Best source: a current measured value
Softener sizing depends directly on hardness, so a guessed number can push the capacity estimate in the wrong direction. A current laboratory or utility value expressed as calcium carbonate is the best starting point.
If you have municipal water
Start with your water utility. The annual Consumer Confidence Report (CCR) is the right place to understand regulated water quality, but hardness is not a federal health-based contaminant and may not always appear in the CCR. Many utilities publish hardness separately, or customer service can provide a current range for your service area.
If you have a private well
A private well can change with geology, season, well condition and nearby activity. EPA recommends annual testing for total coliform bacteria, nitrates, total dissolved solids and pH, plus additional contaminants when local conditions suggest them. Add hardness to the lab panel when you are sizing a softener.
Understand the units
Hardness is commonly reported as mg/L (or approximately ppm in water) as CaCO₃, or as grains per gallon (gpg). For planning, 1 gpg is about 17.1 mg/L as CaCO₃. Our hardness converter handles both directions and shows the USGS class.
What about home test strips?
A strip or drop test can be useful for a quick screening check, but product accuracy and range vary. If the softener purchase is expensive, the water is unusually hard, or iron/manganese is also present, confirm the number with a more reliable test before final sizing.
Do not confuse hardness with TDS
Total dissolved solids and hardness are different measurements. A TDS meter does not tell you how many grains per gallon of hardness you have, so it should not be used as a substitute hardness input.
Use a range when your supply varies
If the utility provides a hardness range, sizing from the upper part of the normal range is more conservative than using the minimum. If the source switches seasonally, ask whether hardness changes with the source.