ScapeMath

CO₂ from pH & KH calculator

If carbonates are the only buffer in your water, pH and KH together tell you how much CO₂ is dissolved. Enter your readings to get an estimate and the pH to aim for.

°C
mg/L
ppm
Dissolved CO₂
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Classic shortcut
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pH for target
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Drop from degassed
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How it works

CO₂ dissolved in water forms carbonic acid, which partly splits into hydrogen and bicarbonate ions. At equilibrium, the ratio is fixed by the first dissociation constant K₁:

CO₂ (mg/L) = [HCO₃⁻] × γ × 44.01 × 10^(pK₁ − pH) [HCO₃⁻] (mmol/L) ≈ KH in meq/L = dKH × 0.3566 pK₁(T) from Plummer & Busenberg (1982): log K₁ = −356.3094 − 0.06091964·T + 21834.37/T + 126.8339·log₁₀T − 1684915/T² (T in kelvin) γ (bicarbonate activity) from the Davies equation, with ionic strength ≈ 2.5×10⁻⁵ × TDS

At 25 °C, pK₁ is 6.35. The hobby shortcut CO₂ = 3 × KH × 10^(7 − pH) is the same equation with the constants rounded, roughly pK₁ = 6.28 with no temperature or activity correction. It reads roughly 10–15% lower than the full calculation at typical temperatures. Both are shown so you can compare with charts you may already use.

The degassed pH method is often more useful than the absolute number. Water left out for a day settles to the low CO₂ level of room air. Every 1.0 drop in pH below that reading means ten times more CO₂. Many planted-tank keepers aim for a drop of about 1.0 when CO₂ is fully on.

Worked examples

KH 4 dKH, pH 6.6, 25 °C

KH = 4 × 0.3566 = 1.427 meq/L. 10^(6.352 − 6.6) = 0.565. With TDS 150 mg/L, γ = 0.936, so CO₂ = 1.427 × 0.936 × 44.01 × 0.565 = 33 ppm. The shortcut gives 3 × 4 × 10^0.4 = 30 ppm.

Which pH gives 30 ppm at KH 4?

pH = pK₁ + log₁₀(1.427 × 0.936 × 44.01 ÷ 30) = 6.352 + 0.292 = pH 6.64. That's a sensible pH-controller set point for this water.

Soft water at 22 °C (72 °F)

KH 2 dKH, pH 6.8, TDS 100 mg/L: 11 ppm. The shortcut says 9.5 ppm, and the difference comes from temperature: pK₁ is 6.37 at 22 °C.

When the answer is wrong

References

  1. Plummer, L.N. & Busenberg, E. (1982). The solubilities of calcite, aragonite and vaterite in CO₂–H₂O solutions between 0 and 90 °C… Geochimica et Cosmochimica Acta 46: 1011–1040
  2. USGS Publications Warehouse record for Plummer & Busenberg (1982)
  3. Stumm, W. & Morgan, J.J. (1996). Aquatic Chemistry, 3rd ed., Wiley (carbonate equilibria; Davies equation)
  4. Langelier, W.F. (1936). The analytical control of anti-corrosion water treatment. JAWWA 28: 1500–1521 (ionic strength ≈ 2.5×10⁻⁵ × TDS)
  5. Barr, T. EI target CO₂ range 20–30 ppm. Barr Report

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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