How To Size A Sump Pump | A Detailed Guide 2021

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Choosing the right-sized pump is essential: pick a small size, and you might end up with a flooded and damaged basement, pick a powerful sump pump, and you may end up with a burnt-out motor.

Let us give you a number outright to help you get some preliminary idea on this matter. For every thousand square feet home that is built on sandy planes, the minimum motor power should be 18 GPM. That means the ideal pump should discharge 18 gallons of water per minute. What if the number goes up and the home you have is now three thousand square feet? Well, the GPM required will have to be multiplied by three as well, bringing the number up to 54 GPM.

However, when it comes to houses on clay soils, you don’t need a pump as powerful. 8 gallons per minute of motor capacity for a thousand feet property should do the trick. So, what are the other things you need to be aware of while sizing a sump pump? Things like how to size your sump pump if you already have a basin, how much GPH is needed, how high you want to pump, and what you need to do if you don’t have a basin. We’re going to break down all these aspects in detail for you to understand the ins and outs of how to Size a sump pump

Things to Know Before Sizing a Sump Pump

Before sizing your sump pump, there are some essential things that you need to know, especially if you want to replace your sump pump with a new one. For instance, if your old sump pump was incorrectly sized, you only have to size the basin regarding how far and high you are pumping. But if you are new to this and your home does not have a basin, then the process might take a bit longer.

However, once you get down to the basics, you need to purchase a sump pump and a sump pump basin that fits your criteria. So, without further ado, let’s jump straight into sizing a sump pump.

How To Size A Sump Pump

What should be the GPH (Gallons Per Hour) number?

To know this, you need to start measuring the diameter of your basin. It might be a duplex or a single unit. An average basin measures up to 18 inches, but you need to check to be sure. You need to measure it for a minute to see how much water gathers in your basin. Here is the basic math: one inch of water intake in an 18-inch basin is a gallon, and one inch in a 24-inch basin is two gallons. This is a crucial step to sizing your sump pump.

Let’s say 24-inch of water is pouring down into the basin in a minute. So, if every inch, as we said earlier, is 2 gallons, the basin taking in 48 gallons of water a minute. So, think of a pump that meets this requirement. After measuring the basin’s diameter, you can move on to measure the amount of water gathered in it. For this, the water level has to be below the sump pump’s starting range.

When the pump switches off after the water reaches its capacity, wait for a minute, measure the point where the water is, and measure how high the water rose in that one minute. Next, multiply the number of inches the water rose in that one minute by 60. With this, you can estimate the number of gallons per hour you will receive at the sump pump basin.

How high do you want to pump?

The second thing to determine is how high you want to pump because without examining the vertical lift, the GPH-number alone cannot handle the process. A vertical lift determines how high the discharge line is from the inlet and stopping when the line becomes horizontal. Measuring it is quite simple; you need to measure your discharge line’s length from the depth of the pit up to the mark where the horizontal line begins.

Suppose if your basin needs a pump of 2700 GPH, then getting one with a maximum flow rate of that amount at 0 feet won’t be enough. Hence, you need to check the sump pump’s maximum flow rate according to the size of the vertical lift. Besides the vertical lift, you also need to check up on the friction produced in the pipes, which might slow down the water. This friction might be caused due to the pipe elbows, narrow pipes, check valves, and longer discharge pipes.

How big of a sump pump do I need?

The size of the sump pump depends on factors such as head height, the volume of water, the functionality of the pump, and other necessary features. Based on these factors you can decide how big of a sump pump you may need.

What size sump pump do I need

when it comes to understanding the correct size of the sump pump that will meet all your needs, you need to first consider a few important factors such as the horizontal and vertical distance the water has to be pushed by the motor. The layout of your drainage system is the determining factor.

  • Vertical piping– if you are using many vertical pipes, then you will need a powerful pump since it will be working against gravity. Also, if the head height is high then it will be difficult for a pump with lesser horsepower to draw water out of the pit. This will inevitably cause flooding.
  • Horizontal piping – if your sump pit is located in the basement, then you will need horizontal piping. Although in this system, the pump doesn’t have to deal with gravity, it does, however, have to work against friction. Also, the longer the distance of the discharge pipes, the more horsepower is needed by the pump.

Do I need a 1/3 or 1/2 HP sump pump?

A 1/3 and a ½ HP pumps are the most commonly used pumps. So when you are looking for a sump pump, you need to consider the pros and cons of both and go for the one that suits you best.

  • 1/3 HP pump – It is ideal for sump pits with low head height and is applicable for pumping water with flow rates below 1,300 gallons per hour. So, if you are facing heavy flooding, then you will need a pump with greater horsepower. A few of the perks of using a 1/3 HP sump pump, is that it is less noisy and consumes very little power.
  • ½ HP pump – if you are having large volumes of water accumulating in your basement, then going for a ½ Hp sump pump is a great choice to go for. It can pump several gallons of water to great vertical head heights. The only issue with using this pump is that it may burn out quickly. Since the motor is very powerful, the pump is not very long-lasting.

How far can a 1/2 HP sump pump push water?

On average, a ½ HP sump pump is capable of pushing water over 10 feet vertical lifts. However, the range may differ depending on the number of 45- or 90-degree angles are there in the piping system. In horizontal pipes, it can pump over 25 feet.

What do you need to do if you don’t have a basin?

This might sound a little hard to believe but not having a basin might make the whole process easier for you because now you are not hindered by the dimensions of a basin as with the case shown above. Most professionals would recommend choosing the sump pump first and then selecting a basin.

In general, a bigger basin is a better basin. However, a big-ass basin can be a problem if you do not adjust the float switch properly. With proper float switch adjustment and a spacious basin, getting a sump pump should become a relatively easy task.

Size the sump pump basin

Now the maximum flow rate you have chosen for your pump will tell you how much water the basin needs to store, and the size of your sump pump will help you determine the dimensions of the basin that you need. If your sump pump comes with a horizontal discharge, it needs a basin that has a bigger diameter.


Keep in mind that the sump pump cannot fully empty your basin because the pump has to be submerged in water to prevent corrosion. Therefore, getting a bigger basin might be beneficial. Once you have picked the right sump pump and basin, the installation process will be a breeze. So, we hope we’ve somehow eased you into the technicalities of sizing a sump pump. If it seems too technical for your taste, you can always look for a professional opinion. Still, doing some research before getting a pump or setting up a basin is the smart man’s route, so keep going!

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