Aquarium CO2 Calculator
The aquarium CO2 calculator estimates the dissolved carbon dioxide concentration in your planted aquarium water based on pH and carbonate hardness (KH). Carbon dioxide is essential for aquatic plant growth, and maintaining 20 to 30 mg/L (ppm) is the standard target for most planted tanks. This calculator uses the well-established formula: CO2 (mg/L) = 3 x KH x 10^(7 - pH). Enter your measured pH and KH values to instantly check whether your CO2 level is in the optimal range for your plants and safe for your fish.
CO2 estimation formula
CO2 (mg/L) = 3 x KH (dKH) x 10^(7 - pH)
Target pH = 7 - log10(Target CO2 / (3 x KH))
Example: pH 6.8, KH 4
CO2 = 3 x 4 x 10^(7 - 6.8) = 12 x 10^0.2 = 12 x 1.585 = 19.02 mg/L
Frequently asked questions
How do I calculate dissolved CO2 in an aquarium?
Dissolved CO2 (mg/L) = 3 x KH (degrees) x 10^(7 - pH). This formula uses carbonate hardness (KH) and pH to estimate CO2 concentration. It works when bicarbonate is the primary buffer in the water, which is true for most tap water sources.
What is the ideal CO2 level for a planted aquarium?
Most planted tank hobbyists aim for 20 to 30 mg/L (ppm) of dissolved CO2. Below 15 mg/L, many aquatic plants grow slowly. Above 35 to 40 mg/L, fish may show signs of CO2 stress such as rapid breathing or staying at the surface. A level of 25 mg/L is a common target.
What is KH in aquarium water chemistry?
KH (carbonate hardness or alkalinity) measures the buffering capacity of water, primarily from bicarbonate and carbonate ions. KH is typically measured in degrees (1 dKH = 17.8 mg/L HCO3). Higher KH means more stable pH but requires more CO2 injection to lower pH and raise CO2 levels.
Does the CO2 formula work for all water types?
This formula is accurate when bicarbonate is the main buffer. It becomes less accurate in very soft water (KH below 2) or when phosphate buffers are used (as in some water conditioners). If you use phosphate-based pH stabilisers, the CO2 estimate from this formula will be inaccurate.
How often should I check CO2 levels?
Many aquarists use a drop checker with a 4 dKH reference solution and bromothymol blue indicator. The indicator turns blue (too low), green (target 25-30 mg/L), or yellow (too high). Check daily when first adjusting CO2, then weekly once stable.
Sources
- EPA: EPA - Aquatic Life Ambient Water Quality Criteria for CO2.
- USGS: USGS - National Field Manual: Carbon Dioxide in Water.
Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. See our methodology.