Convert the units reef test kits, labs, and forums mix up: alkalinity (dKH, meq/L, ppm), salinity (ppt and specific gravity), volume (gallons and liters), phosphate vs phosphorus, and nitrate vs nitrate-nitrogen.
Conversion factors
| Conversion | Factor |
|---|---|
| Alkalinity | 1 meq/L = 2.8 dKH = 50.04 ppm as CaCO₃ (1 dKH = 17.86 ppm) |
| Volume | 1 US gallon = 3.785411784 L (exact) |
| Phosphate | PO₄ = P × 3.066; P = PO₄ × 0.3261 (molar masses 94.97 and 30.97) |
| Nitrate | NO₃ = NO₃-N × 4.427; NO₃-N = NO₃ × 0.2259 |
| Salinity | ppt ↔ specific gravity at 25 °C from the seawater equation of state (35 ppt = 1.0264) |
FAQ
How do I convert dKH to meq/L? Divide dKH by 2.8. 8 dKH is about 2.86 meq/L, and 2.5 meq/L is 7.0 dKH.
How do I convert dKH to ppm? Multiply dKH by 17.86 to get ppm (mg/L) as calcium carbonate. 8 dKH ≈ 143 ppm.
Is my phosphate test reading PO₄ or P? Most hobby kits read phosphate (PO₄). Some, like the Hanna HI736 ultra-low checker, read phosphorus (P) in ppb. Multiply P by 3.066 to get PO₄: 30 ppb P ≈ 0.092 ppm PO₄.
Sources
- Randy Holmes-Farley, "Chemistry and the Aquarium: What is Alkalinity?" (Advanced Aquarist, 2002): 1 meq/L = 2.8 dKH = 50.04 mg/L as CaCO₃
- NIST Handbook 44 (2026), Appendix C: General Tables of Units of Measurement
- Fofonoff & Millard (1983), Algorithms for Computation of Fundamental Properties of Seawater, UNESCO Technical Papers in Marine Science 44