Pump discharge and power: how to calculate what you need for your garden or well

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Pump head and flow rate: how to calculate what you need for your garden or well

When a customer comes into the store and asks for a pump, nine times out of ten they will say one of two things: "I need something for the garden" or "I have a well, I want to pump water from it." Both statements are a good starting point for the conversation, but they are not sufficient on their own to know what to recommend. Two key numbers are crucial, which many customers are not even aware of or underestimate: head (measured in meters or bars) and flow rate (measured in liters per minute or cubic meters per hour). These two values determine whether the purchased pump will be able to irrigate the garden before dusk, or whether it will just sputter and blow air.

This article will guide you through the entire calculation process step by step – from measuring actual conditions (elevation difference, pipe length, required flow rate) through the selection of the correct type of pump to practical examples from real-world projects. I will refer to specific numbers, because theory without numbers is useless in this case.

What is head and why it should not be confused with suction depth

Pump head (in English head, in German Förderhöhe) is the height to which the pump is able to push water at zero flow. It is given in meters of water column (m w.c.) or in bars. The conversion is simple: 10 meters of water column = 1 bar.

Head is not the same as suction depth. Suction depth is a separate parameter – it refers to the vertical distance from the water level to the pump inlet. Standard surface (self-priming) pumps can suction up to a maximum of 7–8 meters. Submersible pumps do not have suction problems because they are directly in the water.

Head is the total "pressure budget" of the pump. From this budget, all losses and system requirements are subtracted:

Geometric height – the vertical elevation difference between the water level (or pump inlet) and the highest point of the system.
Pressure losses in the pipe – friction of water against the pipe walls, resistance from elbows, valves, fittings.
Required residual pressure at the outlet – for example, the pressure needed for proper operation of a garden sprinkler or hose nozzle.

If the pump has a catalog head of 30 meters, but your geometric height is 25 meters and you lose another 8 meters in the pipe, the pump will simply not work at that location – the head is insufficient.

How to calculate the required head – step by step

Calculating the required head is not rocket science, but it does require collecting a few specific pieces of information. Let's break it down into steps.

Step 1: Determine the geometric height (H_geo)

Geometric height is the vertical difference between the water level in the source and the highest point of the system (e.g., the height of a sprinkler on a pole, the inlet to a tank on a hill, the ground floor of a house, etc.). Measure it as accurately as possible – with a tape measure or laser rangefinder. If you have a well with a static water level 5 meters below ground and the highest sprinkler is 1 meter above ground, the geometric height is 6 meters.

For submersible pumps in wells or boreholes: the water level drops during pumping (dynamic water level). Always calculate with the dynamic water level, not the static one. The difference can be up to 3–5 meters in less productive wells.

Step 2: Calculate friction losses in the pipe (H_f)

For standard garden installations, an approximate rule applies: for every 10 meters of pipe length (including all elbows and valves, which are counted as equivalent length) at a flow rate of approximately 1.5–2.5 m/s, you lose about 1–2 meters of head. For more precise calculations, the Darcy-Weisbach equation or loss tables (Moody diagram) are used, but for garden purposes, the following simplification is sufficient:

PE pipe 25 mm (3/4"), flow up to 10 l/min: loss about 0.5–1 m per 10 m length
PE pipe 32 mm (1"), flow up to 20 l/min: loss about 0.3–0.7 m per 10 m length
PE pipe 40 mm (5/4"), flow up to 40 l/min: loss about 0.2–0.5 m per 10 m length
Each 90° elbow adds about 0.5–1 m equivalent pipe length
Ball valve fully open: negligible losses
Check valve: about 0.5–1.5 m loss

If you have a 50-meter PE 32 mm pipe with 5 elbows and 1 check valve, the equivalent length is about 50 + 5×1 + 3 = 58 meters. At a flow rate of 15 l/min, this gives a loss of about 0.5 × 58/10 = 2.9 meters. Round it up to 3 meters.

Step 3: Determine the required residual pressure at the outlet (H_zv)

For a garden sprinkler, 1–2 bars (10–20 meters) is usually sufficient. For a standard garden hose for watering, 1 bar is enough. If you are supplying a house with a pressure tank, a standard requirement is 2.5–3.5 bars. For a shower, at least 1–1.5 bars.

Step 4: Add up all components

Total required head H_total = H_geo + H_f + H_zv

Example: A well with a dynamic water level 8 meters below ground, a sprinkler 1 meter above ground → H_geo = 9 m. Pipe 40 m, 3 elbows → H_f = 2.5 m. The sprinkler requires 1.5 bar (15 m) → H_zv = 15 m. Total required head: 9 + 2.5 + 15 = 26.5 meters. The pump must have at least 27–28 meters of head at the required flow rate (add 5–10% reserve).

