How do I convert dKH to meq/L and ppm?
Divide dKH by 2.8 for meq/L; multiply meq/L by 50 for ppm (mg/L as CaCO₃). So 8 dKH is 2.86 meq/L and 143 ppm; 125 ppm is 7 dKH; 3 meq/L is 8.4 dKH. One dKH is 17.86 ppm. That's the whole converter — the rest of this page is why those numbers, and which one your kit means.
| dKH | meq/L | ppm CaCO₃ | |
|---|---|---|---|
| 4 | 1.43 | 71 | planted tanks, typical |
| 4.5 | 1.61 | 80 | |
| 5 | 1.79 | 89 | |
| 5.5 | 1.96 | 98 | |
| 6 | 2.14 | 107 | |
| 6.5 | 2.32 | 116 | |
| 7 | 2.5 | 125 | natural seawater |
| 7.5 | 2.68 | 134 | |
| 8 | 2.86 | 143 | mixed reef, low end |
| 8.5 | 3.04 | 152 | |
| 9 | 3.21 | 161 | |
| 9.5 | 3.39 | 170 | mixed reef, high end |
| 10 | 3.57 | 179 | |
| 10.5 | 3.75 | 188 | |
| 11 | 3.93 | 196 | LPS/softies, high end |
| 11.5 | 4.11 | 205 | |
| 12 | 4.29 | 214 | |
| 12.5 | 4.46 | 223 | |
| 13 | 4.64 | 232 | |
| 13.5 | 4.82 | 241 | |
| 14 | 5 | 250 |
1 meq/L = 2.8 dKH = 50 ppm as CaCO₃ (hobby convention; the exact factor is 2.804). Reef ranges are the usual consensus; freshwater tanks run 2–8 dKH.
A "German degree" of carbonate hardness is defined as 10 mg of calcium oxide per liter. CaO is 56.077 g/mol and neutralizes two equivalents of acid, so 1 dKH is 0.3567 meq/L — and 1 meq/L is 2.804 dKH, which everyone rounds to 2.8. Ppm "as CaCO₃" expresses the same equivalents as if they were all calcium carbonate: 100.087 g/mol, two equivalents, so 1 meq/L is 50.04 ≈ 50 ppm and 1 dKH is 17.85 ppm. The rounding costs 0.15% — invisible next to a test kit's ±0.3 dKH.
KH, carbonate hardness, alkalinity — are they the same thing?
For aquarium purposes, yes. Alkalinity is the water's capacity to absorb acid, which in a tank is almost entirely bicarbonate and carbonate. KH (from the German Karbonathärte) is the historical name for the same measurement when it was thought of as "carbonate hardness," and it's what freshwater and planted-tank kits print on the box; the unit, dKH, is the same German degree reefers use. Two things it is not: it isn't GH (general hardness — calcium and magnesium; that's the hardness converter's subject), and in a reef it isn't only carbonate — borate and a little else contribute, which is why a reef "alk" of 8 dKH is a slightly different mix than 8 dKH of freshwater KH. The number converts the same way regardless.
Which unit does my test kit use?
| Kit | Reports in |
|---|---|
| Salifert KH/Alk | dKH and meq/L (both on the syringe scale) |
| Hanna Checker HI772 (marine) | dKH |
| Hanna Checker HI755 (marine) | ppm CaCO₃ |
| Red Sea KH/Alkalinity Pro | dKH (meq/L on the chart) |
| API KH (freshwater kit, works in salt) | dKH — one drop ≈ 1 dKH ≈ 17.9 ppm |
| Tropic Marin, Nyos, Fauna Marin | dKH |
| Aquarium controllers / lab meters | dKH or meq/L, sometimes mg/L as CaCO₃ |
Typical of current versions; check your box. Whatever the unit, the dosing calculators on this site take all three.
What should alkalinity be, in each unit?
Natural seawater is about 7 dKH (2.5 meq/L, 125 ppm). Mixed reefs usually hold 8–9.5 dKH (2.9–3.4 meq/L, 143–170 ppm); LPS and soft-coral tanks tolerate up to 11. Freshwater community tanks are happy anywhere from 3 to 8 dKH, planted tanks with CO₂ often deliberately low (3–5) because the pH/KH/CO₂ relationship gets easier there. To move the number, the alkalinity dosing calculator takes any of the three units and gives grams.