Indoor Watering Calculator
Calculate optimal watering frequency, volume, and runoff targets for container-grown indoor plants.
About this calculator
Water Every is built from a baseline frequency for your Growing Medium, then stretched or compressed by three adjustment factors: container size, temperature, and humidity. Container Size works through a square-root relationship rather than a straight line, since a bigger pot holds proportionally more water reserve relative to how fast its surface dries -- raising Container Size always raises Water Every (more days between waterings), but each additional gallon adds a shrinking amount of extra time. Room Temperature and Relative Humidity both shift Water Every by adjusting how fast the medium dries: higher temperature dries the medium faster, so it lowers Water Every (water more often), while higher humidity slows evaporation and raises Water Every (water less often) -- the two work in opposite directions.
Water Amount and Runoff Target, by contrast, do not depend on temperature or humidity at all: Water Amount is set purely by Container Size and Growing Medium (the fastest-draining media, perlite and hydro, hold the least reserve moisture and so need a higher percentage of container volume per watering to reach adequate runoff), and Runoff Target is set purely by Growing Medium. This split reflects that temperature and humidity change how often you need to water, not how much water each individual watering requires.
Inputs
Results
Water Every
3 days
Water Amount
0.45 gal
≈ 5 soda cans
How to Use This Calculator
- Enter your container size in gallons — larger pots hold moisture longer and need less frequent watering.
- Select your growing medium: soil retains moisture the longest, coco coir is moderate, perlite/hydro dries fastest.
- Enter your room temperature in °F and relative humidity % — higher temperatures and lower humidity increase water demand.
- Review Water Every (days) and Water Amount (gal) to establish your watering schedule.
- Note the Runoff Target % — always water to 10–20% runoff to prevent salt buildup in soil and coco.
How the result changes with Room Temperature
| Room Temperature | Water Every | Water Amount |
|---|---|---|
| 60 | 3.9 days | 0.45 gal |
| 71 | 3 days | 0.45 gal |
| 84 | 2.4 days | 0.45 gal |
| 96 | 2.4 days | 0.45 gal |
What each input means
- Container Size
- Pot volume in gallons — larger pots hold moisture longer.
- Growing Medium
- Select the growing medium
- Room Temperature
- Higher temps increase evaporation and water uptake.
- Relative Humidity
- Higher humidity slows evaporation from the growing medium.
How this is calculated
Worked example, using the default values
- Identify Input Parameters4 parametersContainer Size = 3, Growing Medium = 1, Room Temperature = 72, Relative Humidity = 50 = 4 input(s) provided
- Calculate Water EveryWater Every3 = 3
- Calculate Water AmountWater Amount0.45 = 0.45
Engine last updated . Checked against 1 independently-derived test — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.
Frequently Asked Questions
Why do Room Temperature and Relative Humidity not affect Water Amount?
Water Amount answers "how much water per watering," which depends on how much the growing medium can hold and needs to be flushed with -- set by Container Size and Growing Medium. Room Temperature and Relative Humidity instead answer "how often," by changing how fast the medium dries out between waterings, which is why they only affect Water Every.
Does doubling my container size mean I only need to water half as often?
Not quite -- Water Every scales with the square root of container size relative to a 3-gallon baseline, not a straight multiple. Raising Container Size always increases the number of days between waterings, but each additional gallon adds a progressively smaller amount of extra time rather than a proportional one.
Why does higher humidity mean I should water less often?
Higher Relative Humidity slows evaporation from the growing medium's surface, so the medium retains moisture longer under the same conditions. This calculator raises Water Every (more days between waterings) once humidity climbs above a moderate threshold, reflecting that slower moisture loss.
My growing medium is perlite/hydro -- why is the recommended Runoff Target higher?
Perlite and hydroponic media drain very freely and hold little reserve moisture around the roots compared to soil, so a higher runoff percentage (around 20-30%) is recommended to make sure water and dissolved nutrients reach evenly throughout the root zone rather than channeling through a few gaps. Soil, by contrast, retains water well enough that a lower runoff target (around 10-20%) is typically sufficient.
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