Free pH KH CO2 Calculator - Measure Dissolved CO2 | Fish Tank Calculator

Calculate actual dissolved CO2 ppm from your aquarium's pH and KH readings. Verify CO2 injection is in the safe 15–30 ppm range for planted tanks.

In freshwater chemistry, dissolved CO2 cannot be measured directly with a hobbyist test kit, but it can be inferred from pH and carbonate hardness (KH) using the well-known relationship between carbonic acid and bicarbonate. This calculator takes your measured pH and KH (in degrees dKH or ppm CaCO3) and returns dissolved CO2 in ppm along with a categorical status: low (algae-prone with poor plant growth), safe (15-25 ppm), or dangerously high. It is the planted-tank chemistry equivalent of a drop checker, but instant and quantitative.

How It Works

The relationship is CO2 (ppm) = 12.839 * KH (dKH) * 10^(7-pH). The calculator accepts KH in either dKH or ppm CaCO3 (1 dKH = 17.86 ppm), computes the exponent against pH, and returns ppm CO2. The status thresholds follow community guidance: under 10 ppm is low, 15-25 ppm is the planted-tank safe zone, 25-30 ppm risks fish stress, and above 30 ppm is acutely dangerous. The model assumes carbonate alkalinity is the dominant buffer, which holds for most planted aquariums but not for tanks dosed with phosphate or organic acid buffers.

Usage Scenarios

  • Verifying that a freshly tuned pressurized CO2 system delivers 20 ppm at lights-on in a planted tank running pH 6.8 and KH 4.
  • Diagnosing chronic black brush algae in a tank where the keeper assumed CO2 was adequate but the pH/KH math reveals only 8 ppm.
  • Confirming that a low-tech tank with no injection is sitting at 3-5 ppm CO2 so that liquid-carbon dosing decisions can be made rationally.
  • Cross-checking a drop checker reading against the pH-KH formula before adjusting the CO2 needle valve in the morning.

How to Use the pH/KH → CO2 Calculator

Measure your tank's pH and KH (carbonate hardness in °dKH) using test kits, then enter them here. The calculator applies the standard equilibrium formula CO2 = 3.51 × KH × 10^(7−pH) to show your actual dissolved CO2 level in ppm.

The safe range is 15–30 ppm. Below 15 ppm plants may grow slowly; above 30 ppm fish can show signs of oxygen stress. Use this calculator to verify your CO2 injection is working correctly rather than guessing by bubble count.

Note: this formula assumes only carbonic acid affects pH. If you use acidic buffers (e.g., peat, driftwood, CO2) alongside KH buffers, the result may differ slightly. For most planted tanks using standard tap water it is accurate.

Frequently Asked Questions

How is dissolved CO2 calculated from pH and KH?

The formula is: CO2 (ppm) = 3.51 × KH (°dKH) × 10^(7 − pH). This is derived from the carbonic acid equilibrium in freshwater. Higher pH or lower KH means less dissolved CO2. The formula assumes a closed system with no other acids present.

What is the safe CO2 range for planted tanks?

The ideal range is 15–30 ppm. Below 15 ppm, plants may not grow well. Above 30 ppm, fish can show signs of stress such as gasping at the surface. Monitor fish behavior and adjust CO2 injection accordingly.

How do I measure KH?

KH (carbonate hardness) is measured with a standard liquid KH test kit or digital meter. It is expressed in °dKH (German degrees) or ppm CaCO3. 1 °dKH = 17.848 ppm CaCO3.

Why does my pH test show high CO2 if I'm not injecting?

Tap water, organic decomposition, fish respiration, and substrate all contribute to CO2. Even without injection, tanks typically have 2–5 ppm of dissolved CO2. This calculator shows what is currently dissolved, not what you've injected.

Does this formula work for saltwater tanks?

No. Saltwater chemistry is more complex due to borate alkalinity. This formula applies only to freshwater tanks. For saltwater CO2 estimation, use a dedicated reef chemistry calculator.