Pool & spa
Chlorine Calculator
Doses chlorine, where the pH and the stabiliser level decide how well it works.
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. Free chlorine is consumed by sunlight and by whatever is in the water, so a pool needs topping up continually rather than dosing once. Cyanuric acid protects chlorine from UV but also slows it down, which is why the right chlorine level depends on the stabiliser level.
Get a Free Business AuditHow the calculation works
The dose is the gap between your current free chlorine and your target, times the pool volume, converted to the quantity of whichever product you use, since liquid chlorine, trichlor tablets, dichlor and cal hypo all have different available chlorine per unit. Free chlorine is the portion still available to sanitise, as distinct from combined chlorine which has already reacted and is no longer effective, and total chlorine which is both added together.
What moves the number
- Current free chlorine against the target, which is the gap being closed.
- Which product is used, since available chlorine per unit differs widely.
- pH, because chlorine's sanitising effectiveness falls sharply as pH rises.
- Cyanuric acid level, which protects chlorine and simultaneously weakens it.
Where this one goes wrong
Chlorine added while the pH is high is largely wasted, because the proportion existing as the effective sanitising form falls sharply as pH climbs. Correcting pH before dosing chlorine is both cheaper and more effective than adding more chlorine to a pool that is too alkaline. The stabiliser relationship is the other one that matters: cyanuric acid shields chlorine from sunlight and also binds it, so a pool with high cyanuric acid needs a higher free chlorine reading to achieve the same sanitising effect, which is why the two are read together. Never mix chlorine products with acid or with each other, which produces toxic gas.
Before you quote
- Correct pH before dosing chlorine rather than after.
- Read free chlorine against the cyanuric acid level, not in isolation.
- Use the product's available chlorine rather than a generic dose.
- Never mix chemicals; add each to water separately.
Where the rules come from
Questions
About this calculator
Why is my chlorine not working?
Most often pH that is too high, which sharply reduces the proportion of chlorine existing in the effective sanitising form, or cyanuric acid that is too high, which binds the chlorine. Adding more chlorine to either situation is treating the symptom. Correcting pH first is cheaper and more effective.
What is the difference between free, combined and total chlorine?
Free chlorine is still available to sanitise. Combined chlorine has already reacted with contaminants and is largely spent, and it is what produces the strong chlorine smell and eye irritation. Total chlorine is both added together, which is why free chlorine is the reading that matters.
Can I mix pool chemicals to save trips?
No, and this is a genuine safety matter rather than best practice. Mixing chlorine products with acid, or different chlorine products with each other, can produce toxic gas and in some combinations fire. Each chemical is added to the water separately, never combined in a bucket or a feeder.
The formula
ounces of 12.5% liquid = ppm increase x 10.7 per 10,000 gallons
Put this calculator on your site
Free to embed. Paste this where you want it to appear. It stays up to date automatically because it loads from BDEVY.
Please keep the attribution line. It is the only thing we ask in return.
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 0.5.
- Target free chlorine (ppm). Enter your own figure; the page starts at 3.
- Price per pound or gallon ($). Enter your own figure; the page starts at $0.06.
The result updates as you type, so you can try a figure, see what it does to liquid chlorine 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) | 0.5 |
| Target free chlorine (ppm) | 3 |
| Price per pound or gallon | $0.06 |
What comes out
| Result | Value |
|---|---|
| Liquid chlorine to add main result | 53.5 |
| Change needed | 2.5 |
| Estimated cost | $3.21 |
| Dose per 10,000 gallons | 26.8 |
Add about 53.5 to move 20,000 gallons from 0.5 to 3.
How sensitive the answer is
Holding everything else steady and changing only price per pound or gallon, this is what happens to liquid chlorine to add. It is worth knowing which of your inputs the answer actually hinges on, and which barely move it.
| Price per pound or gallon ($) | Liquid chlorine to add |
|---|---|
| $0.03 | 53.5 |
| $0.05 | 53.5 |
| $0.06 example above | 53.5 |
| $0.09 | 53.5 |
| $0.12 | 53.5 |
Every other input is held at the example value while this one changes, which is not how a real job behaves. Treat it as a guide to which number matters most, not as five separate quotes.
What each figure means
- Pool volume (gallons)
- A number, between 0 and 2000000, starting at 20,000.
- Current free chlorine (ppm)
- A number, between 0 and 30, starting at 0.5.
- Target free chlorine (ppm)
- A number, between 0 and 30, starting at 3.
- Price per pound or gallon ($)
- A money amount, starting at $0.06.
What the results tell you
| Result | What it is |
|---|---|
| Liquid chlorine 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. |
Keep going
Related calculators
Pool Shock Calculator
Doses a shock, where the point is reaching a threshold rather than adding a lot.
Open calculatorPool CYA Calculator
Doses cyanuric acid, the one parameter you cannot bring back down with a chemical.
Open calculatorPool Chemical Calculator
Doses the main parameters together, which is how pool chemistry actually works.
Open calculatorEstimating is the easy part
The jobs you never hear about cost more than the ones you mis-measure. BDEVY builds the marketing, systems and automation that bring them in.