01
The purpose
What does this calculator estimate?
Add chlorine but the level barely moves? Measure your pool's hidden chlorine demand and what it'll take to satisfy it.
Water Chemistry · B18
Pool Chlorine Demand Calculator
Add chlorine but the level barely moves? Measure your pool's hidden chlorine demand and what it'll take to satisfy it.
Measure the inputsA big gap between expected and actual FC means organics or algae are eating chlorine fast.
Read the full resultTest 30 minutes after dosing so the chlorine has mixed but demand has only started.
Check the boundaryHigh demand after rain or heavy use is normal — a single satisfying shock usually resets it.
Useful forWater ChemistryPlanningComparisonVerification
Live calculation
Chlorine demand test
✓ Checked May 2026
Enter a known dose and the FC reading after 30 minutes.
Recommended dose
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Enter the required values to calculate.
Enter the known values. Let the engine handle the calculation.
✓ Formula checked↻ Live result○ No account needed
Planning estimate only. Confirm product labels, equipment specifications, qualified-professional advice and local requirements before acting.
The result, explained
Use the Pool Chlorine Demand Calculator result with context.
Add chlorine but the level barely moves? Measure your pool's hidden chlorine demand and what it'll take to satisfy it.
02
The tool
What information does it use?
- Pool volume: enter a measured or verified value
- FC before dosing: ppm, just before you add chlorine.
- Cal-hypo 65% added: The known amount you added, in lbs.
- FC after 30 min: ppm, measured 30 minutes after dosing.
- Pool condition: Helps explain the likely cause.
03
The method
How to use this calculator
- Pool volumeEnter the value you can verify.
- FC before dosingppm, just before you add chlorine.
- Cal-hypo 65% addedThe known amount you added, in lbs.
- FC after 30 minppm, measured 30 minutes after dosing.
- Pool conditionHelps explain the likely cause.
04
The relationship
The formula in plain English
Demand = Expected FC gain − Actual FC gain
Expected gain comes from the known dose; the shortfall is your chlorine demand in ppm.05
Worked through
A worked example: 15,000 gal pool, 2 lb cal-hypo, FC 0 → 0.5 ppm
- Expected FC gain from 2 lb cal-hypo 65% ≈ 4.3 ppm
- Actual gain = 0.5 − 0 = 0.5 ppm
- Demand = 4.3 − 0.5 = 3.8 ppm to overcome
| Condition | Typical demand | Likely cause | Response |
|---|---|---|---|
| Normal | Low | Routine consumption | Standard daily dose |
| Algae present | High | Algae consuming chlorine | SLAM protocol |
| Heavy organic load | Moderate–high | Swimmers, debris | Breakpoint shock |
| After heavy rain | Moderate | Washed-in organics | Shock and re-balance |
06
Why it matters
How the result supports the next decision
A big gap between expected and actual FC means organics or algae are eating chlorine fast.
Test 30 minutes after dosing so the chlorine has mixed but demand has only started.
07
Good inputs
Improve the estimate before relying on it
- A big gap between expected and actual FC means organics or algae are eating chlorine fast.
- Test 30 minutes after dosing so the chlorine has mixed but demand has only started.
- High demand after rain or heavy use is normal — a single satisfying shock usually resets it.
- Persistent demand with no visible cause can mean ammonia or a hidden organic load.
- Once demand is satisfied, FC holds normally again — that's your signal the pool is 'caught up.'
08
The boundary
How to interpret the result
Follow the product label, add chemicals separately and retest before another dose.
The engine reports invalid inputs and safety warnings beside the result. Resolve those messages before using the estimate.
09
Before acting
What should override this estimate?
- Measured site conditions and current test results
- Product labels and manufacturer specifications
- Qualified-professional advice and applicable local codes
10
Common questions
Frequently asked questions
What is chlorine demand?
Chlorine demand is the amount of chlorine your pool consumes on contaminants — organics, algae, ammonia, and debris — before any free chlorine registers. A high-demand pool seems to 'eat' chlorine: you add a normal dose and the level barely rises because the contaminants consume it first.
Why does my pool eat chlorine so fast?
The usual causes are an algae bloom (even early, invisible stages), a heavy organic load from swimmers, leaves, or runoff, ammonia from contamination, or simply low cyanuric acid letting sunlight burn chlorine off. Measuring demand tells you whether it's consumption or UV loss.
How do I measure chlorine demand?
Test free chlorine, add a known amount of chlorine, wait about 30 minutes with the pump running, then test again. Compare the FC gain you should have gotten from that dose to what you actually got. The shortfall is your chlorine demand in ppm, which the calculator converts to the extra product needed.
How do I satisfy a high chlorine demand?
You shock past the demand — add enough chlorine in one go to consume all the contaminants and still leave a free residual. The calculator estimates the extra chlorine needed. Once demand is satisfied, free chlorine starts holding normally, which confirms the pool has caught up.
Is chlorine demand the same as chlorine loss to sunlight?
No. UV loss is sunlight breaking down unprotected chlorine, fixed by maintaining cyanuric acid. Chlorine demand is consumption by contaminants in the water. A daytime FC drop with good CYA points to demand; rapid loss with low CYA points to sunlight.
How do you calculate chlorine demand?
Subtract the actual FC gain from the expected FC gain of a known chlorine dose. The difference is the demand in ppm.
Why won't my chlorine hold?
High chlorine demand from algae or organics, or UV loss from low CYA. Measure demand to tell which.
Does rain increase chlorine demand?
Yes. Rain washes in organics and dilutes chlorine, temporarily raising demand until you shock the pool.
Primary sources
Use the result with current guidance.
These official references support safe planning context. Product labels, local requirements and qualified-professional advice take priority over a general calculator estimate.