Pool & spa
Pool Shock Calculator
Doses a shock, where the point is reaching a threshold rather than adding a lot.
Result
Dose against a test result, never against a schedule. Add chemicals in stages with the pump running, wait for a full turnover, then retest before adding more. Never mix chemicals, and never add water to acid. Where a product label differs from the figure here, the label wins. Shock at dusk or after dark, because sunlight destroys a large share of it within hours. Cal hypo also adds calcium, which matters in hard water; liquid chlorine adds none. Swimmers stay out until free chlorine falls back to normal.
Get a Free Business AuditHow the calculation works
Shocking means raising free chlorine high enough to break down combined chlorine, the spent chloramines that cause the strong smell and eye irritation. The dose is calculated from the combined chlorine present, because the reaction needs free chlorine at a multiple of it to proceed, which is why a shock is a calculated threshold rather than a generous handful. Falling short leaves the combined chlorine in place and the symptoms with it.
What moves the number
- Combined chlorine level, which sets the threshold to reach.
- Volume, which scales the quantity.
- Which product is used, since available chlorine per unit differs.
- Cyanuric acid level, which raises the free chlorine needed for the same effect.
Where this one goes wrong
An under-dose achieves nothing, which is the central point about shocking. The reaction that destroys combined chlorine requires free chlorine to reach a multiple of the combined chlorine present, so adding a partial dose raises chlorine briefly without breaking anything down, and the smell and irritation persist. Calculate from the combined chlorine reading rather than adding a standard bag. Shock after sunset or at night, because sunlight destroys the elevated chlorine before it can do its work. And do not swim until levels have fallen back to normal.
Before you quote
- Calculate the dose from the combined chlorine reading, not a standard quantity.
- Shock at night so sunlight does not destroy the elevated chlorine.
- Keep swimmers out until free chlorine falls back to normal.
Where the rules come from
Questions
About this calculator
Why didn't shocking fix the chlorine smell?
Almost certainly an under-dose. Breaking down combined chlorine needs free chlorine to reach a multiple of the combined chlorine present, so a partial dose raises chlorine briefly without completing the reaction. The smell is the combined chlorine, and it stays until the threshold is reached.
Should I shock during the day or at night?
At night or after sunset. Sunlight destroys chlorine, so an elevated level added in daylight falls before it can break down the combined chlorine. Shocking in the evening gives the reaction all night to proceed.
What does the strong chlorine smell actually mean?
Counterintuitively, usually too little effective chlorine rather than too much. The smell comes from chloramines, which are chlorine that has already reacted and is spent. A properly sanitised pool with adequate free chlorine has very little smell.
The formula
about 0.114 lb of 73% cal hypo per 10,000 gallons raises free chlorine 1 ppm
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How to use this calculator
- Pool volume (gallons). Enter your own figure; the page starts at 20,000.
- Current free chlorine (ppm). Enter your own figure; the page starts at 1.
- Shock target (ppm). Enter your own figure; the page starts at 10.
- Price per pound or gallon ($). Enter your own figure; the page starts at $3.4.
The result updates as you type, so you can try a figure, see what it does to calcium hypochlorite to add, and change it back. Nothing you enter is sent anywhere or saved.
A worked example
These are the figures the calculator opens with. Every number in the results column below is produced by the same calculation the page runs, so you can follow it through before putting your own figures in.
What goes in
| Input | Value |
|---|---|
| Pool volume (gallons) | 20,000 |
| Current free chlorine (ppm) | 1 |
| Shock target (ppm) | 10 |
| Price per pound or gallon | $3.4 |
What comes out
| Result | Value |
|---|---|
| Calcium hypochlorite to add main result | 2.1 |
| Change needed | 9.0 |
| Estimated cost | $6.98 |
| Dose per 10,000 gallons | 1.0 |
Add about 2.1 to move 20,000 gallons from 1 to 10.
How sensitive the answer is
Holding everything else steady and changing only price per pound or gallon, this is what happens to calcium hypochlorite to add. It is worth knowing which of your inputs the answer actually hinges on, and which barely move it.
| Price per pound or gallon ($) | Calcium hypochlorite to add |
|---|---|
| $1.7 | 2.1 |
| $2.55 | 2.1 |
| $3.4 example above | 2.1 |
| $5.1 | 2.1 |
| $6.8 | 2.1 |
Every other input is held at the example value while this one changes, which is not how a real job behaves. Treat it as a guide to which number matters most, not as five separate quotes.
What each figure means
- Pool volume (gallons)
- A number, between 0 and 2000000, starting at 20,000.
- Current free chlorine (ppm)
- A number, between 0 and 60, starting at 1.
- Shock target (ppm)
- A number, between 0 and 60, starting at 10.
- Price per pound or gallon ($)
- A money amount, starting at $3.4.
What the results tell you
| Result | What it is |
|---|---|
| Calcium hypochlorite to add | The headline result. A decimal figure. |
| Change needed | A decimal figure. |
| Estimated cost | A money figure. |
| Dose per 10,000 gallons | A decimal figure. |
Estimating is the easy part
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