A green pool means an active algae bloom. Clearing it is not a single dose — it is a method called SLAM (Shock Level And Maintain) that holds chlorine at a target level until the algae is gone. The first thing you need is your exact water volume, because every step depends on it.
Most shock calculators assume a clean pool and a single maintenance dose. A green pool has a living organic load that consumes chlorine far faster than it can be added in one go. That is why a one-shot "add X gallons" number does not clear algae — the chlorine is eaten before it finishes the job. Clearing a bloom needs a sustained chlorine level held over hours to days, which is what SLAM does.
SLAM stands for Shock Level And Maintain. The idea is to raise free chlorine (FC) to a target tied to your stabiliser (CYA) level and hold it there — testing and re-dosing as the algae consumes it — until three conditions are all met: the water is clear, you lose no more than 1 ppm FC overnight, and there is no measurable combined chlorine. This is the established method documented by the Trouble Free Pool community.
The SLAM target chlorine level scales with your CYA (stabiliser): a common reference holds the shock FC at roughly 40% of the CYA level. That is a target FC concentration in parts per million, not a fixed amount of chlorine — to turn it into an actual dose you must know your current FC (from a test), the strength of your chlorine product, and you must re-dose repeatedly as it is consumed. Because of that, the safe and correct way to run a SLAM is with a proper FC/CYA test kit (such as a FAS-DPD drop test) and a maintained dosing tool — not a one-time calculator figure. Guessing the dose risks both an ineffective treatment and dangerous over-chlorination.
We deliberately do not output a one-shot "add X gallons of chlorine" number for a green pool. A bloom needs a chlorine level held and re-dosed over time, measured with a test kit. A single calculated dose is the wrong mental model and can be unsafe. Get your volume exact here, then run the SLAM with a FAS-DPD test kit.
A mild teal or cloudy pool almost always clears with a SLAM. A very dark, black, or swamp-like pool can sometimes be cleared too, but if you cannot see the bottom at all it may be cheaper and faster to drain and refill — especially if your CYA is very high, since the only way to lower CYA is to replace water. Use your volume to compare: a fill-cost estimate against days of chlorine and effort. Our water fill cost calculator can price a refill from the volume you calculated above.
Only test results tell you that — not the clock and not a calculator. General guidance is to wait until free chlorine has fallen back to the normal range for your CYA and pH is balanced, which you confirm with a test. Do not let anyone swim while the pool is held at shock level.