🌫️ Humidity Pebble Tray Size Calculator
A pebble tray sits under a pot with a layer of pebbles and water below the pot's base, raising local humidity as the water evaporates. Enter your pot size to get a recommended tray size and water amount.
Tray width and water volume
The tray diameter is your pot plus the buffer, and the water is the volume of a disc that wide at the depth you asked for:
tray diameter = pot diameter + buffer
water (cu in) = π × (tray diameter / 2)² × depth
cups = cubic inches / 14.4375A 6 in pot with a 3 in buffer gives a 9 in tray. At half an inch of water that is π × 4.5² × 0.5 = 31.8 cubic inches, about 2.2 cups. Every figure is in inches, and 14.4375 cubic inches is one US cup.
Filling it
- Subtract for the pebbles. The maths treats the tray as empty, but stones fill roughly a third to a half of it. Pour to the calculated depth on the tray wall, not the calculated volume, and you will use less water.
- The pot must not stand in the water. Keep the base on top of the pebbles. A pot wicking water through its drainage hole is being bottom watered continuously, which is how this setup drowns roots.
- The volume assumes a round tray. A square one of the same width holds about 27% more.
Frequently asked questions
How big should a pebble tray be for a 6 inch pot?
Around 9 inches across, using the default 3 inch buffer. The extra width is what gives the water an evaporating surface; a tray the same size as the pot has almost none.
Do pebble trays actually raise humidity?
Only slightly, and only close to the tray. The evaporating surface is small next to the volume of a room, so treat it as a marginal help rather than a fix for a plant that genuinely needs damp air.
How often do I refill it?
Whenever the water drops below the pebble tops, which in a heated room is often weekly. Rinse the stones now and then, since standing water leaves mineral crust and algae.