When you’re picking a submersible pump, the first two numbers you see on the box are maximum head and flow rate. They look straightforward, but most buyers read them wrong. That’s why you often hear about pumps that run but don’t deliver water where it’s needed, or get pushed past their limits and fail early.

In this post, we’ll show you how to read those numbers for the three most common jobs: basement, well, and pool. You’ll get a concrete formula you can use at home, no guesswork involved.

Maximum head and flow rate: two numbers that go hand in hand

Maximum head (Hmax) is the highest point a pump can push water. At that height, flow drops to zero, meaning the pump isn’t actually delivering any water. So in practice, you always want a pump whose Hmax exceeds your real requirement.

Flow rate (Q) is measured in liters per minute (l/min) and always depends on head. The higher the head, the lower the flow. That’s why manufacturers provide a performance curve showing how flow changes with head. If the box only lists a single flow number, it’s usually the maximum at zero head, which isn’t realistic for your setup.

Example: a pump with an Hmax of 10 meters at a head of 8 meters will deliver a trickle, maybe just a few liters per minute. For most jobs, that’s not enough. So it’s better to pick a pump with an Hmax of 15 meters, and at 8 meters it’ll still have solid flow.

Basement: factor in lift height and hose length

For basement drainage, a dirty-water submersible pump is the usual choice, since the water contains sand, leaves, and fine debris. First, measure the height from the basement floor to where the water discharges outside. That’s your base lift height.

Next, add the loss from the horizontal hose. A rough rule of thumb: every 10 meters of horizontal hose adds about 1 meter to the required head. The exact loss depends on hose diameter and flow, so if you’re using a narrow 1-inch hose, the loss is greater than with a 1.5-inch hose. Always use the widest hose the pump can handle.

For a basement 3 meters deep with a 20-meter hose, the math is: 3 meters + 2 meters (20/10) = 5 meters of required head. Add a 20-30% buffer, and you’re looking for a pump with an Hmax of 6 to 6.5 meters. A flow rate of 100 to 150 l/min is enough for quick draining, but the exact number depends on the water volume.

Well: measure from the water level, not the bottom

The most common mistake with wells is measuring the well depth. The pump doesn’t lift water from the bottom—it lifts from the water surface. So the real head is the vertical distance from the water level to the discharge point, plus the loss from the horizontal hose.

Example: a well is 8 meters deep, but the water level sits at 3 meters below ground. The discharge point is 5 meters above ground. That means the head from water to discharge is 8 meters (3 + 5). If the hose runs 20 meters horizontally, add 2 meters, for a total of 10 meters. For that, you’ll need a pump with an Hmax of 12 to 13 meters.

If your well is deeper than 10 meters or narrower than 15 cm in diameter, a standard submersible pump may not be the right fit. For deep, narrow wells, deep-well pumps are the answer—they lower into the casing and lift water from greater depths. Check your well’s diameter before buying.

Pool: submersible pump for draining, not circulation

A submersible pool pump is for draining the pool, not for circulating water through the filter. Circulation is handled by a surface pump that’s permanently connected to the filter and heater. A submersible pump comes in when you need to empty the pool for winter or repairs.

For pool draining, the head is usually low—1 to 3 meters—since the water discharges at the pool’s edge. The main requirement is flow, because you want the pool emptied fast. For a 30-cubic-meter pool, a flow rate of 200 l/min will drain it in about 2.5 hours, but the actual flow depends on head and hose length.

Choose a pump that can handle low water levels, since you’ll need to get those last few centimeters out. Some pumps have a float switch that shuts the pump off when the water gets too low, protecting it from running dry.

Clean or dirty water: a basic distinction you can’t skip

Before you even look at head and flow, figure out what the pump will be moving. For clean water from a well or rainwater, a standard submersible pump with a fine impeller is all you need. For water with sand, mud, or leaves, you need a dirty-water pump with a larger particle passage.

Dirty-water pumps use a vortex impeller that lets most debris pass through the swirl without going through the impeller itself, so they can handle solids up to about 30 mm, though that limit varies by model. If sewage is involved, sewage pumps have a grinder that shreds solid waste. But a grinder isn’t for sand—sand will destroy the cutting mechanism. For sand, go with a vortex pump or one with a rated solids-handling spec.

On the Alatishop site, in the submersible pump category, you can filter by water type and particle size. That’s the first step in your selection, before you even look at head and flow.

Dry-run protection and other details that extend pump life

A submersible pump that runs without water will overheat quickly. So check for thermal protection that shuts off the motor when it gets too hot. Some pumps have a float switch that automatically cuts power when the water level drops below the minimum.

The float needs room to move freely. In a tight sump or shallow basement, the float can get stuck, and the pump won’t work as it should. Think about that before you buy, especially if space is limited.

The housing material affects durability, but there’s no universal rule that metal beats plastic. Stainless steel housings resist corrosion but cost more. Plastic is lighter and cheaper, but it can crack on impact. The choice depends on the medium and operating conditions, so check the manufacturer’s recommendations for your specific use.

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Frequently asked questions

How do I calculate the required head for a well?

Measure the vertical distance from the water level to the discharge point, then add 1 meter for every 10 meters of horizontal hose. Don’t measure the well depth—measure the water level.

What does Hmax mean on a pump?

Hmax is the maximum height a pump can lift water, but at that height, flow is zero. For real-world use, choose a pump with an Hmax 20-30% higher than your requirement.

Can I use a submersible pump for pool circulation?

No. For circulation through the filter and heater, you need a surface pump. A submersible pump is for draining the pool, not for continuous operation.

What’s the difference between a clean-water and dirty-water pump?

A clean-water pump has a fine impeller that clogs if there are particles in the water. A dirty-water pump has a vortex impeller that handles solids up to about 30 mm, making it suitable for basements and wells.

What if my well is 20 meters deep?

For deep wells, you’ll need a deep-well pump that lowers into the casing—it’s a type of submersible pump. Check your well’s diameter, since deep-well pumps come in different sizes: standard 4-inch models fit casings around 100 mm, and narrower options are also available.