Combined Chlorine Calculator: CC = TC − FC, Chloramines

Combined chlorine is chlorine that has already reacted with sweat, urine, and other waste. These compounds are called chloramines. They sanitize poorly and cause most of the "chlorine smell" and red eyes people blame on too much chlorine. Enter both readings from the same test, and your pool size, in the calculator above.

How the calculator works

The math is one subtraction: CC = TC − FC. Your test kit measures free chlorine first and total chlorine second. Whatever total chlorine shows beyond free chlorine is combined chlorine.

With the default numbers (FC 2 ppm, TC 3 ppm, 15,000 gallons), the result is 1 ppm of combined chlorine, which is above the 0.5 ppm action level. The calculator doesn't print a chlorine dose for that. It tells you to follow the shock directions on your product's label. Labels usually give an amount per 10,000 gallons, so it shows the multiplier for your pool: 1.5 times the label amount for 15,000 gallons. That is our rule for every shock on this site, because the label is the legal limit for the product you use.

If TC reads a little lower than FC, that is normal test scatter. The two numbers come from separate steps or pads. The calculator treats it as 0.

What your result means

Combined chlorine bands used by the calculator
CC	What it means	What to do
Below 0.5 ppm	Normal for a pool in use	Nothing for chloramines. Keep FC at the minimum for your stabilizer (CYA).
0.5 ppm or more	Chloramines are building up	Shock with a product labeled for shocking pools, at the amount on its label for your gallons. Retest FC and CC the next day.
Still 0.5 ppm or more after a few days of following the label	Something keeps adding waste, or chlorine runs too low between doses	Check circulation, the filter, and your daily FC. Call a pool professional if it won't clear.

Don't swim after a shock until FC is back at or below the swim limit on your label. Many labels set it at 4 ppm. The pool chlorine levels page shows the daily FC range for your CYA, which is what keeps CC low in the first place.

What chloramines are and where they come from

The CDC describes chloramines as a type of combined chlorine. They form when chlorine binds to the waste swimmers bring in: urine, sweat, and dirt. Ammonia in rain runoff, lawn fertilizer, and some city water also forms them. Some utilities disinfect tap water with chloramine, so a fresh fill can show combined chlorine before anyone swims.

Two things are worth knowing:

Combined chlorine shows up only on tests that read both FC and TC. A strip or kit that reads total chlorine alone can't tell 3 ppm of free chlorine from 1 ppm free and 2 ppm combined. The difference between the two numbers is explained on free chlorine vs total chlorine.
Non-chlorine shock (MPS) can read as combined chlorine. Potassium monopersulfate interferes with the DPD total chlorine step. Test before adding it, or the next day. See the non-chlorine shock calculator.

Are chloramines dangerous?

At home pool levels, the main effects the CDC lists are irritation of the eyes, skin, and airways. Chloramines turn into gas above the water, and the CDC notes that this gas is heavier than air, so it collects just above the surface, where swimmers breathe.

Indoor pools have it worse, because the gas has nowhere to go. At one indoor waterpark, the CDC traced hundreds of reports of red, burning eyes, coughing, and wheezing among guests and lifeguards to chloramine gas in poorly circulating air. The reports stopped after the ventilation was changed. Outdoors, wind clears most of it. For an indoor or enclosed pool, run the ventilation and keep combined chlorine low.

If anyone has trouble breathing, chest tightness, or eye pain after swimming, get them to fresh air and call a doctor. For a chemical exposure emergency, call 911 or Poison Control at 1-800-222-1222.

Breakpoint chlorination, in plain terms

"Breakpoint" is the point where enough chlorine has been added to break chloramines apart. Past it, the inorganic chloramines are destroyed, and the CDC's model code notes that eye irritation and the "chlorine" odor decrease. The CDC's operator guidance calls this superchlorination and describes it for trained public pool operators.

For a home pool, we don't turn that rule into a dose. The amount you add comes from your shock product's label, scaled to your gallons. The label also tells you when the pool can be used again. Two points from the CDC's model code help set expectations:

Ammonia breaks down fast when you shock. Urea, the main part of urine, reacts with chlorine slowly, so a single shock may not clear everything overnight.
Keeping FC at the minimum for your CYA every day prevents most buildup. Shocking fixes the result, not the habit.

Step-by-step shock instructions are on how to shock a pool. If the smell is your main complaint, start with why a pool smells like chlorine.

Test so the number means something

Use a kit that reads FC and TC separately. The CDC recommends DPD test kits over strips for home pools. A FAS-DPD drop kit reads combined chlorine in 0.2 or 0.5 ppm steps, which makes the 0.5 ppm line easy to see.
Strips work in coarse steps, so a gap smaller than one step may not show. Treat a strip result as a hint, and confirm with a drop test before you shock. See how to read pool test strips.
Test both numbers from the same sample, taken about 18 inches below the surface and away from return jets (Taylor's kit card).

Safety. Always follow the product label. It is a violation of federal law to use a pesticide product in a manner inconsistent with its labeling. Never mix chlorine products with each other or with acid; add each one directly to the water with the pump running. Wear gloves and eye protection. Don't swim until FC is back at or below the limit on your label. Poison Control: 1-800-222-1222.

FAQ

How do you calculate combined chlorine?

Subtract free chlorine from total chlorine: CC = TC − FC. For example, TC 3 ppm and FC 2 ppm give CC 1 ppm. Use both readings from the same test.

Are chloramines in a pool dangerous?

The CDC says chloramines irritate the eyes, skin, and airways, mostly when they turn into gas above the water. The effect is strongest at busy indoor pools with poor ventilation. Keep CC under 0.5 ppm and call a doctor if someone has trouble breathing.

Why are chloramines worse at indoor pools?

The gas is heavier than air and collects just above the water. Outdoors, wind carries it away. Indoors, it builds up unless the ventilation removes it.

Does high combined chlorine mean there is too much chlorine?

No, usually the opposite. Combined chlorine builds up when free chlorine runs too low to break down waste. The strong "chlorine smell" is chloramines, not free chlorine.

Sources

CDC, Chloramines and pool operation (what chloramines are, irritation, gas heavier than air): cdc.gov
CDC, MMWR: Respiratory and ocular symptoms among employees of a hotel indoor waterpark resort, Ohio, 2007: cdc.gov
CDC, Model Aquatic Health Code 2024, Annex (definition of breakpoint; urea reacts slowly): cdc.gov (PDF)
CDC, Home pool and hot tub water treatment and testing (DPD kits over strips): cdc.gov
Taylor Technologies, How FAS-DPD tests for free and combined chlorine values: taylortechnologies.com
Taylor Technologies, K-1005 instruction card (sampling depth): PDF
AQUA Tech Notes (PHTA), Potassium monopersulfate non-chlorine shock (reads as combined chlorine): PDF