How to calculate the required flow rate – how much water do I really need

The second key parameter is the flow rate (Q), i.e. the amount of water the pump delivers per unit of time. It is given in liters per minute (l/min) or cubic meters per hour (m³/h). Conversion: 1 m³/h = 16.7 l/min.

Calculating the required flow rate for garden irrigation

The required flow rate depends on how much water you consume and how quickly. For garden purposes, there are standard consumption values:

Garden hose (Ø 3/4"): 10–20 l/min at pressure 1–2 bar
Static sprinkler (pop-up): 0.5–2 l/min per unit, typical section 8–12 sprinklers = 8–24 l/min
Rotary sprinkler (rotor): 2–5 l/min per unit
Drip system for 100 m²: 2–8 l/min
Irrigation for garden 500 m²: typically 20–40 l/min depending on the type of sprinklers

If you have a garden of 800 m² with a combination of rotary sprinklers and a hose for flower beds, the actual consumption can be 35–50 l/min. The pump must deliver this flow rate at the required head – that is why you should always look at the H/Q curve in the catalog, not just at the maximum values.

Calculating the required flow rate for a household or cottage

For water supply to a family house or a cottage, there are typical water consumption values:

Shower: 8–12 l/min
WC (flushing): 6–9 l at once, but peak flow 15–20 l/min
Basin: 4–8 l/min
Kitchen sink: 4–8 l/min
Washing machine: 10–15 l/min during filling
Garden tap (hose): 10–20 l/min

The peak consumption of a family house (4 people) is typically 25–40 l/min when several devices are operating simultaneously. For a cottage without a garden and with a smaller bathroom, even 15–20 l/min is sufficient. These values are critical when dimensioning – if the pump cannot deliver water during the peak, the pressure drops and comfort decreases.

Pump catalog values – how to read them correctly

Manufacturers usually list pump parameters in three numbers: maximum head (at zero flow), maximum flow (at zero head), and motor power in watts. These maximum values are achieved only under extreme conditions and never occur simultaneously in real operation.

Practical example: Gasoline pump BZP-30 has a power of 4850 W and a head of 30 meters. The maximum flow will be at zero head, but when you need a head of 20 meters, the flow will be lower – probably in the range of 60–80 % of the maximum. Always ask the seller for the H/Q curve or a performance table at various heads.

For comparison: Gasoline pump H-BZP-30 with a power of 9690 W and a head of 95 meters is dimensioned for completely different applications – water supply from deep wells, pumping to great heights or long distances. Its operating point at a head of 30 meters will be with a huge reserve and the pump will work inefficiently – unnecessarily high fuel consumption. Overdimensioning is as problematic as underdimensioning.

Types of pumps and their suitability for different situations

Once you have calculated the required head and flow rate values, you need to choose the right type of pump. This choice also depends on the water source, availability of electricity, frequency of use, and whether it is for garden or technological use.

Surface self-priming pumps (gasoline and electric)

These pumps are located outside the water source and self-prime. The suction height is physically limited to about 7–8 meters (practically less, even less with a long suction pipe). They are suitable for garden tanks, surface ponds, streams, shallow wells, or rainwater barrels. The head typically ranges from 20 to 50 meters.

The gasoline versions are independent of the electrical grid – this is their main advantage. Gasoline pump BZP-10 is a compact portable option for garden use, easily moved between the garden and the cottage. For more demanding garden and recreational applications, where you need a higher head or greater flow, it is more suitable to look at BZP-20 with higher power.

Submersible pumps for wells and boreholes

Submersible pumps are lowered directly into the well or borehole. They have no problem with suction height – they operate in a fully submerged state. The head is calculated from the water level to the point of delivery. These pumps are quiet, durable, and suitable for permanent installation.

Submersible pump SKM 100 is a typical representative for home wells with a diameter of 100 mm (4 inches). Its parameters are designed for real depths of typical garden and family wells. The head of submersible pumps for boreholes can be 100 meters or more, which is necessary for deep boreholes.

More detailed information on selection according to well diameter and depth can be found in our article Submersible pump for a well or borehole: which diameter and depth match which pump, where we also discuss the difference between a well and a borehole from the pump's perspective.

Drainage and sewage pumps

These pumps are designed for water containing impurities, sludge, or solid particles. Their head is usually lower (5–15 meters), but the flow is high. Do not use them for clean water supply – they are for pumping out, not for distribution. More about the differences can be read in the article Clean water pump vs. sewage drainage pump: what is the difference and when to use which.

Practical scenarios from practice: what is common and what customers underestimate

Scenario 1: 400 m² garden with automatic irrigation, 4 m deep well

The customer has a dug well with a depth of 4 meters, static water level 2 meters below ground, and dynamic water level drops to 3 meters after pump activation. The garden is on flat ground, the highest sprinkler is at 0.5 meters height. The installation has 4 rotating sprinklers and 8 static sprinklers, total flow at full operation is approximately 30 l/min. Pipe length from the well to the first manifold is 25 meters, pipe diameter 32 mm.

