Short definition
Flow rate is the volume of water delivered per unit of time, measured in gallons per minute (GPM) in US plumbing. It’s the dominant homeowner-facing flow metric: kitchen faucets at 1.5 to 2.2 GPM, bathroom faucets capped at 1.2 GPM (federal max post-1994), showerheads at 1.5 to 2.5 GPM, tankless water heaters rated 4 to 11 GPM whole-house.
What it is
Flow rate and pressure are different concepts often confused. Pressure (psi) is force per unit area; flow rate (GPM) is volume over time. They’re related — more pressure usually drives more flow through the same restriction — but the same fixture can have low flow at normal pressure (clogged aerator) or normal flow at low pressure (oversized supply line at low static).
A useful homeowner test is the bucket and stopwatch: time how many seconds it takes to fill a 1-gallon bucket at the fixture, then divide 60 by that number. Fifteen seconds means 4 GPM. Thirty seconds means 2 GPM.
Why it matters to a homeowner
Flow rate matters most in two scenarios:
- Tankless water heater sizing. Total simultaneous-use GPM determines the unit you need. A whole-house tankless rated at 9 GPM handles a shower and a kitchen sink running together; smaller units don’t.
- Diagnosing low pressure complaints. A fixture might feel low-pressure when the actual problem is low flow from a clogged aerator or scaled showerhead. The bucket test isolates which it is.
For well owners, GPM is also how the well’s yield is measured — a 5-GPM well sustained over an hour delivers 300 gallons, enough for typical household demand if buffered by a pressure tank.
Common variants and what flow rate is not
- GPM vs. LPM. 1 GPM = 3.785 liters per minute.
- Flow rate vs. pressure. Distinct concepts; related but not the same.
- Static pressure vs. flow pressure. Static is no-flow pressure; flow pressure is residual pressure under demand, always lower than static.