How do you calculate CO2 from pH and KH?

CO2 in ppm is about 3 × KH in degrees × 10 to the power of (7 − pH). At 4 dKH and pH 6.6 that gives about 30 ppm, the figure a standard 4 dKH drop checker shows as green.

How much CO2 should a planted tank have?

The Krib chart says 15–25 mg/L works best for plants and warns that over 30 mg/L may cause breathing problems in fish. The Estimative Index guide targets 20–30 ppm. Watch the fish first whenever you raise CO2.

Why is my pH/KH reading wrong?

The formula assumes carbonate is the only buffer in the water. Phosphate buffers, acids from soils or wood, and other additives move pH without changing CO2, so the estimate can be far off. A drop checker filled with 4 dKH reference solution avoids that problem.

Entry 02CO₂

CO₂ from pH and KH

Enter your tank's KH and its pH with the CO₂ running. You get the estimated CO₂ in ppm, where it sits against the published targets, and the pH to aim for.

Last updatedOct 19, 2026

FormulaCO₂ = 3 × KH × 10^(7 − pH)

The chart

The same formula over the usual range. Your reading is outlined (rounded to the nearest whole dKH and 0.2 pH). The shaded cells fall in the 20–30 ppm target from the EI guide (Barr Report forum¹). The Krib table, compiled by Chuck Gadd, is the familiar version of this chart (The Krib (Chuck Gadd's CO2/pH/KH table)²).

CO₂ in ppm = 3 × KH × 10^(7 − pH). Shaded: 20–30 ppm
KH	pH 6.0	pH 6.2	pH 6.4	pH 6.6	pH 6.8	pH 7.0	pH 7.2	pH 7.4	pH 7.6
1 dKH	30	19	12	7.5	4.8	3.0	1.9	1.2	0.8
2 dKH	60	38	24	15	9.5	6.0	3.8	2.4	1.5
3 dKH	90	57	36	23	14	9.0	5.7	3.6	2.3
4 dKH	120	76	48	30	19	12	7.6	4.8	3.0
5 dKH	150	95	60	38	24	15	9.5	6.0	3.8
6 dKH	180	114	72	45	29	18	11	7.2	4.5
7 dKH	210	133	84	53	33	21	13	8.4	5.3
8 dKH	240	151	96	60	38	24	15	9.6	6.0
9 dKH	270	170	107	68	43	27	17	11	6.8
10 dKH	300	189	119	75	48	30	19	12	7.5

What the numbers mean

The Krib table states that 15–25 mg/L of CO₂ works best for plant growth and that concentrations above 30 mg/L may cause respiratory problems in fish (The Krib (Chuck Gadd's CO2/pH/KH table)²). The EI guideline lists a CO₂ target of 20–30 ppm (Barr Report forum¹). The calculator reports against both. Fish gasping at the surface is a sign to cut back now, whatever the number says.

When pH/KH misleads

The formula comes from the carbonate equilibrium, and it assumes carbonate is the only thing buffering the water. Anything else that sets pH breaks that assumption: phosphate buffers, acids from soil or wood, or additives. Then the estimate can be far off in either direction. Soft-water tanks with very low KH are the most sensitive.

Drop checkers

A drop checker isolates a small volume of reference solution, usually 4 dKH water with bromothymol blue, from the tank by an air gap, so only CO₂ crosses over. In the Barr Report thread on drop checkers, Hoppy explains that 4 dKH at pH 6.6 corresponds to 30 ppm, that green indicates roughly 30 ppm (25–40), yellow-green about 50 and blue-green about 20, and that a checker needs about 2 hours to respond to a change (Barr Report forum (Hoppy)³). Green Aqua notes that some solutions sold as 4 dKH turn green at 20 mg/L instead (Green Aqua⁴), so the color is a guide, not a reading.

The same post explains why the older rule of thumb, a pH drop of 1.0 from a degassed sample, falls short: water left out to degas kept losing CO₂ for 2 or 3 days, so the reference point was never fixed.

Bubble counts

We found no published source that converts bubbles per second into ppm. Bubble size depends on the counter, the regulator and the diffuser, and tanks lose CO₂ at different rates. Use a drop checker or this estimate, raise the rate in small steps, and watch the fish.

Sources for this page

1Barr Report forum: EI "light": the less technical aspects of the Estimative IndexRead Oct 19, 2026
2The Krib (Chuck Gadd's CO2/pH/KH table): CO2 chartRead Oct 19, 2026
3Barr Report forum (Hoppy): Drop checkers / CO2 indicators: why and howRead Oct 19, 2026
4Green Aqua: How to determine the CO2 level in your aquariumRead Oct 19, 2026
5Wikipedia: Hardness of water: German degree (1 °dH = 10 mg/L CaO = 0.1783 mmol/L)Read Oct 19, 2026

Every figure above traces to one of these. Formulas are on Methods. Found an error? hello@mosspearl.com