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Dissolved CO₂ from pH and KH

Estimate dissolved CO₂ from carbonate hardness and pH. Approximation for planted tanks; cross-check with a drop checker if you inject CO₂.

For comparable readings over time and with this calculator, keep conditions consistent:

  • KH — more stable than pH; test before a water change or 15–30 min after mixing.
  • pH — test at the same time of day; values differ before and after CO₂ injection.
  • With CO₂, compare pH and CO₂ estimates at the same injection state and time of day.

Compare readings taken at the same time of day under similar conditions (CO₂, feeding).

CO2 (mg/L) = 3·KH·10(7 − pH)
KH
carbonate hardness, °dKH (KH test kit)
pH
water acidity
Assumes alkalinity is mostly from carbonates; phosphates and other buffers skew the result. Does not replace a drop checker when injecting CO₂. Temperature is not included in this calculation.
CO₂ = 3 × KH × 10^(7 − pH) is a practical carbonate-buffer approximation; rigorous pH, alkalinity and CO₂ relationship: Randy Holmes-Farley — alkalinity and pH (Advanced Aquarist)