Why you must measure, not guess
A sprinkler zone is only as good as its weakest assumption. If you plan eight heads around a flow your tap cannot deliver, every head loses radius and the far corners dry out. Measured flow is the ground truth for the Sprinkler Zone Calculator and every run-time schedule you build on top of it.
What you need
- A bucket of known size - a 5-gallon bucket is ideal
- A stopwatch (your phone works)
- The outdoor tap or hose bib you plan to connect to
The test, step by step
- Open the tap fully and let it run for a few seconds to stabilize.
- Time how long the bucket takes to fill to a known line. Start timing when water starts and stop when it reaches the mark.
- Repeat two or three times and average the fill times - municipal pressure wanders.
- Convert with the formula: GPM = gallons ÷ seconds × 60, or just plug the numbers into the GPM Calculator.
Worked example
A 5-gallon bucket fills in 30 seconds: 5 ÷ 30 × 60 = 10 GPM. That single number now drives your design: with 2.5 GPM spray heads and a 20% safety margin, you can run about 3 heads per zone - the zone calculator does the rest.
What your number means
- 6-12 GPM - typical residential outdoor supply. Zone sizes will be modest; lower-flow nozzles help.
- Under 6 GPM - plan small zones or rotary nozzles (0.5-1.5 GPM each), and consider watering schedules that run zones sequentially.
- 12+ GPM - comfortable capacity, but still keep the 10-30% safety margin for pressure drops and future additions.
Common mistakes
- Testing while washing machine or showers run elsewhere in the house.
- Measuring through a kinked hose - test at the tap itself.
- Assuming the number is constant: summer peak demand can reduce it, so re-test in season.