Reading the pump curve: how to select a pump for the correct duty point

The pump curve shows how much water a pump displaces (flow rate) at a given back pressure (head). If you select a pump based solely on its maximum flow rate or maximum head, you will almost always be wrong in practice — because at these extremes, the pump delivers no pressure or no water, respectively. In this article, you will learn to read the curve like an installer and determine the actual operating point of your pump.

What does a pump curve show?

The vertical axis (Y) represents the head in meters, and the horizontal axis (X) represents the flow rate in m³/hour or liters per minute. The downward sloping line is the pump curve: the higher you need to pump, the less flow rate remains. The two extremes are Hmax (maximum head at zero flow) and Qmax (maximum flow at zero head). Neither of these is your operating point — it always lies somewhere in between.

The operating point: where the system and pump intersect

Your installation has its own system curve: the resistance the pump must overcome. This consists of the static head (the fixed height difference) plus pipe friction loss, which increases as more water is pumped. If you overlay the system curve on the pump curve, the intersection point is the operating point: the flow rate and head you actually achieve in practice. The flow rate is not determined by the pump alone, but by the combination of the pump and the installation.

Static head and pipe friction loss

The static head is the vertical height difference between the water level and the discharge point; it does not change with the flow rate. Pipe friction loss (friction in hoses, pipes, and fittings), however, increases approximately quadratically with the flow rate: if you double the flow, the resistance roughly quadruples. Long or thin pipes will therefore shift your operating point significantly to the left — meaning less flow rate.

Common mistakes when reading the pump curve

• Choosing based on Qmax. The maximum flow rate is only achieved at zero back pressure, never in a real installation.
• Forgetting pipe friction loss. Without drawing in the system curve, you will consistently choose an underpowered pump.
• Only looking at height or depth. Also calculate the desired pressure at the tap point: 1 bar is approximately 10 meters of head.
• Incorrectly combining two pumps. Connecting pumps in parallel increases flow rate, while connecting them in series increases head — not the other way around.

How to use the curve when making your choice

First, determine your desired operating point (flow rate and head combined) and then find a pump whose curve extends well beyond that point. You can find the corresponding specifications on each product page in our shop; if you are unsure, browse our range of submersible pumps or let our customer service calculate the operating point for you.

Frequently Asked Questions

What is a pump curve?
A pump curve is a graph that shows how much flow a pump delivers at each head. The line slopes downward: the higher the back pressure, the less water the pump moves.

What is the operating point of a pump?
The operating point is the intersection of the pump curve and the system curve of your installation. This is where the actual flow rate and head you will get in practice lie.

Why don't I achieve the specified maximum flow rate?
Qmax only applies at zero head. As soon as there is height and pipe friction loss, the operating point shifts and the flow rate decreases.