pH Down Calculator for Cannabis
This pH down calculator shows how much pH Down or pH Up to add to your water to hit your target — a starting point you can fine-tune from.
Calculated with PlantClue
Why pH decides how well your plants take up nutrients
Why?
When the pH is off, the nutrients are still in the growing medium, but the roots can’t chemically take them up — that’s called nutrient lockout. The target range depends on your growing medium: soil 6.2–6.8, coco 5.9–6.1, hydro 5.7–6.0.
Why does it matter?
Even with perfect feeding, the plant will show deficiency symptoms if the pH is wrong. Many apparent “deficiencies” are really pH problems — so always check your pH before you add more nutrients.
Important note
This calculation is a starting point, not exact chemistry — the amount you need depends on the alkalinity (carbonate hardness, KH) of your tap water. Always dose in small steps, stir well and re-measure the pH afterward.
pH Chart: Target Ranges & Dosing
Target ranges for each growing medium plus a rough dosing chart (ml of pH adjuster per liter of water for each pH point you correct; multiply by 3.8 for 1 gal) — depending on how concentrated your product is. For your exact amount, use the calculator above.
| Medium | Safe range | Optimal |
|---|---|---|
| Soil | 6.0–7.0 | 6.2–6.8 |
| Coco coir | 5.8–6.3 | 5.9–6.1 |
| Hydro / DWC | 5.5–6.1 | 5.7–6.0 |
| ΔpH | Weak (ml/L) | Medium (ml/L) | Strong (ml/L) |
|---|---|---|---|
| 0.1 | 0.06 | 0.03 | 0.02 |
| 0.2 | 0.12 | 0.06 | 0.03 |
| 0.3 | 0.18 | 0.09 | 0.05 |
| 0.4 | 0.24 | 0.12 | 0.06 |
| 0.5 | 0.30 | 0.15 | 0.08 |
| 0.6 | 0.36 | 0.18 | 0.09 |
| 0.8 | 0.48 | 0.24 | 0.12 |
| 1.0 | 0.60 | 0.30 | 0.15 |
Rough industry guidelines, not exact chemistry — the amount you actually need depends on the alkalinity (carbonate hardness, KH) of your tap water. Always dose in small steps and re-measure.
Download the pH chart as a PDFWhat can go wrong when you adjust pH
- Measuring pH right after adding nutrients, before they’ve dissolved — nutrients shift the pH noticeably. The right order: add nutrients to the water first, stir, then measure and adjust the pH
- Adding too much at once: dumping a big dose of pH Down into a small amount of water → pH crash. Always add in small steps, stir after each step and re-measure
- Not calibrating your pH meter: probes drift over time. Monthly calibration with buffer solutions (pH 4.0 and 7.0) is a must — inaccurate readings are worse than none
- Using the same target for soil and coco: soil handles pH 6.5 just fine, while in coco phosphorus lockout already starts there — always pick the right medium
Cannabis pH — FAQ
What is the ideal pH for cannabis in soil?
The recommended range for soil is 6.0–7.0, with an optimal range of 6.2–6.8. Soil has natural buffering capacity thanks to organic matter and microorganisms — it partly evens out small pH swings on its own. A target of 6.5 works reliably for most strains in soil.
What is the ideal pH for coco?
Coco coir calls for a narrower range of 5.8–6.3. Coco is almost inert (no natural buffer), so the pH of your nutrient solution directly determines what the roots can take up. Below 5.8, you risk iron/manganese toxicity; above 6.3, phosphorus lockout sets in.
What is the ideal pH for hydroponics?
In hydro/DWC, 5.5–6.1 is considered the safe range, with 5.7–6.0 as optimal. Water in a hydroponic system has zero buffering capacity — whatever you measure is exactly what the roots see. Check it more often than in soil (daily).
What is nutrient lockout?
When the pH is off, nutrients in the growing medium are chemically bound in a form the roots can’t take up — even though you’re feeding enough. The symptoms look like a nutrient deficiency (yellow leaves, brown spots), but the real cause is the wrong pH. Always check your pH before you increase nutrients.
Why is the calculation only a starting point?
How much pH Down or pH Up you need depends on the alkalinity (carbonate hardness, KH) of your tap water — and that buffering capacity varies a lot from region to region. Hard water needs much more pH Down than soft water. Always dose in small steps and re-measure after each addition.
