1. Multiply the number of inches of water you need by 0.6. The result is the number of gallons per square foot you need to apply.
2. Multiply those "gallons per square foot" by the square footage of your garden to determine how many gallons you need to apply.
3. Compensate for the watering system's inefficiency by dividing the gallons needed by 0.85 for drip, or 0.70 for sprinkler.
Now, to figure out how long to run the hose, put the hose into an empty 1-gallon jug and time how long it takes to fill the jug. If it fills in 30 seconds, for example, you have a flow of 2 gallons per minute. If it takes a minute, it flows at 1 gallon a minute (and so on).
Divide the total number of gallons your garden needs by the gallon-per-minute rate. That's how many minutes you have to leave your water on. Don't be surprised if your calculations show that it will take as long as 2 to 3 hours to supply the water your garden needs.