Calculation: H_geo = 3 + 0.5 = 3.5 m. H_f at 25 m pipe and 30 l/min = approx. 2 m. H_zv for sprinklers = 1.5 bar = 15 m. Total: 3.5 + 2 + 15 = 20.5 m of head at 30 l/min flow. Recommendation: a surface pump with at least 25 m head at 30 l/min flow – with a margin. A petrol BZP-20 or an equivalent electric pump would work very well here.

Scenario 2: Cottage in the hills, supplied from a stream 60 m away and 15 m lower

The cottage is located 15 meters above the stream. The pump is placed at the stream, water is pumped up to a storage tank at the cottage. Length of pressure pipe is 60 meters, diameter 32 mm. The tank is filled to 1 meter above the roof level (+ 4 m). Total geometric head: 15 + 4 = 19 meters. Pipe losses at 20 l/min and 60 m: approx. 4 meters. Residual pressure in the tank: 0 (gravity tank, pressure is self-generated). Total head: 19 + 4 = 23 meters at 20 l/min flow.

A modest pump with 28–30 m head at 20 l/min is sufficient here. Since the cottage is in a forest without electricity, BZP-30 with 30 m head is a logical choice – it can fill the tank 1–2 times per day, which is completely sufficient for a cottage.

Scenario 3: Family house, supplied from a 60 m deep borehole

The borehole is 60 meters deep, static water level 25 meters below ground, dynamic water level drops to 40 meters during pumping. The house is on flat ground, the pump delivers water to a pressure tank in the basement (0 m elevation). Required pressure in the system is 3 bar (30 m). Pipe length from the borehole is 8 meters, losses negligible. Total head: 40 + 30 = 70 meters. At such a head, a submersible pump directly in the borehole is necessary.

There is no room for a surface pump here – it is simply physically impossible. A submersible pump for a borehole with 70–80 meters head and 15–30 l/min flow is the correct solution. The installation of such a pump is described in detail in the article Installation of a submersible pump in a well or borehole: step-by-step procedure.

Scenario 4: Garden rainwater tank, watering the flower bed

A plastic 2000-liter tank is on the ground, the water level is typically 0.5–1.2 meters above ground (tank on a concrete base). The flower bed is 30 meters away on flat ground, garden hose 3/4". Geometric head: the tank is higher than the flower bed = negative geometry = the pump supports gravity. Practically required head = only pipe losses + a bit for pressure at the nozzle = 5–8 meters total.

Here, even the smallest garden pressure pump or a portable submersible pump with 10–15 meters head is sufficient. Customers often unnecessarily buy powerful pumps here, which is a waste of money and energy.

Most common mistakes when selecting a pump based on head and power

Over the years of practice, we see the same mistakes over and over. Here are the most common ones:

Mixing up maximum head and working head. A pump with a maximum head of 30 m at zero flow will deliver almost nothing at 28 m head. Always check the flow at your specific head.
Ignoring the dynamic water level. A static water level of 5 m does not mean the pump will always pump from 5 meters. After starting, it drops, sometimes dramatically.
Undersizing the pipe diameter. A thin pipe increases friction losses and forces the pump to work at the limit – the pump wears out faster and performance drops.
Forgetting the pump height above the water level. Every extra meter of suction height reduces flow and real head. A pump placed 6 meters above the water level has a real head 6 meters lower than the catalog says.
Selecting based on watt, not based on H/Q curve. Power in kW does not directly say anything about head or flow – it is the input power of the motor, not hydraulic power. Hydraulic efficiency of pumps varies greatly.
Ignoring water temperature. Most clean water pumps are designed for water up to 35–40°C. For warm or hot water, special types are required.

Pressure losses – practical table for quick estimation

Pipe diameter	Flow 10 l/min	Flow 20 l/min	Flow 40 l/min	Flow 60 l/min
PE 20 mm (1/2")	3,0 m/10m	10+ m/10m	not recommended	—
PE 25 mm (3/4")	0,8 m/10m	2,5 m/10m	8 m/10m	not recommended
PE 32 mm (1")	0,25 m/10m	0,7 m/10m	2,2 m/10m	4,5 m/10m
PE 40 mm (5/4")	0,08 m/10m	0,25 m/10m	0,7 m/10m	1,4 m/10m
PE 50 mm (6/4")	0,03 m/10m	0,09 m/10m	0,25 m/10m	0,5 m/10m

Note: values are approximate for smooth PE pipe at water temperature 15°C. For steel or old pipes, losses are 20–50% higher.

