How to use this aquarium ro water mixing calculator
Choose liters, US liquid gallons, or imperial gallons and enter the total batch volume. Select GH or KH as the parameter to solve. Enter both source readings in degrees, then enter a target for the selected parameter between those two source values. Read the tap and RO quantities together: they add to the prepared batch. Test the finished water before considering its use in the aquarium.
How the calculation works
A two-source blend is a volume-weighted average. If the fraction supplied by tap water is f, the chosen reading is f times the tap reading plus one minus f times the RO reading. Rearranging that equation gives the tap fraction. Multiplying the batch volume by each fraction gives the two quantities to measure. This is an approximate mixing balance, not a prediction of all chemical reactions that might occur after preparation.
Only one independent target sets the ratio. GH describes general hardness, while aquarium KH tests commonly report alkalinity in degrees. Their source ratios need not match. A blend that meets a chosen GH can miss a separately desired KH. The tool therefore calculates the other parameter as an outcome rather than pretending that one adjustable ratio can always satisfy two independent targets. Additional treatment or remineralization is outside this model.
Reverse osmosis does not justify automatically entering zero for every reading. Membrane condition, source water, storage, and measurement uncertainty can affect the water available to you. The example uses zero only to make the arithmetic easy to inspect. Replace those values with your own readings. If your test cannot resolve a very low value, interpret the blend as an estimate rather than an exact specification.
A reachable target must lie between the two source readings for the selected parameter. Mixing cannot produce a value above both or below both without another process. At an endpoint, the calculation can return entirely one source and none of the other. Equal source readings cannot determine a unique ratio: if they already equal the target, any fraction meets that one constraint; otherwise the target is unreachable. Both situations are reported rather than divided by zero.
Do not average pH using this formula. pH is logarithmic and its response to mixing depends on buffering and chemistry, not just a linear average of displayed numbers. Nor does a matching GH establish adequate buffering, disinfectant treatment, salinity, temperature, or trace-element composition. Preparing a batch and changing an established aquarium are different decisions. The calculator supplies quantities for the former and does not recommend the size or speed of the latter.
Formulas
Tap fraction for one selected parameter
Batch quantities
Other parameter estimate
Worked examples
A 40 L batch controlled by GH
Tap water is 12 dGH and 6 dKH; measured RO water is 0 dGH and 0 dKH. For a chosen target of 6 dGH, f = (6 − 0) / (12 − 0) = 0.5. Mix 20 L of each source. Estimated mixed KH is 3 dKH, not an independently selected target. These are illustrative measurements, not a species recipe.
RO readings that are not zero
For a 30 L batch, tap GH 14 dGH, RO GH 2 dGH, and target GH 6 dGH give f = 4 / 12 = one third. Tap water contributes 10 L and RO water 20 L. If their KH readings are 8 and 1 dKH respectively, the predicted blend is approximately 3.333 dKH.
Common uses
Prepare a measured batch
Scale a blend from a small test batch to a larger preparation container while keeping the selected source ratio visible. Recheck source readings when the supply or membrane performance changes rather than assuming an old ratio is permanently correct.
Check a second parameter
Use the predicted secondary reading to notice when a GH-controlled ratio does not also meet a separately chosen KH scenario. This comparison identifies a limitation of two-source mixing; it does not recommend a chemical correction or a species target.
Accuracy and limitations
Source test resolution limits the calculation. The model treats readings as additive with volume, ignores volume contraction and reactions, and rounds only the display. Verify the prepared water with a suitable test instead of assuming that a calculated decimal is a laboratory measurement.
Input limits are numerical guardrails, not animal-care ranges. Invalid, missing, or unreachable values hide calculated answers. The calculator does not select a safe transition rate, predict pH, or account for disinfectants and remineralization.
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 set both GH and KH targets?
Not with this two-source, one-ratio model. Choose one target and inspect the other calculated reading. If both desired targets do not lie on the same mixing ratio, mixing these two sources alone cannot meet both.
Does this calculate remineralizer dosage?
No. Powder and liquid products have different compositions and instructions. Do not treat RO liters, GH, or KH as a substitute for a product-specific dosing method.
Why is my target rejected?
A target outside the two selected source readings cannot be obtained by blending those sources. Also check whether a reading was entered in ppm instead of degrees or whether GH and KH were confused.
Can I use the ratio directly in an occupied tank?
The output describes a separate batch only. It does not evaluate the effect or safety of replacing aquarium water. Check reliable husbandry guidance and measured conditions before changing the system.
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.