Plant Watering Calculator
Tell us your pot size and watering habits, and we'll show exactly how much water each plant needs.
📖 How it works & FAQHow much water does a potted plant actually need?
The right amount of water scales with how much soil a pot holds, and bigger pots hold a surprising amount more. Because soil volume grows with the cube of the pot's diameter, a 10-inch pot holds roughly four to five times the soil of a 6-inch pot, so it needs far more water each time you reach for the can. This calculator estimates your pot's soil volume from its diameter, then works out how much water to give per watering and per week.
How the math works
We approximate soil volume by treating the pot as a slightly tapered cylinder whose depth roughly equals its diameter, using a fill factor of 0.6. That gives soil volume in mL as 0.4712 times the diameter (in centimeters) cubed. Your chosen "water per watering" percentage is the share of that soil volume you replace each time, and multiplying by waterings per week gives the weekly total. A common rule of thumb is to water until 20-30% of the soil volume has passed through, which is why 25% is the default.
How to use it
- Measure the pot across the top and enter the diameter.
- Pick inches or centimeters to match your measurement.
- Set the water percentage per watering (25% is a good start).
- Enter how many times per week you water that plant.
- Read the per-watering and weekly amounts in cups and liters.
These figures are general estimates, not professional horticultural advice; always adjust to your specific plant and conditions.
FAQ
- Why does pot diameter matter so much?
- Soil volume rises with the cube of diameter, so small size jumps mean big water changes. Doubling the diameter roughly multiplies the water need by eight.
- What percentage should I use per watering?
- Around 20-30% suits most houseplants. Use a lower value for succulents and cacti, and a higher value for thirsty tropicals or plants in bright, dry rooms.
- Should I really water on a fixed weekly schedule?
- Treat the weekly figure as a target, not a rule. Check the top inch of soil first & water only when it feels dry, since light, heat, and season all shift the true need.
- Does this work for any pot shape?
- It's tuned for standard round pots. Very tall, very shallow, or self-watering containers will differ, so adjust the percentage to fine-tune the result.