RO/DI Water Mixing Calculator: Blend Tap Water to a Target GH/KH

Before you use this: this tool assumes your RO/DI water contributes little to no GH/KH of its own, which is typical but not guaranteed for every unit — test your own RO/DI output if you can. It calculates a blend ratio only; it doesn't recommend a target reading for your species, and any change to your tank's actual hardness should still be made gradually and retested, not applied all at once.

Enter your own tested readings and how much blended water you want to prepare. Nothing here is pre-filled or assumed without telling you.

Your readings

Use the same unit (dGH/dKH or ppm) for both — the blend ratio works out the same either way, since it's a proportion.

Your tap water's reading

Your target reading

Your RO/DI water's own reading (optional)

How much you want to prepare

Total volume

Unit

Your blend

What this calculator is — and isn't

This is a narrow arithmetic tool with one job: turning "I want this target reading, starting from my tap water's actual reading" into a blend ratio and the litres of each you need. It's the same linear-dilution method described in our water hardness guide, just automated. It doesn't hold a database of "ideal" GH/KH targets for any species, doesn't recommend a target for you, and doesn't check whether your specific livestock, plants or invertebrates need a different approach.

If you're not sure what target to aim for, our water hardness guide covers general ranges by tank type, and choosing aquarium shrimp covers why some invertebrates are more sensitive to this than typical community fish.

Common RO/DI blending mistakes

Assuming your RO/DI unit reads exactly zero without ever testing it. A failing membrane or exhausted DI resin can let more hardness through than expected — test your output periodically, especially for a livestock-critical blend.
Applying a calculated blend as one large, sudden water change rather than changing your tank's hardness gradually and retesting.
Mixing up dGH and ppm between your two readings. Use the same unit for your tap and target readings — the ratio comes out the same either way, but only if both figures use one consistent unit.
Using this for a marine salt mix. Mixing salt to a target salinity is a different calculation from freshwater GH/KH dilution — follow your salt manufacturer's instructions instead.
Never retesting the actual blended result. This calculator's output is a starting ratio based on your entered readings, not a guarantee of the final water's exact reading.

How AquaPulse helps

Working out today's blend ratio is one calculation. Knowing whether your tap water's own hardness has actually shifted since the last time you tested it — something municipal supplies genuinely do over time — is a different problem, and it's the one a single calculation can't answer on its own.

AquaPulse works from what you enter — it doesn't test your water or your RO/DI output automatically. Logging your GH and KH readings in AquaPulse keeps a record of your tap water's own baseline over time, so you can tell a genuine shift in your source water apart from ordinary test-to-test variation. Get AquaPulse on the App Store or Google Play to try it.

Keep going

Common questions

Frequently asked questions

Does this calculator assume my RO/DI water reads zero GH/KH?

Only if you don't enter a reading for it, and the calculator always tells you on screen when it's making that assumption. A properly functioning RO/DI unit typically produces water close to zero GH/KH, but "typically" isn't a guarantee for your specific unit. If you've tested your own RO/DI output, enter that reading for a more accurate blend.

Can I use this for a saltwater tank's salt mix?

No — this calculator handles freshwater GH/KH dilution by blending RO/DI with tap water, which is a different calculation from mixing marine salt into RO/DI water to a target salinity. Follow your salt mix manufacturer's own instructions for that, or see our starting a saltwater aquarium guide.

Why does the target reading have to be lower than my tap reading?

Because this calculator only handles diluting hardness downward by blending in RO/DI water, which contributes little to no GH/KH of its own. Raising hardness above your tap water's own reading needs a different approach — adding a remineralising or GH/KH-boosting product — which isn't what this tool does.

Should I change my tank's water hardness all at once using this calculation?

No. Even once you know the right blend ratio for your source water, change your tank's actual hardness gradually and retest rather than making one large correction — a sudden shift in GH or KH can be more stressful to livestock than a slow one, regardless of how precisely the target blend was calculated.

How this calculator works: the target reading is treated as a weighted average of your tap water's reading and your RO/DI water's reading (assumed zero unless you enter one), weighted by the fraction of each in the final blend — solving that relationship gives the tap and RO/DI fractions, which are then applied to your total volume. Volume unit conversions use exact published constants (1 US gallon = 3.785411784 litres, 1 UK gallon = 4.54609 litres). Nothing you enter is saved or sent anywhere.

What we deliberately left out: any built-in "ideal" target reading for any species or tank type, since that depends on what you're keeping and is covered separately in our water hardness guide; any handling of marine salt-mix salinity targets, which follow a different calculation; and any silent assumption about your RO/DI water's own reading — the calculator always states on screen whether it assumed zero or used a figure you entered.

Sources and editorial information

Published: 8 August 2026
Last reviewed: 8 August 2026
Written and maintained by: AquaPulse (Valeon Labs Ltd)

This calculator and its supporting content are written and maintained by the AquaPulse team at Valeon Labs Ltd. The blending method automates the RO/DI-dilution approach already described and sourced in our water hardness guide. The arithmetic uses only the figures you enter — there is no built-in target-reading data for any species or tank type. We'll update this page if we find something we've got wrong.

Good to know: This tool provides arithmetic assistance only. It does not recommend a target GH/KH reading for any species, does not test your water for you, and does not replace testing your actual blended result with a reliable kit.

RO/DI Water Mixing Calculator