Calculate how much refill water to add, the expected EC after mixing, and whether your target fits below the working fill line.
Table of contents
How to use our Hydroponic Reservoir Dilution and Refill Calculator
- Measure the liquid currently in the tank and enter it as Current reservoir volume (gal), not the tank's labeled maximum size.
- Measure the mixed reservoir and the water you will add, then enter Current EC (mS/cm) and Refill-water EC (mS/cm).
- Enter your crop-plan value for Target EC (mS/cm) and the highest normal fill line as Working reservoir capacity (gal).
- Click Calculate. If water is recommended, add the stated amount, mix thoroughly, and recheck EC before making another change.
- Sanity-check that Reservoir volume after this refill does not exceed your fill line and that the action message matches whether your current EC is above, at, or below target.
Definitions
EC (electrical conductivity): A meter reading that estimates how well the solution carries electricity from dissolved salts. This calculator uses mS/cm.
Current EC: The EC of the mixed reservoir solution before a refill.
Target EC: The EC level from your crop-stage plan or nutrient maker's feed chart.
Refill-water EC: The EC of the source water being added, measured before it enters the reservoir.
Working reservoir capacity: The highest normal operating volume, including the headspace needed for circulation and equipment.
Headspace: Empty room between the current liquid level and the working fill line.
Common mistakes and quick fixes
Mistake: Entering the tank's full labeled size as Current reservoir volume (gal).
Fix: Enter the liquid in the tank now. Use the normal fill line only for Working reservoir capacity (gal).
Mistake: Assuming Refill-water EC (mS/cm) is zero because the water looks clear.
Fix: Measure the exact RO, filtered, or tap water source before entering its EC.
Mistake: Using a PPM number in Current EC (mS/cm).
Fix: Enter the EC meter reading in mS/cm. PPM scales are not interchangeable with EC values.
Mistake: Adding Plain water to add now when Current EC (mS/cm) is already at or below Target EC (mS/cm).
Fix: Follow Next refill action. Plain water would lower EC further and is not a nutrient recipe.
Mistake: Treating Water needed to reach target EC as an amount that will always fit.
Fix: Use Plain water to add now, which is limited by Working reservoir capacity (gal), and check Expected EC after this refill.
Limitations & Key Assumptions / Boundary Conditions
- The calculation estimates EC by volume-weighted mixing. Mix the reservoir well and recheck with a calibrated EC meter after adding water.
- It only plans a plain-water dilution when current EC is above target. It does not calculate a nutrient-addition recipe when EC is at or below target.
- The target must be higher than the refill-water EC for water-only dilution to reach it exactly. Water cannot dilute a solution below the EC of the water being added.
- Current volume and working capacity must reflect actual liquid and the safe fill line. Equipment displacement, foam, or a wrong tank mark can change the safe amount.
- EC measures total dissolved salts, not each nutrient. Repeated top-offs can leave EC near target while individual nutrient ratios drift, so reservoir management may require more than EC adjustment. [1]
Methodology
Calculation
The calculator first finds the available headspace. For a valid water-only dilution, current EC must be above target EC, and target EC must be above refill-water EC.
EC-volume amount = current reservoir volume x current EC
Available headspace = working reservoir capacity - current reservoir volume
Water needed to reach target EC = current reservoir volume x (current EC - target EC) / (target EC - refill-water EC)
The water added now is the smaller of the water needed and available headspace. The final EC is the total EC-volume amount after mixing divided by the final reservoir volume.
Expected EC after refill = (current reservoir volume x current EC + refill-water EC x water added) / final reservoir volume
Worked example
With 10 gal at 2.0 mS/cm, a 1.0 mS/cm target, and refill water at 0.2 mS/cm, the exact dilution needs 12.5 gal. If the working capacity is 25 gal, the final volume is 22.5 gal and expected EC is 1.0 mS/cm. If the capacity is only 15 gal, add 5 gal instead; the expected EC at the fill line is 1.4 mS/cm, so the target does not fit in that refill.