Plant Watering Calculator

Most "watering calculators" online are lookup tables — a fixed answer per species. This one runs an actual model: it derives your pot's soil volume from its dimensions, sets a species-specific baseline frequency, then adjusts that baseline with six independent multipliers for your exact environment. Here's every step, in the same order the code runs it.

1. Soil Volume from Pot Geometry

Your pot is treated as a cylinder. Depth is estimated as 85% of diameter (a reasonable approximation for most nursery and decorative pots), and volume follows the standard cylinder formula: π × radius² × height. A 6-inch pot works out to roughly 2.4 liters of soil; a 12-inch pot to roughly 18.9 liters. Because depth is itself derived from diameter, volume scales with the cube of diameter, not the square — doubling a pot's diameter multiplies its soil volume by 8×, which is why a "medium" and "large" pot of the same species can need dramatically different water amounts even though the pots themselves look only modestly different in size.

2. Baseline Frequency by Thirst Level

Each species is classified as low, medium, or high thirst (see the plant directory for every species' rating). That classification sets a baseline interval that also scales gently with pot diameter — bigger pots hold more reserve moisture, so the same species dries out slightly slower in a larger pot before any environmental adjustment is applied.

3. Base Volume: 15% of Soil Volume

A full watering targets roughly 15% of total soil volume — enough to saturate the root zone without oversaturating it. This is floored at 120ml for small pots (below 5 inches) and capped at 6 liters for very large ones, so the formula never recommends an unrealistically tiny or absurdly large single watering.

4. Six Evapotranspiration Multipliers

This is the core of the model, detailed in full in our soil physics guide. Each factor below produces a multiplier; all six are multiplied together into a single evapotranspiration coefficient that speeds up or slows down your baseline interval:

• Light: 0.6× in low light up to 1.65× in direct sun.
• Temperature: 0.7× when cool, up to 1.45× when warm.
• Pot material: 0.9× for plastic (retains moisture), 1.35× for terracotta, 1.5× for fabric pots (evaporate fastest).
• Soil type: 0.75× for water-retentive clay-heavy mixes, up to 1.35× for fast-draining sandy mixes.
• Indoor vs. outdoor: outdoor exposure to wind and sun adds a 1.45× multiplier.
• Season: 0.7× during winter dormancy, 1.2× during active spring/summer growth.

Multiplied together, these six factors span roughly a 40x range between the slowest-drying combination (low light, cool, plastic pot, clay soil, indoor, dormant season) and the fastest (direct sun, warm, fabric pot, sandy soil, outdoor, active growth) — which is exactly why the same species can need watering weekly in one setup and monthly in another.

5. Volume Adjustments and the Drainage Penalty

Separately from the frequency calculation, the base watering volume is adjusted for soil type (clay soil holds water, so volume is reduced 15% to avoid waterlogging; sandy soil drains fast, so volume is increased 15% to fully saturate it) and, critically, for drainage: a pot with no drainage holes gets a flat 40% reduction in recommended volume, since any excess simply has nowhere to go.

6. Safety Clamps

However extreme your inputs, the final interval is clamped to a plausible range per thirst level — 6 to 35 days for low-thirst plants, 3 to 21 for medium, 2 to 14 for high. This exists because the raw multiplier math can otherwise produce implausible extremes (a low-thirst succulent in a worst-case environment shouldn't be told to water every single day); the clamps keep every result inside the range a real plant of that thirst profile could actually tolerate.

For the watering technique that pairs with these numbers — how to actually apply the water, not just when — see our watering best practices guide.