Aquarium CO₂ Calculator (CO₂ ppm from pH and KH)
The dissolved CO₂ in a planted aquarium from its pH and carbonate hardness, and the pH you would need to hold for a target CO₂ level.
Results
The dashed line marks the value you entered.
What this tool does
CO₂ cannot be read with a simple test, but when KH is known and pH is measured the two give it away: CO₂ ≈ 3 × KH × 10^(7 − pH), with KH in °dKH and the result in ppm. At KH 4 the water holds about 12 ppm at pH 7.0 and about 30 ppm at pH 6.6, which is the relation behind pH–KH–CO₂ charts and drop checkers. It holds only when bicarbonate is the water's buffer: peat, phosphate buffers, 'pH down' products and some additives push the result off, so treat it as an estimate. Many planted-tank keepers aim for roughly 20–30 ppm, and watching the fish matters more than the number.
Formula
CO₂ (ppm) = 3 × KH (°dKH) × 10^(7 − pH) · pH for a target = 7 − log₁₀(target ÷ (3 × KH))
Variables
| Symbol | Meaning | Unit |
|---|---|---|
ph | pH of the tank water | — |
kh | Carbonate hardness (KH) | — |
ku | KH is given in | — |
tg | CO₂ level you want | ppm |
C | Dissolved CO₂ | ppm |
TP | pH that gives that CO₂ level | — |
DK | KH in °dKH | °dKH |
LV | Where that sits | — |
Worked example
- pH of the tank water6.8
- Carbonate hardness (KH)4
- KH is given in1
- CO₂ level you want30 ppm
- Dissolved CO₂19.0 ppm
- pH that gives that CO₂ level6.60
- KH in °dKH4.00 °dKH
- Where that sitsThe usual range for a planted tank
Limitations
- The result is an estimate based only on the values you type. Real situations often include factors this calculator does not know about.
- The default values are typical reference figures, not measurements of your situation. Replace them with your own data whenever you have it.