Garden Watering Calculator
The common advice that a garden needs “about an inch of water a week” is easy to say but hard to act on – how many gallons is an inch, and how long should the sprinkler run? This garden watering calculator converts a depth of water into a volume for your lawn or bed, then uses the flow rate of your hose, sprinkler or drip system to estimate the run time per week and per watering session.
How to use this calculator
- Choose imperial (inches, gallons) or metric (millimeters, liters).
- Enter the area being watered by rectangle, circle or known area.
- Enter the weekly water depth. About 1 in (25 mm) a week, including rain, is a common starting point for lawns and vegetables in summer.
- Enter your flow rate. Time how long it takes to fill a bucket of known size, or add up the emitter ratings of a drip system.
- Choose how many times a week you water to see the volume and run time per session. Subtract any rainfall from the weekly depth.
The watering formula
One inch of water over one square foot is 1/12 cubic foot, or about 0.623 US gallons. So gallons = area (sq ft) × depth (in) × 0.623. In metric it is even simpler: one millimeter of water over one square meter is exactly one liter, so liters = area (m²) × depth (mm). Run time = volume ÷ flow rate. These figures assume all the water lands where you want it; sprinklers lose some to wind, evaporation and overspray, so add 10–25% on hot, windy days.
Worked example
A 20 × 10 ft vegetable bed is 200 sq ft. One inch a week is 200 × 0.623 = 125 gallons. With a garden hose delivering 5 gallons per minute that is 25 minutes of watering per week, or two sessions of about 12–13 minutes. A 1,000 sq ft lawn needs about 623 gallons for an inch of water. In metric, a 10 m² bed at 25 mm per week needs 250 liters.
How to measure your flow rate
Put a 5 gallon (or 10 liter) bucket under the hose or sprinkler supply, turn the tap to the position you normally use, and time how long it takes to fill. Flow = bucket size ÷ seconds × 60. For drip irrigation, multiply the number of emitters by their rating – for example 40 emitters at 1 gallon per hour is 40 gallons per hour, or 0.67 gallons per minute. To check how much a sprinkler actually puts down, set out a few straight-sided cans, run it for 15 minutes and measure the depth collected.
How much water do plants really need?
An inch a week is a guideline, not a rule. Sandy soils drain fast and need water more often in smaller amounts; clay soils hold water longer but absorb it slowly, so apply it gradually to avoid runoff. In hot, dry weather, vegetables and lawns may need 1.5–2 inches a week, while in cool spring weather half an inch may be plenty. Newly sown seeds and transplants need frequent light watering, while established shrubs and trees prefer deep, infrequent soakings that encourage deep roots.
Watering efficiently
Water early in the morning, when evaporation is low and leaves dry quickly, reducing disease. Water deeply and less often – soaking the top 6 inches of soil once or twice a week is better than a daily sprinkle. A rain gauge tells you how much nature has already provided, and mulching beds with 2–3 inches of organic material can cut evaporation substantially. Drip irrigation and soaker hoses deliver water straight to the roots and typically use much less water than overhead sprinklers.
Frequently asked questions
How many gallons is an inch of water?
About 0.623 gallons per square foot, or 623 gallons per 1,000 sq ft. In metric, 25 mm over one square meter is 25 liters.
How long should I water my lawn to get an inch?
It depends on your sprinkler. Many oscillating and impact sprinklers apply roughly 0.5–1 inch per hour, so 1–2 hours a week split into two sessions is typical. Measure with tuna cans to be sure.
How much water does a vegetable garden need?
Around 1–2 inches a week in summer, including rain, applied as one or two deep waterings. That is about 62–125 gallons per 100 sq ft per week.
How much water does a garden hose put out?
A typical 5/8 inch garden hose delivers about 9–17 gallons per minute at full pressure, but with a spray nozzle or sprinkler attached it is often 2–5 gallons per minute. Measure yours with a bucket.
Is it better to water every day or a few times a week?
For established lawns and gardens, a few deep waterings a week is better. Daily shallow watering keeps roots near the surface and makes plants more vulnerable to drought.