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CO2 / pH / KH Calculator

Enter your aquarium's pH and KH readings to get the exact dissolved CO2 level — instantly. No charts, no guessing.

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PlantedGuide Editorial Team · Editorial Research
Reviewed for accuracy · About our process

Calculate Dissolved CO2

Measured with a calibrated pH pen or test kit

Degrees of carbonate hardness — from a KH test kit

— mg/L (ppm)
—

010203040+ mg/L

§ 01 · Reference

CO2 Levels at a Glance

CO2 Level Status Effect on Tank
Below 15 mg/L Deficient Slow plant growth; BBA and thread algae likely
15–20 mg/L Acceptable Adequate for low-tech or low-light setups
20–30 mg/L Optimal Strong plant growth, healthy fish at normal O2 levels
30–35 mg/L Elevated Watch fish carefully — surface gasping is a warning sign
Above 35 mg/L Dangerous Fish stress and mortality risk — reduce CO2 immediately

§ 02 · Method

How the Calculation Works

CO2 (mg/L) = 3 × KH × 10^(7 - pH)

This formula derives from the carbonate equilibrium system in freshwater. KH (carbonate hardness) and pH together determine how much CO2 is dissolved, because carbonic acid formed by dissolved CO2 directly drives pH down. The relationship is logarithmic: a 1-unit drop in pH doubles CO2 for any given KH.

Accuracy caveat: this formula assumes carbonic acid is the only buffering agent. It becomes unreliable if you use RO water with non-carbonate remineralizers, peat, CO2-independent pH-down products, or heavily tannic driftwood. In those cases, use a drop checker with a 4 dKH reference solution.

§ 03 · FAQ

Common Questions

The ideal CO2 level for most planted tanks is 20–30 mg/L (ppm). Below 15 mg/L plants will show slow growth and deficiencies. Above 35 mg/L you risk stressing or killing fish. The sweet spot is 25–30 mg/L — high enough to saturate plant demand without harming livestock.
The formula CO2 = 3 x KH x 10^(7-pH) assumes carbonic acid is the only buffering agent in the water. It is accurate for tanks with plain tap water and no pH-altering additives. If you use RO water remineralized with non-carbonate buffers, peat, driftwood, or CO2-independent pH-down products, the calculation will be unreliable. In those cases, a drop checker with 4 dKH reference solution is more accurate.
Pearling is a sign of excess oxygen production, not a requirement for healthy CO2 levels. If your CO2 reads 25–30 mg/L and plants look healthy with good growth, you're fine — pearling may simply require higher light intensity or a more heavily planted setup. Alternatively, check whether your KH is being affected by organic acids from substrate or driftwood, which would skew the pH reading downward and cause the calculator to overestimate CO2.
Most planted tanks do well with a KH between 3–6 dKH. This range gives the pH/CO2 relationship enough sensitivity to be useful. At very low KH (below 2 dKH) small pH swings represent large CO2 changes, making control difficult. At high KH (above 8 dKH) you need more CO2 injection to bring pH down to the target range. A KH of 4–5 dKH with a target pH of 6.8–7.0 is a reliable starting point for most planted setups.

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