Overnight Chlorine Loss Test

Enter your dusk and dawn free chlorine readings — get a pass/fail verdict and, if it's a fail, the plan while it's still cheap to fix.

Verdict
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Overnight loss—
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What to buyPool test kitBuy on Amazon

Test conditions matter: pump running both times, no swimmers between readings, same test kit. Drop-based kits are far more reliable than strips at this precision.

Overnight Chlorine Loss Test — free from poolcarekit.com

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Why the night test works

Daytime chlorine loss has two causes tangled together — sunlight and contamination — so a daytime drop proves nothing. Remove the sun and only consumption remains: healthy water barely uses chlorine overnight, while even invisible early algae metabolizes it measurably. One night separates the two causes for the price of two tests.

A worked example

Dusk 5.0, dawn 4.5: half a ppm — pass, sleep easy. Dusk 5.0, dawn 3.2: a 1.8 ppm fail, and even if the water sparkles, something is feeding. Raising to shock level tonight (about 16 ppm at CYA 40 — the green pool calculator sizes the dose) typically ends it in a day or two. Ignoring it typically costs a green weekend and three jugs more chlorine.

When to run it

Any time chlorine seems hungrier than usual, after every algae fight (passing the OCLT is the official "you won"), after heavy storms or parties, and once in spring before opening chemistry settles. It costs two test-kit drops and zero chemicals — the highest information-per-effort move in pool care.

Frequently asked questions

What is the overnight chlorine loss test?

Test free chlorine after sunset, then again before sunrise, with the pump running and nobody swimming. Sunlight can't take chlorine at night — so overnight loss beyond ~1 ppm means something organic (usually early algae) is consuming it.

How much overnight chlorine loss is normal?

1 ppm or less. Zero-to-half a ppm is a healthy pool; right at 1 is worth re-testing tomorrow; more than 1 is a fail — something's eating chlorine, and it's cheapest to fight before you can see it.

My pool looks crystal clear — can it still fail the OCLT?

Yes, and that's exactly the test's value: algae consumes chlorine measurably days before it's visible. A clear pool failing the OCLT is your 72-hour warning — the difference between a one-evening fix and a week-long green fight.

What do I do if I fail the test?

Treat it as invisible algae: raise chlorine to your SLAM shock level (based on CYA) and hold it — the green pool calculator gives the exact plan. You're done when the water passes the OCLT again: less than 1 ppm lost overnight.

Related calculators

Why Your Pool Loses ChlorineChlorine won't hold? The four causes and how to spot yours.Rain Effect CalculatorWhat N inches of rain just did to your water chemistry.Pool Evaporation CalculatorIs that water loss evaporation — or a leak?Drain & Refill PlannerHow much water to swap to fix a too-high level, and the cost.

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