Motor power vs. hydraulic power: what the wattage means

Motor power (input power) given in watts (W) or kilowatts (kW) is the input power – what the pump consumes from the electrical grid or from fuel. Hydraulic power (actual energy delivered to the water) is always lower – it depends on the hydraulic efficiency of the pump, which for common garden pumps ranges from 40–65%, and for higher quality industrial pumps up to 70–80%.

Hydraulic power (in kW) = (H × Q × ρ × g) / 3600000, where H is the head in meters, Q is the flow in l/h, ρ is the density of water (1000 kg/m³), g is the gravitational acceleration (9.81 m/s²).

Example: a pump with a head of 30 m and a flow of 1200 l/h (= 20 l/min) has a hydraulic power of: (30 × 1200 × 1000 × 9.81) / 3 600 000 000 = 0.098 kW = 98 W. If the motor power is 800 W, the hydraulic efficiency is 98/800 = 12.3%. This is relatively low efficiency, typical for cheap garden pumps. For comparison, high-quality submersible borehole pumps can reach 60–70% hydraulic efficiency.

Therefore: do not buy a pump just by wattage. 1500 W does not necessarily mean a better head or flow than 1000 W – it depends on the impeller design, number of stages, and overall hydraulic design.

Multi-stage pumps – when they are advantageous

If you need high head (over 50 meters) at relatively low flows, or if you are supplying a multi-story house with a pressure tank, multi-stage pumps come into consideration. Each stage (impeller) adds its own head value to the total. A three-stage pump with 25 m head per stage gives a total head of 75 meters.

Multi-stage pumps are typical for submersible pumps in deep wells – that is why a compact submersible pump with a diameter of only 4 inches can achieve a head of 100 meters or more. Surface multi-stage pumps (e.g., type HC or Jetpump with an ejector) are a solution for suction from depths of 15–50 meters, where a standard self-priming pump fails.

Pressure tank (hydrofor) and its influence on pump sizing

A pressure tank (hydrofor) is not just an accessory – it significantly affects the pump flow requirements. The tank stores water under pressure and delivers it during short-term withdrawals without the need to start the pump motor. This reduces the number of pump switching cycles and extends its lifespan.

For a family house, a pressure tank with a minimum volume of 50–100 liters is recommended (effective volume is about 30–40% of the nominal volume). A larger tank = fewer switching cycles. A pump with a pressure tank can be sized for a lower flow, since the tank covers peak withdrawals.

Without a pressure tank (or with a very small one), the pump must respond to every withdrawal immediately and operate continuously. This requires a higher flow, but the pump suffers from frequent switching, which shortens its lifespan.

Common questions (FAQ)

How do I determine what head I need from a well, if I don't know the depth of the dynamic water level?

The most reliable method is to run any (even temporary) pump and measure the height to which it can push water during normal pumping. Alternatively, estimate the drop in the water level during pumping based on what you observe in the well – if the level drops by 2 meters after 10 minutes of pumping, include this reserve in your calculation. For boreholes, contact the company that drilled it – the dynamic level is stated in the drilling report.

Why is my pump not delivering the full flow, even though it has sufficient head according to the catalog?

The most common causes are: clogged suction basket or filter, air in the suction pipe (a leak at some joint), the pump is too high above the water level and working at the limit of its suction height, or the pipe is too thin and pressure losses are higher than expected. More causes can be found in the article Common pump problems: why it doesn't pump, loses pressure or overheats.

Is it better to buy a pump with a larger head reserve or one that is precisely sized?

A reserve of 10–20% above the calculated value is always reasonable – it compensates for measurement inaccuracies, pipe aging, and water level fluctuations. A reserve of more than 30–40% is counterproductive: the pump operates outside its optimal operating point (too far to the right on the H/Q curve), which means higher consumption, more noise, and faster wear. If you are choosing between two models, select the larger one only if the difference in head or flow is less than 25% above your needs.

Can I use a garden pump for supplying drinking water to a house?

It depends on the materials used in the pump's construction. Some garden pumps are made from materials suitable for drinking water (marked as DW – drinking water), others are not. Always check the documentation to see if the pump is certified for drinking water. In addition, the water source (well) must be hygienically clean – the pump itself does not sterilize the water, it only transports it. For supplying a house with drinking water from your own source, regular water quality testing is necessary.

How does the head vary at different water temperatures?

Water density changes with temperature – warm water is less dense, so the pump technically achieves a slightly higher head in meters of water column. However, for practical garden and household applications (water 5–20°C), this difference is negligible – in the order of 1–2%. More important is the opposite effect: at higher temperatures (above 40°C), the suction lift decreases due to higher vapor pressure, which can lead to cavitation and pump failure.

What is cavitation and how can it be prevented?

Cavitation occurs when the pressure at the pump inlet drops below the vapor pressure of water at a given temperature – water begins to boil locally and bubbles form.

Do you have a question about this topic?

Having trouble deciding or dealing with a specific situation in your home? Write to us – we are happy to help.