Pool Shock Calculator
Shocking is a level you hold for days, not a dose you add once. This works out the target, what the whole job costs rather than just the first pour, and — where stabilizer is high — whether replacing water first would have been cheaper.
This sets the target. Test it first.
A green pool is usually at or near zero.
A light bloom takes 2. A bad one, 5 or more.
Water plus sewer, off your bill.
Shocking is a level you hold, not a dose you add. The target is about 40% of your cyanuric acid reading, and you keep topping back to it until the pool stops eating chlorine overnight — which is why the total matters more than the first dose.
Result
Enter your pool volume
The fields start on ordinary values so you can see what each one wants. Nothing is worked out until you put your own numbers in.
Shocking is a level you hold for days, not a dose you add once. This works out the target, what the whole job costs rather than just the first pour, and — where stabilizer is high — whether replacing water first would have been cheaper.
Getting an accurate answer
- Test cyanuric acid before anything else. It sets the target, and shocking without knowing it is the most common reason a pool gets shocked three times and stays green.
- Add at dusk. Unstabilized chlorine in full sun has a half-life measured in tens of minutes, so a midday dose is largely gone before it works.
- You are done when the overnight chlorine loss test comes back under 1 ppm — not when the water looks clear. Clear water with a live bloom in it turns again in days.
- Liquid chlorine for this job unless calcium hardness is low. Cal hypo adds about 0.7 ppm of hardness for every 1 ppm of chlorine, and a long hold puts a lot in.
The target comes from stabilizer, not from pool size
The number on the bag assumes a pool with no cyanuric acid in it. Almost no outdoor pool is that.
Stabilizer protects chlorine from sunlight by binding most of it, so the more you carry, the more total chlorine you need for the same killing power. The shock target is about 40% of your CYA reading — 16 ppm at 40 ppm CYA, 32 at 80, 40 at 100.
Which means two pools of identical size can need wildly different amounts, and the only way to know is to test. Shocking without testing CYA first is the single most common reason a pool gets shocked three times and stays green: the dose was right for a pool with 30 ppm of stabilizer and the pool had 90.
The first dose is not the job
This is what the usual dose calculators miss, and it is where the money goes.
A pool with a live bloom eats chlorine. You get it to shock level at dusk, and by morning the algae has pulled it most of the way back down — so you dose again, and again, until the consumption stops. Each round is a full swing back to the target, not a top-up.
A light bloom takes about two rounds. A properly green pool takes five or more. So the total chlorine is two to five times whatever the first pour looked like, and buying one jug on the way home is how people end up making three trips.
Buy the whole job up front. Running out halfway is worse than not starting: the level drops, the algae recovers, and the chlorine already spent is wasted.
If what you actually want is a single dose to move free chlorine from one number to another — routine dosing rather than a bloom — the chlorine dosage calculator is the simpler tool, and it covers trichlor and dichlor as well.
When replacing water beats buying chlorine
At high stabilizer this becomes a real arithmetic question rather than a rule of thumb, and the honest answer depends on what water costs where you live.
Diluting CYA from 100 ppm down to 40 means replacing 60% of the pool. That is a lot of water — but it also drops the shock target from 40 ppm to 16, so every dose from then on is under half the size, and so is every dose for the rest of the pool’s life.
On cheap water and a stubborn bloom, that pays back during the job itself. On expensive water and a quick clear-up, shocking through it is cheaper and diluting is a convenience rather than a saving. The calculator above runs both with your own prices instead of assuming.
One thing the arithmetic does not capture: a pool left at 40 ppm CYA is easier to hold all season, and a pool left at 100 will be back here. Lowering stabilizer is the other half of that decision.
Finishing, and not repeating it
Clear water is not the finish line. The test is the overnight chlorine loss: free chlorine at dusk, again at first light before the sun reaches the water. Under 1 ppm lost and nothing is living in there. More than that and something still is, whatever the water looks like.
Brushing matters more than people expect, especially on plaster, because algae anchored to a surface is shielded from the water around it. So is running the pump continuously and cleaning the filter as pressure climbs — you are removing what you kill.
Afterward, the question is why the pool got there. Nine times out of ten it is chlorine that was too low for the stabilizer being carried, which is a maintenance target problem rather than an accident. The free chlorine target is 10 to 15% of your CYA, every day, and holding it is what stops this recurring.
Questions about this calculator
How much shock do I need for my pool?
It depends on cyanuric acid rather than on pool size alone. The target is about 40% of your CYA reading, so a pool at 40 ppm CYA shocks to 16 ppm of free chlorine and one at 80 ppm shocks to 32. In a 20,000 gallon pool, 16 ppm is about 2.6 gallons of 12.5% liquid chlorine and 32 ppm is about 5.1 — before any topping up.
How many times do I need to add shock?
Until the pool stops consuming it. A light bloom usually takes two rounds, a bad green pool five or more, and each round is a full swing back to the target because the algae eats the chlorine down overnight. That is why the total cost of shocking is two to five times what the first dose suggests.
Is it cheaper to drain the pool than to shock it?
Sometimes, and it turns entirely on your water price and how long the bloom runs. Diluting cyanuric acid from 100 ppm to 40 means replacing 60% of the water, and it cuts the shock target from 40 ppm to 16 — so every subsequent dose is under half the size. On cheap water and a stubborn bloom that pays back quickly. On expensive water and a quick clear-up it does not. The calculator above compares the two with your own numbers.
How do I know when shocking is finished?
The overnight chlorine loss test. Record free chlorine at dusk, retest at first light before the sun is on the water, and compare. Losing under 1 ppm means nothing is living in there and you can let the level drift back down. Losing more means keep going, however clear the water looks.
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