When chlorine reacts with nitrogen compounds in pool water (from sweat, urine, and organic matter), it forms chloramines — also called combined chlorine. Chloramines come in several forms:
When you add chlorine to a pool with existing chloramines, something counterintuitive happens: the chloramine concentration initially increases as free chlorine combines with more nitrogen compounds. Only after enough chlorine is added — the "breakpoint" — does the chemistry shift. Above the breakpoint, excess chlorine oxidizes and destroys chloramines rather than forming more of them.
Required FC increase = Combined Chlorine × 10
| Combined Chlorine Reading | Free Chlorine Increase Needed | Cal-Hypo (65%) per 10K gal |
|---|---|---|
| 0.3 ppm | 3 ppm | ~0.8 lbs |
| 0.5 ppm | 5 ppm | ~1.3 lbs |
| 1.0 ppm | 10 ppm | ~2.5 lbs |
| 2.0 ppm | 20 ppm | ~5 lbs |
Combined chlorine = Total Chlorine − Free Chlorine. You need a DPD drop kit (not test strips) to measure combined chlorine accurately. Test strips measure only free chlorine and total chlorine and cannot reliably distinguish the difference at low levels.
This is the key insight that most pool owners miss: adding chlorine below the breakpoint dose temporarily increases chloramine levels before they come back down. If you shock at half the required dose, you're spending money on chemicals that make the problem temporarily worse before providing partial benefit.
This is why some pools still smell after a shock treatment that seemed to work — the dose was below breakpoint, so chloramines were partially reduced but not eliminated. The solution is always to calculate and meet the full breakpoint dose.
The harsh "pool smell" that people associate with over-chlorinated pools is actually the smell of chloramines — and it's a sign of under-chlorination relative to the bather load. A properly shocked pool with high free chlorine and low combined chlorine has virtually no smell. The smell means the chloramine level is high and breakpoint chlorination is needed.
SplashLens logs free chlorine, total chlorine, and calculates combined chlorine so you know exactly when breakpoint chlorination is needed — and exactly how much to add. Stop guessing at shock doses.
Open SplashLens Free →Multiply your combined chlorine reading by 10 — that's how much you need to raise free chlorine. Combined chlorine = Total Chlorine − Free Chlorine. Test with a DPD drop kit, not strips, for accuracy at this level.
Combined chlorine (chloramines) forms when free chlorine reacts with nitrogen from bather waste. It is a weak sanitizer — only 1/80th the potency of free chlorine — and causes the harsh smell, eye irritation, and respiratory irritation falsely attributed to "too much chlorine." The fix is more chlorine (to breakpoint), not less.
When combined chlorine exceeds 0.3 ppm, when the pool smells "chloriney," after heavy bather use, after rain, or as a weekly preventive measure in summer. Don't wait for problems — regular shocking keeps combined chlorine consistently low.
Any free chlorine source works if you add enough — but calcium hypochlorite and liquid sodium hypochlorite are standard for shocking because they raise FC quickly without adding CYA. Trichlor tablets cannot realistically achieve breakpoint at safe doses. Use unstabilized chlorine for shocking.
The dose was likely below breakpoint — you partially reduced chloramines but didn't destroy them. Measure combined chlorine again, calculate the correct 10x dose, and re-shock. Also confirm pH is at 7.2–7.4 before shocking — high pH significantly reduces chlorine effectiveness.