Water chemistry
Aquarium Calcium & Magnesium Calculator
Calculate magnesium from GH and calcium, calculate calcium from GH and magnesium, or estimate aquarium GH and the Ca:Mg ratio from known mineral values.
- Calcium and magnesium
- °dGH and ppm
- Hardness breakdown
Calculate
Choose the values you know
Your water
Calculated water values
Estimated from calcium and magnesium.
Ca:Mg ratio
—The ratio is secondary context; absolute mineral concentrations remain more important.
Mg-high relative to CaCa-high relative to Mg
Hardness contribution
Calcium and magnesium
- Calcium
- — °dGH —
- Magnesium
- — °dGH —
What this result means
GH is an estimate of combined divalent-ion hardness. Calcium and magnesium account for most GH in typical freshwater aquarium water, but other multivalent ions can also contribute. GH alone cannot identify calcium and magnesium separately.
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Water chemistry guide
Understanding calcium, magnesium, and GH
What is the calcium-to-magnesium ratio?
Calcium and magnesium are essential plant nutrients and both contribute strongly to general hardness. The Ca:Mg ratio describes their relative concentrations, but it is not a standalone indicator of aquarium health.
How do you calculate magnesium from GH and calcium?
Mg = (GH − Ca / 7.144) × 4.356
First subtract calcium's hardness contribution from GH. Convert the remaining hardness contribution to magnesium in mg/L.
Worked example
At 8 °dGH and 40 mg/L calcium: calcium contributes 40 / 7.144 ≈ 5.6 °dGH. The remaining 2.4 °dGH × 4.356 ≈ 10.5 mg/L magnesium. The ratio is 40 / 10.5 ≈ 3.8 : 1.
How do you calculate calcium from GH and magnesium?
Ca = (GH − Mg / 4.356) × 7.144
Subtract magnesium's hardness contribution from GH, then convert the remaining hardness contribution to calcium in mg/L.
How do you calculate GH from calcium and magnesium?
GH = Ca / 7.144 + Mg / 4.356
This gives the estimated GH attributable to calcium and magnesium. The calculator also converts °dGH to ppm as CaCO3 using 17.848 ppm per °dGH.
Does GH tell you how much calcium and magnesium are in the water?
No. GH measures combined hardness and does not uniquely identify how much comes from calcium versus magnesium. If GH and one mineral concentration are known, the other can be estimated.
What Ca:Mg ratio is suitable for a planted aquarium?
Around 3:1 to 4:1 is commonly used as a practical planted-aquarium reference, and ratios around 2:1 are also used successfully. There is no single mandatory biological ratio. Absolute calcium and magnesium concentrations matter more than chasing an exact ratio, and water chemistry should be considered as a whole.
Where can I find calcium and magnesium values?
Look in municipal water reports, laboratory water reports, or aquarium-specific mineral test results where available. Water suppliers often list calcium and magnesium separately.
Aquarium Calcium & Magnesium Calculator FAQ
What is a good Ca:Mg ratio for a planted aquarium?
Around 3:1 to 4:1 is a commonly used practical reference, while ratios around 2:1 are also used successfully. There is no mandatory ratio; the absolute calcium and magnesium concentrations matter more.
Can I calculate magnesium from GH?
Only when calcium is also known or reasonably estimated. GH alone cannot separate the calcium and magnesium contributions.
Can I calculate calcium from GH?
Only when magnesium is also known or reasonably estimated. Enter GH and magnesium to estimate calcium.
Can GH tell me my calcium level?
No. GH measures combined hardness and does not identify how much comes from calcium versus magnesium.
How do I calculate GH from calcium and magnesium?
Estimate GH in °dGH with Ca / 7.144 + Mg / 4.356, using calcium and magnesium concentrations in mg/L.
Does KH affect this calcium and magnesium calculation?
No. KH represents carbonate and bicarbonate alkalinity. It is separate from this calcium-and-magnesium GH estimate.
Where can I find calcium and magnesium values for my tap water?
Check your municipal or local water-quality report. Laboratory reports and aquarium-specific mineral tests may also list calcium and magnesium separately.
Is ppm the same as mg/L?
In dilute freshwater solutions, ppm and mg/L are commonly treated as approximately equivalent for dissolved mineral concentrations. GH reported as ppm here specifically means ppm as CaCO3.