How to Choose a Water Pump: Understanding GPH, GPM and Head Height

When shopping for a water pump, you may come across specifications such as GPH and GPM. But what do these numbers actually mean? Is a 720 GPH pump more powerful than a 10 GPM pump? And how do you know what flow rate you need?
Understanding the difference between GPH and GPM can help you choose the right water pump for rain barrels, garden irrigation, basement drainage, water transfer, and other applications.

GPH vs GPM: What's the Difference?

GPH stands for "Gallons Per Hour." It indicates how many gallons of water a pump can move in one hour under specified conditions.
For example, a pump rated at 720 GPH has a maximum flow rate of 720 gallons per hour.

GPM stands for "Gallons Per Minute." It tells you how many gallons of water a pump can move in one minute.

For example:

• 5 GPM = 5 gallons per minute
• 8 GPM = 8 gallons per minute
• 10 GPM = 10 gallons per minute
• 12 GPM = 12 gallons per minute

The main difference between GPH and GPM is simply the time period used to measure the flow rate.

The conversion is straightforward:

GPH = GPM × 60

GPM = GPH ÷ 60

So a pump rated at 720 GPH is equivalent to 12 GPM.

GPH and GPM are not two different measurements of pump power. They are simply two ways of describing the same flow rate.

Why Maximum GPH Can Be Misleading

The biggest mistake when comparing water pumps is assuming that the advertised maximum flow is the flow you will always get.

It isn't.

Maximum flow is typically measured under specific conditions. Once you add vertical lift, a hose, fittings, and other restrictions, the actual flow can decrease.

This is why two pumps with similar maximum GPH ratings can perform differently in the same setup.

For example, if you are transferring water from a rain barrel to a garden, the pump may only need to move water a short distance with little elevation change.

If you are pumping water from a basement up to ground level, the same pump has to overcome significantly more head height.

The application matters as much as the number on the specification sheet.

Flow Rate vs Head Height

These are two of the most important specifications to look at together.

Flow rate tells you how much water the pump can move.

Head height tells you how high the pump can lift the water.

A pump's maximum flow rate is generally measured under specific test conditions. When the pump has to lift water vertically, the flow rate can decrease.

This vertical distance is commonly referred to as "head height" or "maximum head."

For example, a pump may have:

• Maximum flow rate: 720 GPH
• Maximum head height: 26 ft

This does not mean the pump will deliver 720 GPH while lifting water 26 feet vertically.

As the required lift increases, the pump has to work against greater resistance, and the actual flow rate generally decreases.

Other factors can also affect flow, including:

• Vertical lift
• Hose length
• Hose diameter
• Number of bends or fittings
• Filters and other restrictions
• Water temperature and conditions

For this reason, it is important to look at both flow rate and head height when comparing water pumps.

What Flow Rate Do You Actually Need?

Start with the job, not the pump.

Ask three questions:

How much water am I moving?

A 50-gallon container and a 500-gallon tank have very different transfer requirements.

How quickly do I need to move it?

If speed matters, a higher flow rate can reduce transfer time.

How far and how high does the water need to go?

This determines how important head height and hose resistance will be.

For example, a homeowner transferring rainwater to a nearby garden may prioritize portability and flow control. Someone draining a large amount of standing water may place greater importance on maximum flow and head height.

Why Adjustable Flow Can Be More Useful Than Maximum Flow

Maximum output isn't always the goal.

When transferring a large amount of water, you may want the highest available flow.

When watering plants or directing water through a smaller hose, a lower flow can provide better control.

This is where adjustable-speed pumps have an advantage.

A pump offering 12, 10, and 8 GPM settings, for example, allows the same tool to handle different water-transfer tasks without relying on maximum output all the time.

A Better Way to Compare Water Pumps

Instead of asking:

"Which pump has the highest GPH?"

Ask:

"Which pump has the right combination of flow rate, head height, and control for my application?"

A useful comparison should include:

This gives you a much more realistic picture of pump performance.

Final Takeaway

GPH and GPM are useful starting points, but they shouldn't be the only numbers you look at.

A pump rated at 720 GPH may be an excellent choice for one application and unnecessary for another. The right choice depends on the amount of water you need to move, the required transfer speed, the lifting height, and how much control you need over the flow.

Don't choose a water pump by GPH alone. Choose it based on the job.