Watering Calculator
Work out how much water your plants need and how often — less guessing, better watering.
Calculated with PlantClue
Why watering is more than just adding water
Dry-back cycles matter
Roots need oxygen — and oxygen only gets into the growing medium when it dries out between waterings. Constantly wet soil pushes the air out of the pores and starves the roots of oxygen (overwatering). The dry-back isn’t a mistake, it’s part of an ideal watering cycle.
Your medium sets the interval
Soil holds water much longer than coco. Coco has hardly any water reserve and needs watering daily or even several times a day — in smaller amounts. Perlite improves drainage and shortens the dry-back. Knowing how your medium behaves is the first step to the right watering rhythm.
Always plan for runoff
10–20% runoff when you water isn’t wasted water — it flushes salt and nutrient buildup out of the medium (salt buildup) and makes sure the whole pot gets evenly wet down to the bottom. Always remove runoff right away so the roots don’t sit in water.
What can go wrong when watering
- Watering on a schedule instead of by how dry the medium is — your plant tells you when it needs water, not the calendar.
- Too small a pot and too much water — in small pots you can easily flood the entire root zone with no dry-back at all.
- Leaving runoff standing — constant contact with runoff promotes root rot and fungus gnat larvae.
- Ignoring the pH of your water — even a perfect watering amount won’t help if the pH is off and blocks nutrient uptake.
Watering amounts — what growers want to know
How much water does a cannabis plant need per watering?
Rule of thumb: 20–30% of the pot volume in soil, 15–25% in coco. For a 2.6 gal (10 L) pot, that’s about 0.5–0.8 gal (2–3 L). More important than the exact amount is getting some runoff out of the drainage holes — that ensures even moisture and prevents salt buildup.
How often should I water cannabis?
In soil: only water once the top inch (2–3 cm) is dry — usually every 2–4 days. In coco: daily or twice a day, because coco holds almost no water reserve. Growth stage and plant size affect the interval as well.
What happens if you water too much?
Overwatering is the most common beginner mistake. Symptoms: drooping leaves (even with enough water in the pot), yellowing and poor root growth. Constantly wet soil promotes fungus gnat larvae and root rot.
Do you need runoff?
Yes — about 10–20% runoff prevents salt and nutrient buildup in the medium. Remove standing runoff right away so the roots don’t sit in water. This matters most in coco and with mineral (synthetic) nutrients.
Does water demand change during flowering?
Yes. In early flowering, water demand rises as the plant grows. In late flowering (flush), many growers switch to plain water. In the last 1–2 weeks before harvest, some growers let the medium dry back slightly to concentrate terpenes.
