pH Up / Down Calculator (Reservoir Dose)
Estimate how much pH Up or Down your reservoir needs.
Result
Result is an estimate — water chemistry and product strength vary widely. Always add part of the dose, re-measure, and approach the target gradually.
History
What it is
This estimator predicts how much pH Up or pH Down to add to move your reservoir from its current pH to a target, scaled to your solution volume and the strength of your adjuster. It also tells you which direction (acid or base) you need.
Who should use it
Hydroponic growers correcting reservoir pH who want a sensible starting dose rather than adding blindly drop by drop.
How to use it
- Enter your reservoir volume.
- Enter the current pH from your meter.
- Enter the target pH for your system (commonly 5.5–6.5 in hydroponics).
- Set the adjuster strength — how many mL per unit of water shifts pH by 1.0 (start conservative).
- Read the estimated dose and whether you need pH Up or pH Down. Add about half, mix, and re-measure.
Example calculation
Worked example
For a 20-gallon (≈76 L) reservoir going from pH 6.8 down to 5.9, with a strength of ~2 mL per gallon per pH unit, the estimate is about 36 mL of pH Down. Because water buffering varies, add half (~18 mL), stir, re-measure, and top up as needed.
Formulas used
Dose (mL) = reservoir_litres · |target_pH − current_pH| · strength_per_litre
How to read your result
This is an estimate only. Source water alkalinity, buffers in your nutrients and the concentration of your specific pH product all change the real dose. Always approach the target gradually and re-measure rather than dosing all at once.
Recommended ranges
| System / stage | Target pH |
|---|---|
| Hydroponics (general) | 5.5 – 6.5 |
| Vegetative | 5.8 – 6.2 |
| Flowering | 6.0 – 6.5 |
| Coco coir | 5.8 – 6.2 |
Common mistakes to avoid
- Adding the full estimated dose at once instead of approaching the target in steps.
- Adjusting pH before nutrients are fully mixed (nutrients shift pH).
- Mixing pH Up and pH Down repeatedly, which builds up salts.
- Ignoring source-water alkalinity, which strongly resists pH change.
Frequently asked questions
How much pH Down per gallon?
It depends entirely on your water and product, but often well under 1 mL/gal for a small adjustment. Start low, mix, and re-measure — this tool gives a scaled starting estimate.
What pH is best for hydroponics?
Generally 5.5–6.5, with many growers targeting around 5.8–6.2 for balanced nutrient availability in soil-less systems.
Why won't my pH stay stable?
High-alkalinity source water and biological activity push pH back. Use RO or partially RO water, and let freshly mixed reservoirs settle before final adjustment.
Do I adjust pH before or after nutrients?
After. Add and mix nutrients first, because they change pH, then adjust to target.
Is pH Up or pH Down more concentrated?
It varies by brand. Because strength differs, always start with a partial dose and re-measure rather than trusting a fixed amount.