Aquarium CO2 Calculator: CO2 ppm from KH & pH
Free aquarium CO2 calculator: enter KH and pH to get dissolved CO2 in ppm via 3 × KH × 10^(7−pH). Target 20-30 ppm for planted tanks.
An aquarium CO2 calculator estimates the dissolved carbon dioxide in your planted tank from two numbers you can test at home: pH and carbonate hardness (KH, in dKH). The tool above runs the standard hobby formula, CO2 (ppm) = 3 × KH × 10^(7 − pH), in two directions. In measure mode, enter your pH and KH to read CO2 in ppm. In target mode, enter your KH and a desired ppm and it returns the pH to aim for. It then flags where you land: under 10 ppm is low for demanding plants, 20-30 ppm is the planted-tank sweet spot, and over 35 ppm pushes fish toward stress. Because the math assumes carbonic acid is the only thing moving your pH, treat every result as an estimate, not a lab reading — the sections below explain when the chart lies and how a drop checker keeps it honest.
How much CO2 should a planted aquarium have?
A planted aquarium should hold 20-30 ppm of dissolved CO2, with 30 ppm treated as the safe ceiling for a tank containing fish or shrimp. That range gives plants enough carbon for strong photosynthesis while staying below the roughly 35 ppm point where livestock begin gasping and losing color. Heavily stocked tanks should target the low 20s; plant-dominant aquascapes with few animals can sit at the top of the range.
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Aquarium CO₂ Calculator (pH/KH)
Key Information
| Parameter | Details |
|---|---|
| CO2 Formula (measure mode) | 3 × KH × 10^(7 − pH) = ppm |
| Target-pH Formula | pH = 7 − log10(ppm ÷ (3 × KH)) |
| Planted-Tank Target | 20-30 ppm (30 ppm safe cap) |
| Non-Injected Equilibrium | ~2-3 ppm CO2 |
Frequently Asked Questions
How much CO2 should a planted aquarium have?
A planted aquarium should hold 20-30 ppm of dissolved CO2, with 30 ppm treated as the safe ceiling for a tank containing fish or shrimp. That range gives plants enough carbon for strong photosynthesis while staying below the roughly 35 ppm point where livestock begin gasping and losing color. Heavily stocked tanks should target the low 20s; plant-dominant aquascapes with few animals can sit at the top of the range.
How do I calculate aquarium CO2 from KH and pH?
Calculate dissolved CO2 with the formula CO2 (ppm) = 3 × KH × 10^(7 − pH), where KH is carbonate hardness in dKH. For example, KH 4 and pH 6.6 gives 3 × 4 × 10^0.4 = about 30 ppm. Every 1.0 drop in pH at fixed KH multiplies CO2 tenfold. The calculator above does this instantly, and its target mode reverses it to tell you which pH produces the ppm you want.
Is 30 ppm of CO2 safe for fish?
Thirty ppm is generally safe for most community fish and shrimp, but it is a ceiling, not a comfort zone. Safety depends on oxygen: a tank with strong surface agitation and high dissolved O2 tolerates 30 ppm well, while a still, oxygen-poor tank can stress fish at the same level. Watch for surface gasping, and always keep CO2 off at night, when levels build up unused.
Are these calculators free to use?
Yes, all calculators on CalcCorp are completely free — no registration, no login, no hidden charges. Results are calculated instantly in your browser and we do not store any of your data.
How accurate are these calculations?
Our calculators use standard financial formulas updated with the latest tax rates, interest rates, and government policies for 2026. Results are accurate for planning purposes but should be verified with a professional for final decisions.
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Last updated: March 2026