly enough to broadcast.
Until free chlorine falls back below about 4 ppm, which after a full shock usually means overnight with the pump running. Test rather than guessing, and never rely on smell — a strong chlorine smell means chloramines, which is a sign you needed the shock, not that it worked.
Keep going
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Results are based on the values you enter and on typical material
properties. Densities, coverage rates and product sizes vary between
suppliers, so confirm quantities against your supplier's data before
ordering. Structural work must be signed off by a qualified
professional and meet your local building code.