How to use this calculator
- Choose whether you have Ca and Mg results or EDTA titration data.
- Enter the values from your report or burette.
- Read total hardness, unit conversions and the IS 10500 check.
How is water hardness calculated?
TH = Ca × (100.09 ÷ 40.08) + Mg × (100.09 ÷ 24.31) = 2.497 Ca + 4.118 Mg
TH (EDTA) = VEDTA × M × 100,086 ÷ Vsample
1 mL of 0.01 M EDTA equals 1.0 mg of CaCO₃. Method: APHA 2340 C and IS 3025 (Part 21). Classes follow the USGS scale.
Worked example
With data you have: Calcium and magnesium results, calcium (ca²⁺) 60 mg/L, magnesium (mg²⁺) 20 mg/L:
Total hardness: 232 mg/L as CaCO₃
The calculator also gives calcium hardness 150 mg/L as CaCO₃; magnesium hardness 82 mg/L as CaCO₃; millimoles per litre 2.32 mmol/L.
Assumptions and limits
- Strontium, iron and other polyvalent ions add a little hardness but are ignored here.
- Reports in ppm and mg/L are the same for dilute water.
Frequently asked questions
What is the hardness limit for drinking water in India?
BIS IS 10500:2012 sets 200 mg/L as CaCO₃ as the acceptable limit and 600 mg/L as the permissible limit in the absence of an alternate source.
How do I convert ppm hardness to grains per gallon?
Divide ppm (mg/L as CaCO₃) by 17.12. For example, 300 ppm is about 17.5 gpg.
What is the difference between temporary and permanent hardness?
Temporary (carbonate) hardness is the part balanced by bicarbonate and drops out as scale when water is boiled. Permanent hardness is linked to sulphate and chloride and stays after boiling.
References
- BIS IS 10500:2012, Drinking Water Specification
- BIS IS 3025 (Part 21):2009, Total hardness
- APHA, Standard Methods 2340, Hardness
- USGS, Hardness of Water