pH Adjustment Dosing Calculator
Do a small test dose, note how far the pH moved, and this tool scales it to your whole reservoir. It never guesses a dose from scratch, because that depends on your water.
How the calculation works
The test tells you how many pH units one millilitre moves one unit of volume. That response is scaled to your reservoir and to the change you want:
Dose = test dose × (reservoir volume ÷ test volume) × (wanted pH change ÷ pH change in test)
Example: 0.5 ml of pH Down in a 1 litre sample moved the pH from 7.2 to 6.6, a change of 0.6. Your 20 litre reservoir reads 7.0 and you want 6.0, a change of 1.0. Dose = 0.5 × (20 ÷ 1) × (1.0 ÷ 0.6) = 16.7 ml.
Because the real response is curved, the tool shows half of the dose as a safe first step. Add it, stir, wait, re-measure, then add the remainder only if you still need it.
Use the same units for the test sample and the reservoir (litres with litres, gallons with gallons). To find the dose for a nutrient mix instead, see the hydroponic nutrient calculator.
pH Up and Down products are strong. Follow the safety instructions on the label, add the product to the water rather than the other way round, and keep it away from roots and skin.
Frequently asked questions
Why do I need a test dose?
How much pH Up or pH Down a reservoir needs depends on the alkalinity (buffering) of your water and on the strength of your pH product, and neither is something a calculator can know. A small test dose tells the calculator how strongly your water reacts, and it scales that to your full reservoir.
Is the pH response really linear?
Only roughly. pH is a logarithmic scale and buffered water resists change, so the response curves. The estimate is reasonable for changes of about one pH unit or less. For bigger moves, add part of the dose, mix, re-measure and repeat.
How long should I wait before measuring?
Stir well and give the water a few minutes. Measure pH again once the reading has settled, and avoid adding a second dose before you have a new reading.
What pH range should I aim for?
Most soilless crops do well at a slightly acidic pH, commonly quoted as around 5.5 to 6.5. Check the guidance for your crop and nutrient.
Can I use it for pH Up and pH Down?
Yes, but run each separately with its own test dose. If your target is on the opposite side from your test dose, the tool will tell you to use the other product.