Determining the gallons-per-minute (GPM) rating for things like pumps, pool filters, and other water flow systems is an important part of proper sizing, maintenance, and troubleshooting. With some simple measurements and calculations, you can figure out the GPM yourself. Here’s how to do it.
Measuring Flow Rate
The key to determining GPM is to accurately measure the flow rate. There are a few different ways to do this:
Using a Flow Meter
A flow meter is a device that directly measures GPM. Flow meters can be installed inline with the piping system to get an accurate real-time measurement. Some flow meters display the GPM reading on a digital display.
To use an inline flow meter:
- Install it according to the manufacturer’s instructions.
- Turn on the pump or water flow.
- Read the GPM number on the display.
Flow meters provide the easiest and most precise way to determine GPM. However, not everyone has immediate access to one.
Bucket Test
A simple bucket test can also measure GPM:
- Get a large bucket that has volume markings on it, such as gallons or liters. 5 gallon buckets work well.
- Position the bucket under the water flow.
- Using a stopwatch or timer, start the water flowing into the bucket and start timing at the same time.
- After exactly 1 minute, stop the water flow and check the water level on the bucket. This is the GPM.
For example, if the bucket fills to the 2 gallon mark after 1 minute, the flow rate is 2 GPM.
Do a few bucket tests and average the results for greater accuracy.
Marked Container Test
This is similar to the bucket test, but a bit more precise.
- Get a container marked with precise volume units, such as a laboratory grade graduated cylinder. 500ml cylinders work well.
- Position the container under the water flow.
- Using a stopwatch or timer, start timing as you start the water flowing into the container.
- Stop the water after exactly 1 minute.
- Note the final water volume. This is the GPM in the units marked on the container (convert liters to gallons as needed).
As with the bucket method, do a few tests and average the results. The smaller volume container allows greater precision.
Calculating GPM
In some cases you may not be able to directly measure the flow rate, but you can calculate the approximate GPM based on the pump specifications.
Using Pump Horsepower
For centrifugal pumps, you can calculate the GPM from the horsepower rating.
Use this formula:
GPM = (Pump Horsepower x 500) / Total Dynamic Head
Where total dynamic head is the length of pipe plus the vertical rise plus losses through fittings like elbows, expressed in feet.
For example:
- The pump motor is 1 HP
- Dynamic head is 30 feet
- Using the formula:
- GPM = (1 x 500) / 30
- GPM = 16.67
So the expected GPM from a 1 HP pump with a dynamic head of 30 feet is around 16.7 GPM.
Using Pump Speed
For positive displacement pumps, you can calculate GPM from the pump speed:
GPM = Pump Speed (RPM) x Displacement per Revolution
Where displacement per revolution is how much water volume is moved through the pump for each shaft revolution. This spec should be provided by the pump manufacturer.
For example:
- Pump speed is 600 RPM
- Displacement is 0.5 gallons per revolution
- Using the formula:
- GPM = 600 x 0.5
- GPM = 300
So a 600 RPM positive displacement pump with 0.5 gallon per revolution displacement will produce 300 GPM.
Typical GPM Values
Here are some approximate GPM flow rates for common applications as a reference:
- Household water pressure – 8-12 GPM
- Kitchen faucet – 1.5-2.5 GPM
- Bathroom faucet – 1-2 GPM
- Shower head – 2-5 GPM
- Garden hose – 4-8 GPM
- Whole house water filter – 8-15 GPM
- Large bathtub – 20-30 GPM fill rate
- 1″ water main pipe – 150-200 GPM
- Small pool pump – 30-60 GPM
- Large pool pump – 75-150 GPM
- Small well pump – 5-15 GPM
- Large well pump 60-100 GPM
- Commercial fire sprinkler – 75-250 GPM
Factors That Affect GPM
When determining the actual GPM flow rate, there are some factors that cause it to vary from the rated pump capacity or calculated theoretical GPM:
- System pressure – Higher pressure allows more flow. Lower pressure reduces GPM.
- Pipe size – Larger pipes have less friction and allow more GPM. Smaller pipes greatly reduce flow.
- Number of fixtures – More open faucets or outlets reduces pressure and splits overall GPM.
- Valves and filters – Partially closed valves or clogged filters add restriction and lower GPM.
- Elevation – Pumping uphill decreases pressure and GPM due to gravity.
