Calculate head and flow for your situation

What are delivery head and flow rate?

Flow rate is how much water a pump moves, in liters per hour (l/h) or cubic meters per hour (m³/h; 1 m³ = 1,000 liters). Delivery head is how high the pump can push the water, in meters. These two are related: the higher the water needs to go, the fewer liters per hour the pump achieves. This relationship is shown in the pump curve.

Step 1: Calculate the delivery head

1. Height difference: measure from the water surface (or the bottom of the well) to the highest point of the pipe or outflow.
2. Pipe loss: water encounters resistance in hoses, bends, and couplings. Rule of thumb: add 1 meter per 10 meters of hose. For a thin hose, many bends, or a high flow rate, you should add more.
3. Pressure at the end (only for sprinklers or taps): 1 bar ≈ 10 extra meters. See choosing a well pump.

Horizontal counts too. Pumping water 30 meters through the garden without a height difference still costs about 3 meters of delivery head due to pipe loss.

Step 2: Calculate the flow rate

Standing water (basement, swimming pool, construction pit): volume = length × width × water height. Divide that by the time you want it removed.

Continuously flowing water (groundwater, rain, drainage): the pump must be able to handle at least the inflow per hour, plus a margin. Measure the inflow by observing how quickly the water rises again after pumping it dry.

Step 3: Choose the pump with the pump curve

In the pump curve, find the delivery head you calculated in step 1 and read off the flow rate. Is that enough for step 2? Then the pump fits. Preferably choose a pump where your operating point is in the middle of the curve. At the very end of the curve, it only pumps a trickle, which is also not good for the pump.

Calculation examples

Example 1: Flooded basement

Basement 5 × 4 m, 15 cm water: 5 × 4 × 0.15 = 3 m³ = 3,000 liters. The water needs to go up 2.5 m and 20 m of hose to the street: 2.5 + 2 = 4.5 m delivery head. If you want to finish within an hour, you need a pump that can deliver at least 3,000 l/h at 4.5 m. More: basement flooded.

Example 2: Sump pit with inflow

2,000 liters per hour of groundwater flows in. The discharge pipe goes up 3 m and 10 m horizontally: 3 + 1 = 4 m delivery head. Choose a pump with a float that delivers well over 2,000 l/h at 4 m, for example 4,000 l/h, so it doesn't have to run continuously.

Example 3: Emptying a swimming pool

Swimming pool 8 × 4 m, 1.2 m deep: 38.4 m³. Outflow to a pit 15 m away without height difference: approximately 1.5 m delivery head. With 10,000 l/h at that height, you'll be done in about 4 hours. A flat suction pump then removes the last millimeters.

Common mistakes

• Using the maximum flow rate from the box. That applies at 0 meters; in practice, the pump will deliver less.
• Forgetting pipe loss with long hoses or a thin pipe.
• Using a hose that is too thin; that significantly reduces the flow rate.
• Underestimating inflow from groundwater or heavy rain.

Recommended pumps per capacity

UP TO 6,000 L/H – FLAT SUCTION

Pentair Simer 6

For basements, roofs and crawl spaces, up to 2 mm dry.

€269.50

View pump

UP TO 35 M³/H AT 14.5 M

HOMA TP48 (50 mm passage)

Dirty water pump with 50 mm passage for larger volumes (M-version).

from €1,465.95

View pump

CONSTRUCTION AND CONTINUOUS

HOMA HBP 512

Jacket-cooled construction pump, suitable for sand and sludge.

from €2,626.95

View pump

Still no idea what type of pump? Start with the submersible pump selection guide.

Frequently asked questions

How do I calculate the delivery head of a pump?

Add the height difference to the highest point to the pipe loss. Rule of thumb: approximately 1 additional meter per 10 meters of hose.

How many liters per hour do I need?

Divide the volume to be removed by the desired pumping time. For inflowing water, the pump must be able to handle at least the inflow per hour, with a margin.

Why does my pump pump less than stated on the box?

The maximum flow rate applies at 0 meters of delivery head. The higher and further the water needs to go, the less the pump delivers. Check the pump curve.

What is 1 m³ in liters?

1 cubic meter is 1,000 liters. So a flow rate of 6 m³/hour is 6,000 liters per hour.