KH + pH estimate
Enter carbonate hardness and pH to estimate dissolved CO2 in ppm for freshwater planted aquariums.
Tools
CO2 calculator
Estimate dissolved CO2 from KH and pH values. Use the chart to visualize target ranges and confirm your reading.
KH + pH to CO2 ppm
Freshwater focus
Calculate dissolved CO2
Use the standard KH/pH relationship for planted freshwater aquariums.
CO2 reference chart
Cross-reference KH and pH values to compare your result with typical CO2 ranges for planted tanks.
KH (rows)pH (columns)
Chart for quick comparison
Use the reference chart to compare nearby KH and pH values and understand how small pH shifts affect CO2.
Safety-first interpretation
High CO2 can stress fish and shrimp. Make changes gradually and monitor livestock closely.
Planted tank CO2 guide
Estimate CO2 from KH and pH, then verify with livestock behavior.
The KH/pH CO2 relationship is a useful planted tank reference, but it is still an estimate. Use it with observation, surface agitation, drop checker context, and careful changes to injection rate.
How the CO2 calculator works
The calculator uses the common planted aquarium formula CO2 ppm = 3 * KH * 10^(7 - pH). KH is entered in dKH and pH is the measured aquarium pH.
This relationship assumes carbonate buffering is the main factor affecting pH. Other acids, buffers, active substrates, and water chemistry can make the estimate less accurate.
Using CO2 estimates in planted tanks
Many high-light planted tanks aim for a stable CO2 range before and during the photoperiod, but livestock safety comes first.
If fish gasp, shrimp become inactive, or livestock gather near the surface, reduce CO2 and increase aeration even if the calculator suggests a target range.
Why pH timing matters
CO2 changes throughout the day. A pH reading before CO2 turns on, during peak injection, and after lights out can tell very different stories.
For useful comparisons, test at consistent times and log the readings together with bubble rate, drop checker color, and plant response.
When KH and pH are not enough
Active aquasoils, peat, driftwood, acids, remineralizers, and non-carbonate buffers can change pH independently of dissolved CO2.
Use the calculator as a reference, not a guarantee. Combine it with a calibrated pH test, KH test, livestock behavior, and gradual adjustments.
FAQ
CO2 Calculator FAQ
What CO2 level is good for a planted aquarium?
Many planted tanks aim somewhere around 20 to 30 ppm, but the right level depends on livestock, oxygenation, plant demand, light, and stability.
Is the KH/pH CO2 calculator accurate?
It is an estimate. It is most useful when carbonate hardness is the main buffer and pH is measured consistently with reliable tests.
Can too much CO2 hurt fish?
Yes. High CO2 can stress or kill fish and shrimp. Watch behavior closely and increase aeration or reduce injection if livestock show distress.
Why did my pH drop after adding CO2?
Dissolved CO2 forms carbonic acid, which lowers pH. The size of the pH drop depends on KH, injection rate, gas exchange, and timing.
Does this calculator work for saltwater aquariums?
No. This tool is intended for freshwater planted aquariums. Saltwater and reef systems need different interpretation.
When should I measure pH for a CO2 estimate?
Measure pH at a consistent point in the CO2 schedule, such as before injection and again near peak injection. Comparing readings at the same time each day makes trends easier to interpret.
Related aquarium planning
Continue with the next aquarium decision.
Use the calculator result with logs, guides, and maintenance tools so one number becomes part of a repeatable aquarium care routine.
Nitrate & Phosphate Ratio CalculatorCheck nutrient context when troubleshooting plant growth.Water Parameters TrackingTrack KH, pH, CO2 context, and repeated readings over time.Aquarium Test Results LogLearn why repeated measurements are more useful than one test.Aquarium CO2 Tracking GuideConnect KH, pH, injection timing, plant response, and livestock safety.
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