- Pump wear – Worn impellers and seals reduce pump performance and GPM over time.
When troubleshooting low flow issues, checking the system GPM can help narrow down where the problem is occurring.
Measuring GPM of Irrigation Systems
Determining the GPM for irrigation systems like sprinklers and drip lines is also important for proper operation and scheduling.
Catch Can Test
A catch can test can measure GPM for a sprinkler zone:
- Place small empty cans in a grid pattern throughout the zone coverage area. Tuna cans work well for this.
- Run the sprinkler zone for a set time, such as 15 minutes.
- Measure the water depth in each can, in inches. Calculate the average depth.
- Convert the average depth to gallons using the can surface area.
For example:
- Use 5″ diameter cans
- Average water depth is 1.5″
- Can surface area is 5 x 5 x 3.14 = 19.6 square inches
- 1.5″ depth in a 19.6 in2 area is 0.245 gallons
- In 15 minutes, each can collected 0.245 gallons
- So GPM = 0.245 x (60 / 15) = 9.8 GPM
Do this test a few times each season to monitor changes in flow.
Flow Meter
For drip irrigation, connect an inline flow meter to the supply header. Run each zone and check the GPM reading on the meter. Multiple heads on a zone will split the flow across them.
Manufacturer Specs
Refer to the sprinkler head or drip emitter manufacturer specs for the rated GPM of each unit. Multiply this by the number of units on the zone to estimate total GPM.
Why GPM Matters
Getting an accurate gallons-per-minute measurement is useful for several reasons:
- Properly size pumps and pipes – Size the system components to handle the required GPM flow rate.
- Set timers correctly – Irrigation schedules are based on zone GPM and area covered.
- Diagnose issues – High or low GPM indicates problems restricting flow.
- Compare actual to theoretical – Ensure the system performs to specifications.
- Determine fill times – Calculate how long it takes to fill based on GPM.
- Calculate water usage – Multiply GPM by time to get volume used.
So take a few minutes to determine the GPM rating where needed. It’s an important metric to better understand water flow systems.
Frequently Asked Questions About GPM
How is GPM typically measured?
The most accurate way to measure GPM is with an inline flow meter installed as part of the plumbing system. GPM can also be determined by simple timed tests using a container like a bucket or graduated cylinder.
What pumps have the highest GPM rating?
Large centrifugal pumps used for municipal water supply, irrigation, and fire suppression systems have very high GPM ratings, ranging from 500 to over 2000 GPM for some large pumps.
How can I calculate GPM from pump horsepower?
Use the formula: GPM = Pump HP x 500 / Total Dynamic Head
Where total dynamic head is the piping system length plus vertical rise plus losses in feet.
What is a normal GPM range for household water supply?
A typical household water system operates between 8 to 12 GPM during normal use across all fixtures. Kitchen faucets are around 1.5-2.5 GPM. Showers range from 2-5 GPM.
How do I figure out GPM for a sprinkler system?
Perform a catch can test to measure sprinkler zone GPM. Place cans in the coverage area, run for a set time, measure depth, and calculate volume to determine GPM. Drip systems can use an inline flow meter.
What causes lower than expected GPM?
Restrictions like undersized pipes, partially closed valves, clogged filters, kinked hoses, elevation changes, and worn pump components can all reduce actual GPM versus calculated or rated flow.
How is GPM used for an irrigation system?
Knowing the GPM for each zone along with the area covered is needed to properly set watering schedules and runtimes. It ensures adequate watering without waste.
Can GPM be increased for higher flow?
Increasing pipe size, reducing elevation, adding booster pumps, opening valves fully, and removing flow restrictions can improve GPM if hydraulic conditions allow it.
How often should I confirm the GPM?
It’s good to check GPM periodically, such as seasonally. Changing flow conditions due to pump wear, debris buildup, or water demand can alter GPM over time.
Conclusion
Determining a system’s gallons per minute flow rate is important for calculating capacities and water usage, diagnosing problems, and setting schedules. While a flow meter provides the greatest accuracy, simple manual tests using timed container filling also work. Understanding typical GPM values for various applications provides a reference point for evaluation. Calculating the theoretical GPM from pump curves or specs is also an option when direct measurement is not possible. Getting the actual GPM helps optimize water systems and identify any issues restricting flow. Knowing this key metric leads to better performance and efficiency.