Flowrate vs. Head Pressure

Flowrate vs. Head Pressure

When you’re selecting a pump, flow rate and head pressure are the two variables that define almost everything about performance. They’re linked—if one goes up, the other usually goes down.

💧 Flow Rate (How much water moves)

Flow rate is the volume of water a pump moves over time.

  • Measured in: GPM (gallons per minute), LPM, or m³/h
  • Think of it as: “How fast can I move water from point A to point B?”

Example:

  • 100 GPM = moving 100 gallons every minute
  • Higher flow = faster dewatering, filling, or transfer

⛰️ Head Pressure (How hard the pump pushes)

Head pressure (or just “head”) is the force required to move water, expressed as height.

  • Measured in: feet of head or meters
  • Think of it as: “How high or how far can the pump push water?”

Includes:

  • Vertical lift (height)
  • Friction loss (pipes, bends, valves)
  • Pressure requirements at discharge

🔗 The Relationship (This is the key)

Pumps don’t give you max flow and max head at the same time.

👉 It follows a pump curve (based on the centrifugal pump performance curve):

  • At low head → you get high flow
  • At high head → you get low flow

📉 Simple visualization

  • Open hose (no resistance):
    👉 High flow, low pressure
  • Hose spraying uphill:
    👉 Lower flow, higher pressure needed

⚙️ Real-World Example

Let’s say you’re using a dewatering pump:

Scenario A:

  • Water is flat, short distance
  • Head = 10 ft

👉 You might get:

  • 200 GPM

Scenario B:

  • Pumping uphill 50 ft + long pipe

👉 Now:

  • Flow might drop to 80–100 GPM

🧠 Why This Matters for Your Customers

Most buyers make this mistake:

👉 They choose pumps based only on flow rate

That’s wrong.

They should calculate:

  1. Total head (height + friction)
  2. Then find a pump that delivers the needed flow at that head

🧮 Basic Formula (Simplified)

Total Dynamic Head (TDH) =

  • Vertical lift
  • Friction loss
  • Pressure requirements

🔧 Practical Buying Rule

When selecting a pump:

👉 Don’t ask:

  • “What’s the max GPM?”

👉 Ask:

  • “What flow do I get at my required head?”

🧩 Quick Analogy

Think of it like a truck:

  • Flow rate = how much cargo you carry
  • Head = how steep the hill is

A truck carrying a heavy load (high head) will move slower (lower flow).