Hold a container of known size under the shower, faucet or hose, open it fully and time how long it takes to fill. Enter the volume and the seconds, and the calculator gives the flow in gallons per minute (litres per minute in Metric), how it compares with shower head limits, and the yearly use at the minutes per day you enter.
How the calculation works
Flow rate is volume divided by time. The timer runs in seconds and flow is quoted per minute, so the result is multiplied by 60:
gallons per minute = container gallons ÷ seconds to fill × 60
The yearly figure is the flow multiplied by the minutes of use per day and then by 365. The shower line compares your result with two benchmarks: the 2.5 gpm (9.5 L/min) federal maximum for new shower heads, and the 2.0 gpm (7.6 L/min) WaterSense level.
Worked example: 1 gallon in 30 seconds
The calculator starts with a 1 gallon container that fills in 30 seconds, used for 10 minutes a day.
- Flow: 1 ÷ 30 × 60 = 2 gpm, which is 7.6 litres per minute
- As a shower: at or below the 2.0 gpm WaterSense level
- Yearly: 2 × 10 × 365 = 7,300 gallons
Press Metric and it starts with 4 litres in 30 seconds instead. That is 8 L/min, or 2.11 US gallons per minute, which as a shower sits within the 2.5 gpm federal limit but above the 2.0 gpm WaterSense level. Ten minutes a day comes to 29.2 cubic metres a year.
Getting a reading you can trust
- Use a container of known volume: a 1 gallon jug for faucets, a 2 or 5 gallon bucket for showers, tubs and hoses. Bigger is more accurate, since a second's slip on the timer matters less.
- Open the outlet fully and let it run for a few seconds before you start.
- Start timing as the container goes under the flow and stop the moment it reaches the mark. Do it twice and use the average.
- Keep everything else off, unless you want to see what happens when two outlets run together.
- For a shower, a plastic bag held around the head and fed into the bucket catches the spray that would otherwise miss.
Reading the result
These are the benchmarks the calculator and most product listings refer to:
| Fixture | Benchmark | Where it comes from |
|---|---|---|
| Shower head | 2.5 gpm (9.5 L/min) maximum | US federal limit for new heads |
| Shower head | 2.0 gpm (7.6 L/min) or less | EPA WaterSense label |
| Kitchen faucet | 2.2 gpm (8.3 L/min) maximum | US federal limit |
| Bathroom faucet | 1.5 gpm (5.7 L/min) or less | EPA WaterSense label |
A reading well below these on one outlet usually points to that outlet: a shower head or aerator furred up with scale, which the guide to hard water and limescale shows how to clear. A weak reading on every outlet suggests a supply or pressure problem instead, and the guide to low water pressure causes works through the checks in order. A shower reading well above 2.5 gpm usually means an older head made before the limit, or one whose flow restrictor has been taken out.
The yearly line is where small differences show. At 10 minutes a day, a 2.5 gpm shower uses 9,125 gallons a year and a 2.0 gpm one 7,300, a gap of 1,825 gallons (about 6,900 litres). Much of that is hot water, and a measured shower flow is the right input for the water heater size calculator when sizing a tankless unit.
Limits, and when to call a plumber
The test measures one outlet at one moment. Flow changes with pressure, so a reading during the morning rush can differ from one at midday.
It is not a pipe flow rate calculator either. It tells you what a pipe is delivering at the outlet, not what it could carry; that depends on pressure, diameter, length and fittings.
The benchmarks are US federal and WaterSense figures. Some states, California among them, set lower shower head limits, and other countries have their own rules.
Call a plumber when:
- every outlet in the house has dropped at the same time and nothing has changed in the street
- the flow collapses as soon as a second tap opens
- low flow arrives with brown or rusty water, a sign of corroded galvanised pipe
- the water meter keeps turning with every outlet closed
Where the figures come from
The 2.5 gpm shower head limit has applied to new shower heads in the US since the Energy Policy Act of 1992, measured at 80 psi (5.5 bar). The 2.2 gpm kitchen faucet limit comes from federal efficiency standards and is measured at 60 psi (4.1 bar). The 2.0 gpm shower and 1.5 gpm bathroom faucet figures are the maximum flows for EPA WaterSense labelled products. The 3.785 litres per US gallon used for the metric figures is a defined value.
FAQ
How many gallons per minute does a shower use?
A shower head sold new in the US flows at most 2.5 gpm (9.5 L/min), and WaterSense labelled heads 2.0 gpm (7.6 L/min) or less. Older heads can use more. To check yours, time how long it takes to fill a 1 gallon jug: 24 seconds means 2.5 gpm and 30 seconds means 2 gpm.
How many gallons per minute is a garden hose?
It depends on your water pressure and the hose's diameter and length, so the only reliable answer is to measure it. A 5 gallon bucket that fills in 40 seconds means 7.5 gpm (28.4 L/min).
How do I work out gallons per minute without the calculator?
Divide 60 by the number of seconds it takes to fill a 1 gallon container. Twenty seconds gives 3 gpm and 30 seconds gives 2 gpm. With a 5 gallon bucket, divide 300 by the seconds instead.
Can I use this as a pipe flow rate calculator?
Only to measure what an existing pipe delivers at the outlet today. It cannot predict the capacity of a pipe you have not fitted yet. Pipe sizing for new work uses code tables based on pressure, run length and fixture demand, and a plumber or designer should do it.