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What Is Pump Head?

If you remember one thing from this page, make it this: head is how high a pump can push water, measured in feet.

A pump's job is to add energy to water. We could measure that energy as pressure (pounds per square inch, or PSI), but the pump industry measures it as head instead — the height, in feet, that the pump could lift a column of water straight up.

So if a pump is rated for 150 feet of head, it means: if you stood the pump at the bottom of a tall, thin pipe and turned it on, it could push water up to a height of 150 feet. That is true no matter how fast the water is flowing at that moment.

Why feet instead of PSI?

Because head is honest about the pump and ignores the fluid. A pump produces the same head whether it is pumping light gasoline or heavy brine — but it would produce very different pressure readings for each, because pressure depends on how heavy the fluid is. Measuring in feet lets engineers compare pumps directly without worrying about what is flowing through them.

When you do need pressure, the conversion for water is simple:

The Main Event

How to Read a Performance Curve

Here is a real pump performance curve. It looks busy, but every line on it answers one specific question. We'll take them one at a time.

The two axes: flow and head

Everything starts with two measurements:

• Flow runs along the bottom (horizontal) axis, measured in gallons per minute (GPM). This is how much water the pump moves.
• Head runs up the left (vertical) axis, measured in feet. This is how hard the pump is pushing.

Pick any point on the chart and it tells you a flow-and-head pair: "at this many gallons per minute, the pump produces this many feet of head."

The head curves (the arcs sloping down to the right)

The smooth arcs that start high on the left and slope down to the right are head curves. Each one shows the fundamental trade-off: as flow increases, head decreases. Push more gallons and you can't push them as high — exactly like the elevator carrying a heavier load to a lower floor.

You'll usually see several head curves stacked together on one chart. Each represents a different impeller diameter — the spinning part inside the pump. A bigger impeller produces more head (a higher arc); a smaller one produces less (a lower arc). Manufacturers machine, or "trim," the impeller to a specific size to land your job right where you need it. The top arc is usually the largest impeller the pump can hold; the bottom arc is the smallest.

The efficiency islands (the concave lines that loop back)

Those curving lines with numbers like 66, 74, 80, 84 are iso-efficiency curves — "iso" just means "equal." Each line connects all the points where the pump runs at that exact efficiency. The 80 line means "the pump is 80% efficient everywhere along this line."

Two important truths about these numbers:

• They are not a fixed set. One pump might show 57–80; a more efficient one might show 66–87. The highest number you see is that pump's peak efficiency.
• You will essentially never see 100. No pump is lossless. Real pump peaks land in the high 80s to low 90s for large units, lower for small ones.

Where a head curve crosses an efficiency island tells you the efficiency at that operating point. That single fact is how you judge whether a pump is a smart, low-cost choice or a wasteful one.

The Best Efficiency Point (BEP)

The center of the innermost efficiency island — the very top of the hill — is the Best Efficiency Point, or BEP. This is the flow-and-head combination where the pump runs at its absolute best: lowest energy cost, least vibration, longest life.

A good pump selection puts your operating point at or very near BEP. The healthy operating window is roughly 70% to 120% of BEP flow. Run far below BEP and you risk recirculation damage; run far above it and you waste energy and edge toward cavitation. A pump running far from BEP costs more to operate for its entire life.

The NPSH curve (the line climbing along the bottom)

NPSH stands for Net Positive Suction Head. It's about the inlet side of the pump — making sure water arrives with enough pressure that it doesn't boil into vapor bubbles and damage the pump (a destructive effect called cavitation).

• NPSH required (NPSHr) is the line on the curve — how much suction pressure the pump needs. Notice it rises as flow increases: the harder the pump works, the more it demands at its inlet.
• NPSH available (NPSHa) is what your system supplies. It is not on the pump curve — it comes from your installation.

The horsepower lines (gray diagonals)

Some curves add gray diagonal lines labeled with horsepower (75 hp, 100 hp, 125 hp…). These are constant-power lines — they show how much power the pump draws at any flow-and-head point, which tells you what size motor you need.

The system curve (the line rising from the bottom-left)

The line that starts low on the left and climbs steeply to the right is the system curve — and this one describes your piping, not the pump. The pump maker draws the pump curves; you (or your engineer) supply the system curve from your installation's static lift, pipe size, length, and fittings.

It climbs steeply because system head is mostly friction, and friction grows with the square of flow. The single point where your system curve crosses a pump head curve is the operating point — the actual flow and head you'll get in real life. A pump doesn't choose its own flow; it runs wherever its curve meets your system curve. That's a law of the installation.

Glossary

Key Terms in One Table

Complete Reference

Full Pump Glossary

Every common term used to describe pump performance and data — including ones that don't appear on the curve but show up on datasheets, quotes, and submittals. Grouped by what they describe.