How to use the aquarium co2 calculator
- Choose the unit your KH test uses, dKH or ppm as calcium carbonate, and enter the carbonate hardness.
- Enter the pH of the tank water.
- Optionally enter a CO2 level you want. The page then finds the pH that matches it at your KH.
- Read the estimated dissolved CO2 in ppm and the table of CO2 across pH values at your KH.
Formula
CO2 (mg/L, same as ppm) = 3 x KH (in dKH) x 10^(7 - pH)
KH in dKH = KH in ppm as CaCO3 / 17.848
pH for a target CO2 = 7 - log10(CO2 / (3 x KH))
Each 0.1 drop in pH multiplies CO2 by 1.26 at the same KH
The rule comes from the carbonate balance of water. It holds only when bicarbonate is the main buffer, and the number 3 includes a conversion that is built in, so use the KH in dKH.
KH 4 at pH 6.8
With a KH of 4 dKH (71.392 ppm) and a pH of 6.8 the estimate is 3 x 4 x 10^0.2 = 19.02 ppm. At pH 7.0 it is exactly 12 ppm, and at pH 6.0 it is 120 ppm. A KH of 71.4 ppm at pH 7.0 gives 12 ppm again because it is the same hardness.
To reach 30 ppm at a KH of 4 dKH the pH must be 7 - log10(30 / 12) = 6.602. The table lists pH 6.0 to 7.6 in steps of 0.2 for your KH: 120, 75.71, 47.77, 30.14, 19.02, 12, 7.571, 4.777 and 3.014 ppm at a KH of 4.
How reliable is the estimate
The pH and KH method is a rule of thumb used in planted tanks. It works for tap water where carbonate sets the buffer, and it can be far off when peat, driftwood tannins, phosphate buffers, acids or other additives are present. A drop checker or a direct test is more reliable, so use the estimate as a guide.
For the water in the tank see the aquarium volume calculator, and for partial changes the aquarium water change calculator. The aquarium salt calculator and the aquarium heater size calculator cover other setup numbers.
Reading pH and KH
pH is a scale, so a small change in pH is a large change in CO2: each drop of 1 multiplies the level by 10. KH is the carbonate hardness, which is different from general hardness (GH). Use a KH test and a pH reading taken at the same time, ideally from the same water sample, and record the tank conditions along with the numbers.
Assumptions and limits
- Carbonate (bicarbonate) is the main buffer in the water.
- KH in ppm is as calcium carbonate, and one dKH is 17.848 ppm.
- One ppm of CO2 in water is the same as one milligram per liter.
- The page gives an estimate, and it does not advise on animal or plant health.
Frequently asked questions
How do I calculate CO2 from pH and KH?
Use CO2 (ppm) = 3 x KH (dKH) x 10^(7 - pH). A KH of 4 at pH 6.8 gives about 19 ppm.
Why is the pH and KH chart unreliable sometimes?
It only works when bicarbonate is the main buffer. Peat, some water conditioners and other buffers break the link, so the real CO2 can differ.
How do I convert KH from ppm to dKH?
Divide the ppm as calcium carbonate by 17.848. A KH of 71.4 ppm is almost exactly 4 dKH.
What pH gives a certain CO2 level?
Use pH = 7 - log10(CO2 / (3 x KH)). For 30 ppm at a KH of 4 dKH it is 6.602.