Plant Watering Calculator

How much water a pot actually takes

Watering a plant in the ground and watering a plant in a pot are different problems. A bed has an effectively infinite reservoir under it and the plant’s roots go looking. A pot has exactly the water you put in it, held in a volume you can measure, and once that is gone the plant has nowhere to go.

So the question is not really “how much water does this plant need.” It is “how much water does this pot hold, and how fast does the plant empty it.”

Step one: what the pot holds

Soil volume = π × (radius)² × depth × 0.77

The 0.77 is the taper. Almost no pot is a cylinder, and one that narrows to about three quarters of its rim diameter at the base holds roughly 77% of what the straight-sided version would. Pots are also usually about three quarters as deep as they are wide, which is where the calculator’s depth estimate comes from when you only give it a diameter.

Step two: how much to pour

Water thoroughly, until it runs from the drainage holes. Going from properly dry to fully wet takes roughly 20 to 25% of the soil volume, and thirstier plants sit at the top of that range because they are being watered before the pot has dried out as far.

A 20 cm (8 in) pot holds about 3.5 litres of mix, so a proper soak is somewhere near 800 ml. Not the 100 ml a small jug delivers, which is the single most common way a houseplant dies: watered often, watered a little, and never actually watered at all. Water reaches the top inch, the bottom two thirds stay bone dry, and the roots down there quietly give up.

Step three: how often

Frequency is the part that changes with the plant and the season. Volume barely should, because you always water until it drains.

Hot dry air cuts those intervals by about a third, and cool humid air stretches them by about the same. A pot in a south-facing window in August is a different climate from the same pot in a north-facing room in October, so the calculator’s climate setting matters more than people expect.

The one rule that beats any schedule

Push a finger into the soil to the second knuckle. Dry at that depth means water. Damp means wait, whatever the calendar says. A calculated interval is a starting point for a plant you do not know yet, not an instruction to follow past the evidence in front of you.

Terracotta dries faster than plastic, a root-bound plant dries faster than a freshly potted one, and a pot with no drainage hole cannot be watered correctly at all. Fix that first if it applies: no volume is the right volume for a pot that cannot drain.