Plant Watering Calculator
Calculate watering frequency and amount from plant type, pot size, soil mix, and climate.
Covers succulents, tropicals, vegetables, herbs, and garden beds.
Garden watering schedule balances the water needs of plants with rainfall, soil type, and evapotranspiration rates. The goal is to maintain consistent soil moisture in the root zone without waterlogging (anaerobic) or drought stress.
Weekly water need formula: Water Needed (inches/week) = ET₀ × Crop Coefficient (Kc) − Effective Rainfall
Where:
- ET₀ = Reference Evapotranspiration (how much a reference grass crop would use); available from local weather data or US extension services; typically 1–2 inches/week in summer
- Kc (Crop Coefficient) = plant-specific multiplier: 0.3–0.5 for low-water plants, 0.8–1.2 for thirsty vegetables
- Effective Rainfall = rain that actually infiltrates (excludes heavy rain that runs off)
Volume of water formula: Gallons per Application = Bed Area (sq ft) × Irrigation Depth (inches) × 0.623 (0.623 = gallons per sq ft per inch of water)
Drip system runtime formula: Runtime (minutes) = (Water Volume in gallons ÷ Emitter Flow Rate in GPH) × 60
What each variable means:
- Irrigation depth — water each time deeply enough to moisten the root zone; light frequent watering encourages shallow roots; deep infrequent watering develops drought-resistant roots
- Soil type and drainage — sandy soil: water more frequently in smaller amounts; clay soil: water deeply and infrequently to avoid surface runoff
- Watering time of day — early morning is ideal; evening watering leaves foliage wet overnight, promoting fungal disease; midday wastes water to evaporation
Reference: weekly water needs by vegetable (summer, full sun):
- Tomatoes: 1.5–2 inches/week (consistent moisture prevents blossom end rot)
- Cucumbers: 1–1.5 inches/week
- Beans: 1 inch/week
- Squash/Zucchini: 1–1.5 inches/week
- Lettuce/Spinach: 0.75–1 inch/week
- Corn: 1.5 inches/week (especially during tasseling)
Worked example: Raised bed: 4 ft × 8 ft = 32 sq ft. Tomatoes in July (ET₀ = 1.5 inches, Kc = 1.1, effective rainfall this week: 0.5 inches).
- Water needed = (1.5 × 1.1) − 0.5 = 1.65 − 0.5 = 1.15 inches this week
- Volume = 32 × 1.15 × 0.623 = 22.9 gallons
- With 2 GPH drip emitters per plant (6 plants = 12 GPH total):
- Runtime = (22.9 ÷ 12) × 60 = 1.91 hours = 115 minutes
- Split into 2 sessions of ~58 minutes, 3–4 days apart, for ideal moisture retention.