Pool Reading Measurements

How To Read The Numbers

A pool reading is not just a number. To understand it, keep the value, unit, and measurement basis together.

Value: the number recorded from a test kit, meter, controller, or operator check.
Unit: how the value is measured, such as ppm, mg/L, mV, degrees, psi, GPM, percent, pass/fail, or a written note.[4]
Basis: what the unit is measured as, such as ppm as CaCO3 for alkalinity or hardness, ppm as Cl2 for chlorine, and ppm as Br2 for bromine.[4][5]

Common Logbook Readings

These are the readings people most often see in day-to-day pool logs.

Reading	Typical unit	Common measurement methods	What it means
pH	pH, unitless	Phenol red color match, photometer, digital pH meter, or controller probe.	An acidity/basicity reading. It is unitless, not ppm.[4]
Free chlorine (FCL or FC)	ppm or mg/L as Cl2	DPD color test, FAS-DPD titration, test strip, photometer, or chlorine sensor.	The chlorine sanitizer residual available in the water.[1][4][5]
Total chlorine (TCL or TC)	ppm or mg/L as Cl2	DPD total chlorine test, photometer, or strip.	Free chlorine plus combined chlorine compounds measured by the test.[4][5]
Combined chlorine (CC)	ppm or mg/L as Cl2	Usually calculated as total chlorine minus free chlorine; some kits report it directly.	Chlorine that has reacted with contaminants; often calculated from total and free chlorine.[5]
Total bromine (TB)	ppm or mg/L as Br2	DPD bromine color test, photometer, or strip.	The bromine sanitizer residual for bromine-maintained water.[2][4]
ORP	mV	ORP electrode from a controller or portable meter.	A probe-based oxidation signal in millivolts, not a direct sanitizer ppm conversion.[4]
Water temperature	deg F or deg C	Thermometer, heater sensor, automation system, or handheld meter.	Useful for comfort, spa checks, and saturation index calculations.[2][4]
Total alkalinity	ppm or mg/L as CaCO3	Acid titration drop test, strip, or photometer.	A water-balance reading reported as CaCO3.[4]
Calcium hardness	ppm or mg/L as CaCO3	Hardness titration, strip, or photometer.	Water balance input; not the same as total hardness unless the test says so.[4]
Cyanuric acid (CYA)	ppm or mg/L	Turbidity view-tube test, photometer, or strip.	Most relevant to stabilized chlorine pools.[3][4]
Salt or salinity	ppm, ppt, or g/L	Salt test strip, salinity meter, conductivity meter, or salt-chlorine generator controller.	Salt concentration. The unit matters because ppm, ppt, and g/L are different scales.[4]
Total dissolved solids (TDS)	ppm or mg/L	Conductivity or TDS meter.	Broad water-balance signal, not an ingredient breakdown.[4]
Water clarity	Pass/fail, visibility note, or NTU	Visual main-drain visibility check or turbidity meter.	Describes whether the water is visibly clear or measured by turbidity.[4]

Situational Readings

These are useful when a property uses a relevant system, has a recurring issue, or needs a startup or troubleshooting report.[4]

Reading	Typical unit	When it matters
Phosphates	ppb or ppm as PO4	Algae and nutrient troubleshooting.
Nitrates	ppm or mg/L	Source-water or persistent algae investigation.
Copper	ppm or mg/L	Staining, source water, heaters, algaecides, or copper ionizers.
Iron	ppm or mg/L	Source-water staining and discoloration issues.
Silver	ppm or mg/L	Silver or copper-silver ionizer systems.
Borates	ppm or mg/L	Pools intentionally maintained with borates.
Monopersulfate (MPS)	ppm or mg/L	Non-chlorine oxidizer use; may matter because MPS can interfere with some sanitizer tests.
Ozone residual	ppm or mg/L	Ozone systems and safety checks; usually not a normal pool-water target.
Bacterial testing	Method-specific result	Regulatory, lab, remediation, or incident workflows rather than daily entry.

Operational Readings

These are not chemistry readings, but they help explain the equipment condition recorded beside water readings.[4]

Filter pressure Force per area from a pressure gauge, reported in units such as psi, kPa, or bar.

Flow rate Volume per unit of time from a flow meter or controller, reported in units such as GPM, L/min, or cubic meters per hour.

Pump state On/off, schedule, speed, RPM, percent, or watts.

Chemical feeder output Percent, dial setting, pump rate, or dose volume.

References

This page uses CDC Healthy Swimming guidance, the 2024 Model Aquatic Health Code, and the MAHC Annex for baseline terminology and measurement context.

CDC. Guidelines for Keeping Your Pool Safe and Healthy. Used here for disinfectant residual and pH as core pool checks.
CDC. What You Can Do to Stay Healthy in Hot Tubs. Used here for hot tub sanitizer, pH, and temperature context.
CDC. Model Aquatic Health Code edition page. Used here for CDC's 2024 5th edition posting information.
CDC. 2024 Model Aquatic Health Code, Code Language, 5th Edition. Used here for testing methods, ORP units, water clarity, equipment readings, and common reading types.
CDC. 2024 Annex to the Model Aquatic Health Code, Scientific and Best Practices Rationale, 5th Edition. Used here for available chlorine terminology, ppm as Cl2 basis, and combined chlorine context.