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Self-Watering Pot Calculator — Reservoir Size & Refill
Calculate reservoir size and watering frequency for self-watering pots.
Measure across the top of the pot, rim to rim
Internal soil depth — exclude the reservoir gap below the false floor
Average daily temperature in °C
Reservoir as proportion of pot volume (default 0.2)
Daily water use as proportion of pot volume (default 0.05)
How We Calculate This
Pot volume is estimated as a brimful cylinder: π × r² × depth (in litres). A real pot tapers and the reservoir takes up the lower portion, so the usable compost volume is a little less — treat the figure as an upper bound.
Reservoir size = pot volume × reservoir ratio. The default ratio is 0.20 (20%), which sits at the cautious end of the common 25-33% rule-of-thumb for self-watering and wicking systems; adjust it in Advanced Options to match your build.
Daily water use = pot volume × base rate (default 5%, a generic average) × a temperature factor (0.7 cool at 15°C and below, 1.0 moderate above 15°C up to 20°C, 1.2 warm above 20°C up to 25°C, 1.5 hot above 25°C). These are estimates — real use depends on plant species, growth stage, light and humidity, not temperature alone.
Refill interval = reservoir size ÷ daily water use, rounded down to whole days.
Frequently Asked Questions
Water is stored in a reservoir at the bottom. A wick or capillary mat draws water up to the roots as needed, providing consistent moisture.
As a rule of thumb, sizing the reservoir at roughly a quarter to a third (25-33%) of the total planter volume works well, which usually means a reservoir 75-100mm (3-4 inches) deep beneath the false floor. This calculator defaults to 20% — at the cautious end — so you refill a touch more often but leave more room for roots. Actual use varies with plant species, growth stage and humidity, so treat the figure as a starting point.
Depends on pot size, temperature, and plant type. In summer, a 30cm pot may need refilling every 3-5 days. In cooler weather, weekly.
Plants that like steady moisture do best: leafy herbs such as basil, parsley and chives, fruiting crops like tomatoes, and thirsty houseplants such as peace lilies, ferns and pothos. Avoid succulents, cacti and Mediterranean herbs (rosemary, thyme, oregano, lavender) — they need their compost to dry out between waterings and will rot in a permanently moist reservoir system.
Yes. Use a container within a container, with a wicking system (fabric strip) connecting the reservoir to the soil. Many online tutorials available.
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Last updated: February 2026
All calculations are estimates based on standard gardening guidelines. Results may vary with soil conditions, climate, and plant varieties.