Water hardness & alkalinity converter
Test kits, salt makers and water reports all use different units for the same thing. Enter a KH or GH reading in whatever unit you have and get every other unit at once.
How it works
Every unit here is a way of counting the same ions. A German degree (dKH or dGH) is defined as the equivalent of 10 mg/L of calcium oxide (CaO). ppm on test kits and water reports means mg/L as calcium carbonate (CaCO₃). Alkalinity is counted in milliequivalents (meq/L), the amount of acid the water can neutralise, while hardness is counted in millimoles of calcium plus magnesium.
Why meq for one and mmol for the other? Bicarbonate (HCO₃⁻) carries one charge, so for alkalinity meq/L and mmol/L are the same number. Calcium and magnesium carry two, so 1 mmol/L of hardness is 2 meq/L. That's also why 1 dKH (0.357 meq/L) and 1 dGH (0.178 mmol/L) look different but both equal 17.85 ppm as CaCO₃.
Worked examples
A reef alkalinity test reads 3.0 meq/L
3.0 × 2.804 = 8.4 dKH, and 3.0 × 50.04 = 150 ppm as CaCO₃. That's inside the 7–11 dKH range Holmes-Farley suggests for reef tanks.
A tap water report says hardness 120 mg/L as CaCO₃
120 ÷ 17.848 = 6.7 dGH, or 120 ÷ 100.09 = 1.20 mmol/L. That's the top of the USGS "moderately hard" class (61–120 mg/L).
GH from a calcium and magnesium test
Ca 40 mg/L and Mg 10 mg/L: 2.497 × 40 + 4.118 × 10 = 99.9 + 41.2 = 141.1 ppm as CaCO₃, which is 7.9 dGH (hard).
Typical ranges
| What | Range | Source |
|---|---|---|
| Reef tank alkalinity | 7–11 dKH (2.5–3.9 meq/L, 125–196 ppm); 7–8 dKH for some low-nutrient SPS tanks | Holmes-Farley |
| Soft water | 0–60 mg/L as CaCO₃ (0–3.4 dGH) | USGS |
| Moderately hard | 61–120 mg/L (3.4–6.7 dGH) | USGS |
| Hard | 121–180 mg/L (6.8–10.1 dGH) | USGS |
| Very hard | above 180 mg/L (above 10.1 dGH) | USGS |
Limits
- A KH test measures total alkalinity, so borate, phosphate and other bases count too. In reef salt mixes borate adds a little. The unit conversion is exact either way.
- A GH test counts calcium and magnesium together. It can't tell you the split, which matters for some plants and for reef tanks, so test calcium and magnesium separately if you need it.
- "ppm" without "as CaCO₃" can mean something else. A calcium reading of 420 ppm on a reef test is mg/L of calcium itself, not hardness. Use the "GH from Ca & Mg" tab for that.
- To change KH or GH by mixing waters, use the blend tab of the water change calculator. Hardness mixes in a straight line.
Common questions
How many ppm is 1 dKH?
1 dKH is 17.85 ppm (mg/L) as CaCO₃, or 0.357 meq/L. The same 17.85 ppm factor applies to 1 dGH, because both German degrees are defined as 10 mg/L of calcium oxide.
What is 8 dKH in meq/L and ppm?
8 dKH is 2.85 meq/L and 142.8 ppm as CaCO₃. To go the other way, multiply meq/L by 2.8: a reef test reading 3.0 meq/L is 8.4 dKH.
What is the difference between KH and GH?
KH (carbonate hardness, or alkalinity) measures bicarbonate and carbonate, which buffer pH and set how much CO₂ a given pH means. GH (general hardness) measures dissolved calcium and magnesium. They are separate tests and don't have to match. Use KH in the CO₂ from pH & KH calculator.
What alkalinity should a reef tank have?
Randy Holmes-Farley suggests about 7–11 dKH, which is 2.5–3.9 meq/L or 125–196 ppm as CaCO₃. Keep it steady rather than chasing a number, and check salinity at the same time with the salinity calculator.
Is 150 ppm hard water?
Yes. 150 mg/L as CaCO₃ is 8.4 dGH (1.50 mmol/L), which the USGS classes as hard (121–180 mg/L). Up to 60 mg/L is soft, 61–120 moderately hard, and above 180 very hard.
References
- Hardness of water: Measurement. Wikipedia (unit definitions: 1 °dH = 10 mg/L CaO = 0.1783 mmol/L = 17.848 mg/L CaCO₃; gpg, °fH; conversion table)
- APHA, AWWA & WEF. Standard Methods for the Examination of Water and Wastewater, 2320 Alkalinity and 2340 Hardness (mg CaCO₃/L; hardness = 2.497 [Ca] + 4.118 [Mg])
- U.S. Geological Survey Water Science School: Hardness of water (soft 0–60, moderately hard 61–120, hard 121–180, very hard over 180 mg/L as CaCO₃)
- Holmes-Farley, R. What is alkalinity? Reefkeeping Magazine, reprinted on Reef2Reef (suggested reef range 7–11 dKH, 2.5–4 meq/L)
- IUPAC Commission on Isotopic Abundances and Atomic Weights: Standard atomic weights (CaO 56.077, CaCO₃ 100.087, HCO₃ 61.017 g/mol)
- NIST Handbook 44, Appendix C (1 grain = 64.79891 mg; US gallon = 3.785411784 L)
Formulas on this page are checked by automated unit tests against the reference values above. Last reviewed October 29, 2026. Found an error? Tell us.
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