A planted tank CO2 calculator uses the pH-KH relationship to determine the exact CO2 concentration in your aquarium. Getting CO2 wrong can kill fish: too little and algae overwhelms plants, too much and your fish suffocate. Enter your pH and KH readings for an instant CO2 reading, bubble rate estimate, and safety assessment.
Tank Parameters
Measure several hours after lights on
Degrees of carbonate hardness
Typical target: 20–30 ppm
pH-KH-CO2 Reference Table (ppm)
| pH \ KH | 3 dKH | 4 dKH | 5 dKH | 6 dKH | 8 dKH |
|---|---|---|---|---|---|
| 6.4 | 57 | 76 | 95 | 114 | 152 |
| 6.6 | 36 | 48 | 60 | 72 | 96 |
| 6.8 | 23 | 30 | 38 | 45 | 61 |
| 7.0 | 14 | 19 | 24 | 29 | 38 |
| 7.2 | 9 | 12 | 15 | 18 | 24 |
| 7.4 | 6 | 7 | 9 | 11 | 15 |
| 7.6 | 4 | 5 | 6 | 7 | 10 |
How to Use the Planted Tank CO2 Calculator
CO2 injection is the single most impactful upgrade you can make to a planted aquarium — but it requires careful management. The planted tank CO2 calculator uses the proven pH-KH relationship to give you an instant CO2 reading without a drop checker, along with the exact pH target needed to hit your desired CO2 level.
Step 1: Test pH and KH
Measure pH several hours after your lights turn on, when CO2 is at its peak. Morning readings taken before lights on will be lower CO2 (higher pH) due to overnight respiration. A quality digital pH pen gives more precise readings than a liquid test kit. For KH, a titration test kit is accurate and inexpensive. Test both the same water sample for a consistent calculation.
Step 2: Enter Values and Calculate
Enter your measured pH and KH values. The calculator applies the formula CO2 = 3 × KH × 10^(7 − pH) to determine CO2 concentration in parts per million. This formula is widely used in the aquascaping community and is accurate when your KH is composed primarily of carbonate hardness (not silicates or other buffering compounds).
Step 3: Interpret the Safety Status
The result includes a safety rating. Under 15 ppm is too low for optimal plant growth and algae will likely outcompete your plants. 20–30 ppm is the sweet spot for lush growth. 30–35 ppm is borderline — watch your fish for surface gasping. Over 35 ppm is dangerous territory; reduce your injection rate immediately. If fish are gasping or listing sideways, aerate immediately and reduce CO2.
Step 4: Dial In Your Bubble Rate
The bubble rate estimate gives you a rough starting point based on tank volume. Actual bubble rates vary enormously by diffuser type, water movement, and CO2 system pressure. Start at the estimated rate, then measure pH 4–6 hours after lights on. Adjust up or down in small increments (0.5 bubbles per second) until your pH stabilizes at the target value for your desired CO2 level.
Step 5: Set a CO2-Off Schedule
Always run CO2 during the photoperiod — typically turning on 1 hour before lights and off 1 hour before lights out. This prevents overnight CO2 accumulation when plants cannot photosynthesize. A pH controller connected to your CO2 solenoid automates this perfectly, shutting off CO2 when pH drops to your target and resuming when it rises above it.
Understanding the pH-KH-CO2 Relationship
CO2 dissolves in water to form carbonic acid, which lowers pH. The degree of pH drop depends on how much buffering capacity (KH) your water has. High KH water resists pH changes, so you need more CO2 to achieve the same pH drop. The reference table shows exact CO2 values at common pH and KH combinations — use it as a quick field guide without needing to run a calculation.
Frequently Asked Questions
Is this CO2 calculator free?
Yes, this planted tank CO2 calculator is completely free with no signup required. All calculations run locally in your browser using the standard pH-KH-CO2 relationship formula.
Is my data private?
Absolutely. Everything runs client-side in your browser. Your pH, KH, and tank parameters never leave your device — no server, no tracking.
What is the ideal CO2 level for a planted aquarium?
Most planted tanks target 20–30 ppm of CO2. Below 15 ppm, plant growth is limited and algae often wins. Above 35–40 ppm, fish and shrimp become stressed and risk of death increases significantly. The sweet spot for lush plant growth without harming livestock is 25–30 ppm.
How does the pH-KH-CO2 relationship work?
CO2 dissolves in water to form carbonic acid, which lowers pH. The relationship is governed by the Henderson-Hasselbalch equation: CO2 (ppm) = 3 × KH (dKH) × 10^(7 − pH). This means for any given KH, a lower pH indicates higher CO2 and a higher pH indicates lower CO2.
Why does the calculator warn about high CO2?
CO2 levels above 35 ppm can cause fish to gasp at the surface, become lethargic, or die — especially sensitive fish like tetras, rasboras, and shrimp. The calculator flags readings above 30 ppm as high and above 35 ppm as dangerous so you can reduce your injection rate before harming livestock.
How accurate is the bubble rate estimate?
The bubble rate estimate is a rough guideline based on typical diffuser efficiency. Actual bubble rate varies widely depending on diffuser type, water flow, tank depth, CO2 cylinder pressure, and tubing length. Use it as a starting point, then fine-tune based on your actual pH readings taken a few hours after lights-on.
Can I use this calculator with a CO2 drop checker?
Yes. A drop checker uses a dKH reference solution (usually 4 dKH) and a pH indicator dye. If you know the color corresponds to a target pH of 6.6–6.8 in 4 dKH solution, you can reverse-engineer the CO2 level. This calculator lets you enter your KH and observed pH to get the actual CO2 concentration.
What KH should I target for a planted tank?
A KH of 3–6 dKH is typical for planted tanks. Very low KH (under 2 dKH) makes pH unstable and CO2 levels hard to control. Very high KH (above 10 dKH) makes it difficult to achieve the pH drop needed for adequate CO2 without over-acidifying the water. Aim for 4–5 dKH for the most manageable CO2 control.