Water Flow Converter

Water flow rate conversion factors.

Base conversions:

• 1 GPM (US gallon/min) = 3.7854 LPM (liters/min)
• 1 GPM = 0.2271 m3/h (cubic meters/hour)
• 1 GPM = 0.002228 ft3/s (cubic feet/second)
• 1 LPM = 0.2642 GPM
• 1 m3/h = 4.4029 GPM = 16.667 LPM

Common flow rates:

• Shower head: 2.0-2.5 GPM (7.6-9.5 LPM)
• Kitchen faucet: 1.5-2.2 GPM (5.7-8.3 LPM)
• Garden hose: 5-10 GPM (19-38 LPM)
• Fire hydrant: 500-1,500 GPM (1,893-5,678 LPM)
• Swimming pool pump: 40-80 GPM (151-303 LPM)

US vs Imperial gallons:

• 1 US gallon = 3.785 liters
• 1 Imperial gallon = 4.546 liters
• 1 Imperial GPM = 1.201 US GPM
• Both are listed above, because a UK plumbing spec quoted in “gallons per minute” means the larger one.

Water flow rate is what decides whether a shower feels strong or a pump can keep up with demand. It’s worth separating from water pressure, which people often blame by mistake. Pressure is the push (psi or bar); flow is how much actually comes out per minute. A narrow or partly clogged pipe can show high pressure yet deliver poor flow, so a weak shower isn’t always a pressure problem, and a flow restrictor in the showerhead is often the real culprit.

Flow also isn’t constant through a system. Every fitting, bend, and length of pipe adds resistance that saps flow by the time water reaches the tap, which is why the rate at a far bathroom runs lower than at the one nearest the main. When sizing a pump or judging a fixture, the flow that matters is the one measured at the point of use, not the headline number on the box.

Measuring your own flow rate

You do not need a meter. Put a bucket of known size under the tap, open it fully, and time how long it takes to fill. Flow in gallons per minute is 60 divided by the seconds taken for one gallon; in litres per minute it is 60 divided by the seconds per litre. A 5 gallon bucket filling in 50 seconds is 6 GPM.

Do this at the fixture furthest from where the water enters the house, and do it with nothing else running. That is the number that decides whether two showers can run at once.

What the regulations say

US federal law has capped showerheads at 2.5 GPM since 1994, and California and a few other states go further at 1.8. Bathroom faucets are capped at 2.2 GPM federally, kitchen faucets at 2.2 as well. Most modern showerheads therefore sit at exactly 2.5 or 1.8 GPM, and any older head you find in a house will be 3.5 or higher.

That matters when a shower feels weak. Before blaming the plumbing, unscrew the head and look for the flow restrictor, a small plastic disc in the neck. Removing it is legal on a head you already own in most places, and it is a two-minute job, but it also roughly doubles your hot water use.

Flow versus pipe size

A pipe carries a sensible flow only up to a velocity of about 5 to 8 feet per second; push more through and you get noise, water hammer and erosion at the fittings. In practice that means roughly 6 GPM through 1/2 inch copper, 12 GPM through 3/4 inch, and 22 GPM through 1 inch. If a fixture needs more than its supply pipe can carry, no pump fixes it. The pipe is the limit.