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What Size Sump Pump Do I Need? A Complete Guide

sump pump in garden park


If you live next to a large body of water or in an area where it rains a lot, you’ll certainly want a sump pump in your basement. This pump helps move excess water out of your home and to a safe location. However, if you don’t have the right sized sump pump, your home might not be fully protected.

That’s why it’s important to find the correct size sump pump for your home. Every home is different. So, you have to consider your home’s size unique design to find the best size for you.divider 4

Short Answer: It’s Complicated

Unfortunately, finding the right sump pump size for your home is a bit complicated. You need to look at several factors, multiple of which involving math. Since every home is different, there is no rule of thumb that everyone can follow.

Credit: Amelia Martin, Shutterstock

How to Find Out What Sump Pump Size You Need

Here's a quick rundown of how to estimate what sump pump size you need based on your home's unique structure:

1. Estimate Your Capacity Needs

The first step in knowing what size sump pump you need is to calculate the capacity. The capacity tells you how many gallons of water are moved from the sump pump during one hour.

If you get a sump pump with a lower capacity than what is needed, the sump pump will pump water out slower than it is coming in. In simple terms? A low capacity means that the pump won’t pump out as much water as needed.

This step requires some math, but it is pretty easy to follow. You will need a tape measurer and a calculator. The one on your phone is fine. Don’t worry. This formula is straightforward to follow.

To get started, you need to perform this on a wet and rainy day. Allow the sump pump to run until the float switch is deactivated and the water recedes. Unplug the sump pump for one minute. This will allow the water to rise back up.

Measure how much water accumulated in the sump pit while it was unplugged. Most sump pits will be 18 inches across. Measure yours to confirm the diameter. Every inch of water accumulation will represent about 1 gallon of water in total.

Multiply the number of inches of accumulated water by 60. This number is the estimated volume of how much water would collect in one hour. Finally, multiply that number by 1.5. This gives you some extra wiggle room for extra rainy days. The final number you land on is how many gallons per hour you need.

  • Formula for an 18-inch diameter sump pit: 1 x Water Accumulation x 60 x 1.5 = Estimated GPH

If you find that your sump pit accumulates 30 gallons of water or more, you might want to opt for a 24-inch diameter sump pit instead. For every inch that accumulates in this sized pit, that is worth 2 gallons of water. That changes the formula to the following:

  • Formula for a 24-inch diameter sump pit: 2 x Water Accumulation x 60 x 1.5 = Estimated GPH


Reading math can be a bit difficult. Here is an example to make the formula a bit easier to understand. Let’s assume that an 18-inch diameter sump pit acquired 20 inches of rainwater while it was turned off. You would multiply 20 by 60. That equals 1,200. For extra wiggle room, multiply 1,200 by 1.5. That equals 1800.

In this example, the home would need a sump pump with 1800 gallons per minute capacity.

2. Pick the Right Horsepower

In addition to the capacity needed, you need to look at the motor power. The horsepower determines how powerful the sump pump’s motor is. Sump pumps for a regular household will range between a ½ horsepower and a ¾ horsepower motor.

For most homes, the ⅓ horsepower motor is enough. However, if you live that floods often, you might want to opt for a ½ horsepower or ¾ horsepower pump instead. To be safe, it’s always smart to get a motor that overperforms than underperforms.

3. Look at the Vertical Lift or Static Head

The vertical lift, sometimes called the static head, is the height the water must travel through the discharge pipe. The further the distance between these points, the harder the pump has to work. You can measure your home’s vertical distance by measuring from where the water enters the pump to where the discharge pipe changes from vertical to horizontal.

There are recommended gallon per hour capacities at specific static head measurements. Find the recommended capacity level based on the measurement for your basement.

4. Consider the Friction Head

Whenever water goes through the discharge pipe, friction is created. The friction comes from when the water rubs along the side of the pipe. Your pump must be able to overcome this friction since it slows down the water flow.

The narrower the pipe, the more friction that will be created. Additional pipe fittings create friction as well. If you think that friction head is an issue for your system, you might want to get a friction head estimate. This will involve a little math too, but it is super simple.

All you need to do is add the friction head amount and the static head amount together. This tells you the total dynamic head. You can find a sump pump with a capacity specific to your total dynamic head finding.

  • Formula: Friction Head + Static Head = Total Dynamic Head

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Maintain Your Sump Pump

No matter how much effort you put into selecting the right sump pump, the effort could be futile if you don’t maintain it properly. Maintaining your sump pump ensures that it lasts throughout the years and continues to work as it should.

Because sump pumps are exposed to so much water, they require more maintenance than other parts in your home. Every three or four months, check the power cord and make sure it is still plugged in. Check the float switch as well to ensure that it is functioning as it should.

Once every year, it is a good idea to take the sump pump out and inspect it thoroughly. This allows you to find any broken parts or pieces. Rinse out any parts as needed because debris may have built up over the year.

Consider adding a topper to your pit too. This will catch any debris, children, or pets from slipping into the pit in the ground.

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The right sump pump size depends entirely on your basement and home. You will need to estimate the capacity needs, find the right horsepower, look at the vertical lift, and consider the friction head. Of these four considerations, determining the capacity and horsepower is the most important. For a regular-sized home, you won’t need to look at the vertical lift or friction head.

Also, remember to maintain your sump pump. After a little bit of use, you might find it slacking. Chances are, it’s not that you picked the wrong sump pump, especially if you took the time to research it before. It’s that the sump pump needs a little maintenance.

Featured image credit: SomTaste, Shutterstock


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