Water Hardness Test Kits: How to Choose
Choose a test that gives you the level of detail you need, in units you can use.
Source review: September 15, 2026. This is a comparison of manufacturer information. We have not independently lab-tested these kits.
Hach
5-B Total Hardness Test Kit
Checking untreated water or softener performance with a numerical reading
Useful for: Drop-count titration; Reports total hardness in grains per gallon.
Limitations: Requires careful counting; Check the measuring range and reagent expiry.
API
General & Carbonate Hardness Test Kit
Users who want both hardness and alkalinity measurements and can convert units
Useful for: Separate GH and KH tests; Manufacturer provides a conversion chart.
Limitations: GH is reported in German degrees; KH is alkalinity, not a substitute for total hardness.
Titration or test strips?
| Method | Useful for | Limitation |
|---|---|---|
| Drop-count titration | A numerical hardness reading for settings or before/after checks | Sample volume, drop size and endpoint judgement affect the result |
| Colour strips | A quick hardness band | Colour bands usually do not resolve a precise softener setting |
| Multi-parameter kit | Screening several water characteristics | Check the included hardness range; a hardness result does not establish drinking-water safety |
Check what the kit actually measures
Total hardness measures calcium and magnesium expressed as calcium carbonate. A TDS meter measures conductivity and estimates dissolved solids; it does not directly measure total hardness. In an aquarium kit, GH is general hardness, while KH is an alkalinity measurement.
Hach's 5-B is a total-hardness kit. It does not separately report calcium hardness. Check the kit's current instructions for its measuring range, sample size and conversion.
Convert your result correctly
- GPG × 17.1 = ppm (mg/L) as calcium carbonate.
- ppm ÷ 17.1 = GPG.
- For API's German-degree scale: °dGH × 17.9 = ppm, then divide by 17.1 for GPG.
For example, 10 °dGH is 179 ppm, or about 10.47 GPG. The German-degree-to-GPG multiplier is approximately 1.047.
API manufacturer instructions and conversion chart · GPG–PPM converter
Get a representative sample
Follow the instructions for your particular kit, including sample volume, mixing and reading time. For sizing a softener, test untreated cold water upstream of the unit. For a performance check, compare untreated and treated samples. Do not assume that leaving water in the pipes overnight improves a hardness measurement.
Repeat an unexpected result with fresh reagents and a clean container. Recheck after a change in source water or softener performance. Use a suitably accredited laboratory when you need certified results.
Already have a reading?
Use the measured hardness and your household water use to estimate a softener's required capacity.
Calculate softener requirements