Garden Water flow rate calculator
You cannot design an irrigation zone, choose a sprinkler, or judge whether a pressure washer will run properly without knowing how much water your supply actually delivers. Pressure and flow are different things, and flow is the one that limits what you can run at once. The test takes two minutes.
How this is calculated
GPM = bucket size (gallons) × 60 ÷ seconds taken to fill. Design zones to about 75% of measured flow, leaving headroom.
Source: Standard bucket test method used in irrigation design; the 75% design margin is common practice
Worth knowing: Test from the outdoor spigot you will actually use, with everything else in the house off, and open the tap fully. Flow drops when other fixtures run, and it varies through the day as neighbourhood demand changes — if you irrigate in the early morning, test then. A pressure gauge reading is static pressure with no water moving; it will fall once flow starts.
Common questions
What is the difference between pressure and flow?
Pressure (psi) is the force pushing the water; flow (GPM) is how much actually arrives. A narrow supply pipe can show high static pressure yet deliver very little flow once a tap opens. Sprinkler heads are specified by both, and it is almost always flow that limits how many you can run together.
Why design to only 75% of what I measured?
Because measured flow is a best case. Friction in pipes and fittings, a slightly dirty filter, or someone running a tap indoors all eat into it. Leaving a quarter in reserve means the zone still performs on a bad day rather than only on a good one.
Do I need a pressure gauge?
Not for this test, but a cheap screw-on gauge is genuinely useful. Most sprinkler heads want 30-50 psi at the head, and drip systems usually need a regulator to bring supply down to around 25 psi or they blow fittings apart.
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