Enter your batch volume in litres and the measured concentration of each source. This calculator estimates how much reverse-osmosis (RO) water and tap water to combine. Measure the RO water rather than assuming its concentration is zero.
Use one linearly mixable parameter throughout. For an approximate general-hardness calculation, enter all three concentrations in mg/L as CaCO₃. Do not mix measurement units or combine GH and KH readings. Water hardness reflects dissolved calcium and magnesium; treating it as additive is an approximation that assumes no relevant chemical changes during mixing. USGS: Hardness of Water.
Never enter or average pH. pH is logarithmic, so this linear calculation cannot predict the pH of mixed water. USGS: pH and Water.
Enter a batch volume from 0.01 to 10,000 L, and concentrations from 0 to 10,000 mg/L as CaCO₃, in increments of 0.01. These are calculator entry limits, not recommended aquarium conditions.
The target must lie between the two measured source concentrations, including either endpoint. The source concentrations must differ for a unique mixing ratio. Either source may have the higher concentration.
The calculator does not automatically enforce the relationship between the target and the source readings. A negative output or an output larger than the batch means the requested target is impossible with those sources: do not use that result as a mixing instruction. Equal source readings produce unavailable results, not zero volumes. If both equal the target, any mixing ratio meets that concentration; otherwise no ratio can meet it.
Outputs describe a separate batch of new water, not the resulting concentration of an aquarium after a water change. Mix and test the batch before use. Display rounding does not represent measurement accuracy.
Concentration multiplied by volume gives the amount of a conserved constituent. Add the amounts from the two sources and assume their volumes add: C_target × V = C_RO × V_RO + C_tap × V_tap, with V = V_RO + V_tap. Substitution and rearrangement give the displayed formula.
This is a two-source derivation from the concentration and solute-conservation principles explained in OpenStax Chemistry 2e, section 3.3: Molarity. Applying it to hardness assumes the measured quantity remains additive: no relevant precipitation, dissolution or other reactions. It predicts only the selected parameter.
| Batch (L) | RO hardness | Tap hardness | Target hardness | RO volume (L) | Tap volume (L) |
|---|---|---|---|---|---|
| 50 | 0 | 200 | 80 | 30 | 20 |
| 30 | 20 | 200 | 80 | 20 | 10 |
| 40 | 10 | 150 | 10 | 40 | 0 |
| 40 | 10 | 150 | 150 | 0 | 40 |
For the first example, the tap fraction is (80 − 0) ÷ (200 − 0) = 0.4. A 50 L batch therefore needs 50 × 0.4 = 20 L tap water and 50 − 20 = 30 L RO water. The mixed concentration is (200 × 20 + 0 × 30) ÷ 50 = 80 mg/L as CaCO₃.
For the second example, the RO water has a nonzero reading. The tap fraction is (80 − 20) ÷ (200 − 20) = 1/3, giving 10 L tap water and 20 L RO water.
An impossible example is a target of 250 with sources of 0 and 200. For a 50 L batch the arithmetic returns 62.5 L tap and −12.5 L RO. Those values signal an unattainable target, not usable volumes.