water
CO₂ estimator
Estimate dissolved CO₂ from pH and KH — and see exactly when the estimate stops being trustworthy.
≈ 19 mg/L CO₂
The range usually targeted in an injected planted tank
Working — every step, so you can check it
- Formula
- 3 × 4 × 10^(7 − 6.8)
- Exponent
- 10^0.2 = 1.5849
- Result
- 19 mg/L
Formula
- CO₂ (mg/L) ≈ 3 × KH (°dKH) × 10^(7 − pH)
Assumptions
An estimate whose assumptions are invisible is indistinguishable from a guess.
- Carbonate and bicarbonate are the ONLY significant buffers in the water.
- The KH measurement reflects carbonate alkalinity rather than another acid-neutralising species.
- The system is at equilibrium.
Limitations
Where this calculation stops being reliable. Read these before acting on a result.
- The assumption fails in any tank containing tannins from wood or leaves, phosphate buffers, or a commercial pH adjuster — and it fails in the direction of over-reading, often by a large factor.
- A blackwater tank at pH 5.5 and KH 1 is not carrying 90 mg/L of dissolved CO₂; the acidity comes from organic acids, not carbonic acid.
- A drop checker with a 4 °dKH reference solution avoids the assumption entirely and is the more reliable instrument.