We use cookies to improve your experience and analyze site usage. See our Privacy Policy.
Calculate how much pH up or down to add using carbonate buffer chemistry
Water Type
Alkalinity (ppm CaCO\u2083)
ⓘYour water's resistance to pH change. Found on your water report or measured with a test kit.Starting pH
Target pH
Volume
Acid (pH Down)
Add approximately
3.5 mL
(~70 drops)
pH Scale
Nutrient Impact
Adds ~19 ppm phosphorus
Stability
low drift riskModerate alkalinity. pH is well-buffered — changes will require more acid but will hold longer.
Add 3.5 mL (~70 drops) while stirring. Wait 5 minutes, then retest. Adjust in small increments if needed.
Track your plants with Meridian
Save calculator results, sync custom products across devices, and log recipes directly to your plant journal. Free forever.
Create Free Account →This calculator uses the Henderson-Hasselbalch equation combined with a carbonate buffer model — the same chemistry that determines how your water resists pH changes. Unlike simple "mL per gallon" estimates, it accounts for your water's alkalinity (carbonate hardness), which is the dominant factor in how much acid or base is needed.
Alkalinity (ppm CaCO₃) measures your water's buffering capacity — its resistance to pH change. RO water (near-zero alkalinity) can be adjusted with just a few drops of acid. Hard tap water at 200+ ppm may need 10-20x more acid to reach the same target pH. This is why "add 1 mL per gallon" advice often fails — it doesn't account for the single most important variable.
| Method | Target pH |
|---|---|
| Hydroponics (DWC, NFT) | 5.5 – 6.0 |
| Coco Coir | 5.8 – 6.2 |
| Soil | 6.0 – 7.0 |
How much pH down should I add?
It depends on your water volume, alkalinity (buffering capacity), and current pH. RO water needs very little — often just a few drops. Hard tap water may need several mL per gallon due to carbonate buffering. Always add gradually and test after 5-10 minutes.
Why does my pH keep rising?
High alkalinity (carbonate buffering) in your water causes pH to bounce back. The alkalinity acts as a buffer that resists pH changes. Using RO water eliminates most pH instability. This calculator accounts for alkalinity to give more accurate estimates.
Which pH adjuster is best for hydroponics?
Phosphoric acid is the most common pH down — it adds a small amount of phosphorus. Nitric acid adds nitrogen instead. For pH up, potassium hydroxide is standard. Avoid citric acid in recirculating systems as it breaks down and pH drifts back up.
What pH should I target?
For hydroponics: 5.5-6.0. For coco coir: 5.8-6.2. For soil: 6.0-7.0. Slightly acidic conditions maximize nutrient availability for most plants.
What is alkalinity?
Alkalinity (measured as ppm CaCO₃) is your water's resistance to pH change. RO water has near-zero alkalinity and shifts easily. Hard tap water can have 200+ ppm, requiring much more acid to lower pH. Your water report or a test kit will tell you.
Theme
Width
Height (px)
<iframe src="https://growmeridian.com/embed/ph-calculator" width="100%" height="500" frameborder="0" style="border-radius:12px;" title="ph calculator"></iframe>
📡 Use via API
Access this calculator programmatically with our REST API