RO and tap water mixing calculator for hydroponics and indoor growing

This RO and tap water mixing calculator helps growers blend reverse osmosis water and tap water to reach a target starting EC before adding nutrients. It is useful for hydroponics, coco, DWC, recirculating systems, drip irrigation, and indoor growing where source water quality affects nutrient stability and plant performance.

Use it to estimate the right blend ratio, prepare reservoir water more consistently, and reduce the guesswork when you want softer starting water than straight tap water but more mineral content than pure RO.

How this water mixing calculator works

The calculator uses weighted blending based on source water EC values. Because EC behaves approximately linearly when two water sources are mixed, the tool can estimate the tap-water fraction needed to reach a selected target EC.

What each parameter means

• Tap water EC: electrical conductivity of your tap water before nutrients.
• RO water EC: electrical conductivity of your reverse osmosis water.
• Target mixed EC: the EC you want to start with before adding feed.
• Target water volume: final total amount of mixed water.
• Tap water percentage: chosen ratio of tap water in the blend.
• Alkalinity weight: rough weighting factor showing how strongly tap water may dominate final pH.

Why growers mix RO and tap water

Pure RO water is very clean but may start too soft for some feeding strategies, while tap water may contain too much mineral content, sodium, or bicarbonate. Mixing the two gives growers more control over starting EC and often creates a more manageable base for nutrients, CalMag, and pH adjustment.

This is especially helpful when tap water is too hard for direct use but fully pure RO is not the desired starting point.

About the pH estimate

Unlike EC, pH does not mix in a simple straight-line way because alkalinity and buffering capacity matter a lot. This tool provides only a practical weighted estimate in advanced mode. Real mixed pH can differ depending on bicarbonates, dissolved minerals, and source-water chemistry.

Always measure actual pH after blending and again after adding nutrients.

Best practice for growers

1. Measure both tap and RO water with a calibrated EC meter.
2. Blend water first, then add nutrients.
3. Check EC again after mixing to confirm the estimate.
4. Adjust pH only after the final nutrient solution is complete.
5. Keep notes on the blend ratios that work best for your system and source water.