How to use this aquarium water hardness converter
Select the parameter actually measured by your test. Choose the unit on its result or report, then enter a nonnegative reading. The answer uses the other unit for that same parameter. Changing the input unit converts the current valid reading, while changing GH to KH clears it because a new measurement is required. Check that a ppm label refers to CaCO3 equivalents before using it here.
How the calculation works
General hardness and alkalinity describe different properties. GH commonly reflects hardness contributed by calcium and magnesium ions. Aquarium KH tests commonly describe acid-neutralizing capacity or alkalinity using a degree scale, even though the name carbonate hardness can suggest a narrower chemical definition. Similar-looking numbers do not make the two parameters interchangeable. A unit conversion can preserve a measured property; it cannot create a missing measurement of another property.
Both readings can be expressed using calcium-carbonate equivalents. The words as CaCO3 identify a reporting basis, not a statement that all the measured material is physically calcium carbonate dissolved in the sample. A value of 100 mg/L as CaCO3 is therefore not the same thing as 100 mg/L of elemental calcium. Do not use this converter to determine calcium, magnesium, remineralizer, or buffer doses from that number.
This tool multiplies degrees by 17.86 for the forward conversion and divides mg/L as CaCO3 by the same factor in reverse. The factor follows a published rounded aquarium alkalinity convention, which is useful when comparing test-kit scales. It is not presented as an exact SI definition or an improvement over laboratory precision. If a report specifies a different factor or a specialized definition, follow that documented method instead.
The interface describes ppm using the common dilute-water reporting convention of mg/L. Strictly, parts per million is a mass fraction, while milligrams per liter is mass per volume. Density can make them differ, particularly outside ordinary dilute-water assumptions. This is not a density-corrected analytical chemistry tool. A laboratory report or test method takes precedence over a convenient label when precision or a different sample matrix matters.
TDS is not hardness. A conductivity meter may estimate total dissolved solids using a device-specific conversion factor, and dissolved salts other than hardness contributors affect that reading. There is no universal TDS-to-GH or TDS-to-KH conversion. Likewise, pH cannot be derived from a hardness reading alone. This calculator intentionally offers none of those shortcuts, because displaying a precise number would conceal missing chemical information.
Formulas
Degrees to calcium-carbonate equivalents
Calcium-carbonate equivalents to degrees
Parameter identity
Worked examples
A GH reading in degrees
A measured 8 dGH becomes 8 × 17.86 = 142.88 mg/L as CaCO3 under this rounded convention. It is still a GH reading. It does not establish KH, pH, or an elemental-calcium concentration, and the decimal output does not imply that the original test resolved hundredths.
An alkalinity report
If a compatible alkalinity report gives 100 mg/L as CaCO3, dividing by 17.86 gives approximately 5.599 dKH. Selecting KH keeps that identity visible. If the report instead states calcium in mg/L or uses another reporting basis, this example does not apply and its value should not be entered as alkalinity.
Common uses
Compare test-kit scales
Express one measured parameter on a common reporting scale when a test kit uses degrees and a compatible report uses mg/L as CaCO3. Keep the date, sample, and parameter alongside the converted value so a change of units is not confused with a change in water chemistry.
Prepare inputs for a mixing example
Convert compatible source measurements into degrees before using the RO mixing calculator. Confirm both GH readings describe GH and both KH readings describe alkalinity. Unit normalization removes a reporting mismatch, but cannot resolve missing measurements or inconsistent sampling.
Accuracy and limitations
Outputs are rounded for readability, with the published factor explicitly shown beneath the result. Repeatedly converting a displayed rounded number may introduce additional rounding. Keep the original test result for records and use a report-specific convention if required.
A nonnegative reading is mathematically convertible but not a statement of suitability. The converter accepts neither a density correction nor a dissolved-ion analysis. A device's TDS estimate, pH reading, or elemental calcium concentration cannot be entered as a substitute for the chosen property.
Sources and methodology
References support unit conversions, relevant material or energy calculations, and husbandry context where listed. NumUtility's geometric and arithmetic models are explained above; they do not establish safe stocking, maintenance requirements, or structural capacity.
Frequently asked questions
Can I calculate KH from GH?
No. They are distinct measurements. The converter clears the number when you change the measured parameter so that a GH result is not silently relabeled as KH.
Why does another converter use 17.848?
Published factors can use different definitions or rounding. This interface explicitly uses 17.86, the rounded aquarium convention cited below. For a laboratory workflow, use the factor required by that report or method and avoid claiming more precision than the measurement supports.
Does ppm mean calcium?
Not in this tool. It means the selected property expressed as CaCO3 equivalents under a dilute-water mg/L convention. Elemental calcium is a different quantity and needs a different interpretation.
Can a TDS meter replace a hardness test?
Not through this conversion. A TDS or conductivity reading includes effects from multiple dissolved substances. Use a suitable test or report for the actual parameter you need.
Are my measurements saved?
No. Calculation happens in your browser without an account. NumUtility does not receive or store the values entered into these fields. Keep your own measurement log if you want to compare observations later.