How To Size A Sump Pump

Homeowners usually complain about how unreliable sump pumps are while they didn’t size the pump appropriately and caused it to burn out or corrode in the basin.

That’s why I think it’s very important to explain to you how to size your sump pump. Whether you’re replacing or installing a new sump pump, you will want to read this.

Sizing a sump pump is a complicated process, it’s not the same for every homeowner. If you already have a basin, you need to size your basin first, however, if you don’t have a basin, you’ll need to choose a pump first then pick a basin appropriate for that pump.

All this will be explained thoroughly in a minute, I’ll make sure to cover every possible situation.

How do sump pumps work?

I think it’s important to clarify some things about pumps before answering the main question. Those that had their fair share of sump pumps can skip this part.

A sump pump is required when water forms under the foundation of your house, sometimes water only occurs during rains but some homes get water even if it doesn’t rain because of the naturally wet ground.

A sump basin collects the water from your foundation, then the sump pump sucks that water and sends it to a safe discharge spot via discharge lines.

Now that we got this out of the way, we can continue.

What do I need to know before sizing?

You can’t start sizing before gathering some crucial information about your house. Maybe you’re replacing a pump, maybe you’re a new homeowner, we need to get this out of the way.

If your previous sump pump was inappropriately sized and that is the reason why you are changing your sump pump, you only need to size your basin, how high and far you are pumping. 

However, if you’re a new homeowner and your house doesn’t even have a basin, the process may take longer for you. In this case, you need to know why, how much and where water is entering your house.

Once you know that, you will need to pick a sump pump adequate for your situation, and then pick a basin adequate for your sump pump. By doing this, you will escape further troubles.

Enough procrastinating, let’s start with the actual problem.

How to size a pump if you already have a basin?

In this section, I will explain everything that you need to do in order to find out what kind of sump pump you need. It’s a simple task, believe me.

I hope this puts an end to the endless process of replacing pumps, your next pump will work as the manufacturer described it will do.

So, without any further ado, let’s start sizing.

How much GPH do you need?

Now, we need to start with measuring the diameter of the basin, it may be a single unit or a duplex. I’m guessing you have the average 18 inches basin, however, to be sure, measure the diameter.

You will need to measure for a minute, how much water comes in your basin, that is why we needed to know the diameter of your basin. 1 inch in 18 inches basin is 1 gallon, 1 inch in 24 inches basin is 2 gallons.

If you know the diameter of your basin, you can proceed with measuring how much water comes in your basin, this is crucial information for the size of your pump.

To do this, the water in your basin will need to be below the pump’s starting level, so, choose a particularly rainy day and run your sump pump until the water reaches the turn-off level.

After the pump stops working, measure the point where the standing water is, wait for a minute, and measure how high did the water rise in that minute, remember 1-inch equals 1 gallon.

Now, grab a calculator if you’re bad at math. Take the number of how many inches did the water rise in a minute and multiply it by 60. You will get an estimate of how many gallons per hour will come in your basin.

However, that estimate is not enough, you need to consider the safety factor, that is, multiply the estimate with 1.5 to get a more accurate number. Let me show you an example,

Now you know how many gallons per hour flow in your sump basin, however, if your home is at a low-point in the neighborhood or located on naturally wet ground, increase the safety factor.

We are done with measuring the GPH, we can move on to the vertical lift.

How high are you pumping?

The GPH alone doesn’t mean anything without considering the vertical lift, more precisely, how high is your discharge line starting from the inlet and ending where the line is horizontal.

Measuring the vertical lift is actually a very simple process. As I said, all you need to do is measure the size of the discharge line from the bottom of the pit to the point where the horizontal line starts.

You may be asking why this is important, well, if your basin requires a pump with 2700 GPH, getting a pump that has a maximum flow rate of 2700 GPH at 0 feet will not be enough.

That’s why you need to see the maximum flow rate of the pump for the size of your vertical lift. All will be clear with an example, let’s say you are pumping 10 feet high and you need 2700GPH, so, look for a pump with that much GPH at 10 feet.

However, the vertical lift is not the only thing that matters, you need to consider the friction created in the pipes that slow down the water.

Now, this is a complicated process, and not everyone can do it, more importantly, the friction head is not a problem for the average home.

Friction is created from narrow pipes, longer discharge pipes, pipe elbows, and check valves. So, you need to figure out if this will be a problem for you.

For a 10 feet discharge line and 2 check valves, you don’t need to calculate the friction head, you can get a pump with 100 GPH more than the desired estimate.

However, if you have a long discharge line with elbows almost everywhere, I recommend calling a plumber to calculate the friction head, a professional is needed in your case.

Well, now that you know how to size your sump pump, you can go do that and get the sump pump you need, good luck. In the meanwhile, let’s help those without a basin.

What to do if you don’t have a basin?

This may not make sense to you but if you don’t have a basin, the process will be way easier for you. You are not limited by the basin’s dimensions as with the case above.

Most manufacturers recommend first getting a pump then choosing a basin. Working around

your basin leaves you with few options and they are not always good.

Don’t worry if all this looks confusing to you, all will be explained in the following section.

Pick your pump

I’m guessing you are a new homeowner and that’s why you don’t already have a sump basin, or maybe water under the foundation of your house is a new problem for you.

Now, the thing to do if you don’t have a basin is to first pick a sump pump and then choose a basin appropriate for that pump, doing this will prevent further problems in your basement.

You may think that picking a pump without knowing the discharge lift and GPH need is pointless, however, this doesn’t matter if your pump and basin fit perfectly.

Of course, you need to take into account how much rain you get in a year and the location of the house in the neighborhood but nothing else matters.

I recommend buying any average sump pump or a sump pump system, you can never go wrong with these choices, you can see our best sump pump lists if you need help.

Once you’ve done that, you can start sizing your basin, most manufacturers recommend some basins for the pumps they offer but feel free to improvise.

Size your basin

Now, the maximum flow rate of the pump you’ve chosen will show you how much water the basin will need to store, and the size of the pump will show you the dimensions of the basin you need.

It’s important to consider the discharge option of the pump. If the pump has a horizontal discharge, it requires a basin with a bigger diameter.

Basically, that’s all the information you need but I want to tell you some details that you may want to know.

Keep in mind that you don’t want the pump to empty the basin completely, the pump needs to be submersed in order to avoid corrosion, so maybe consider a bigger basin.

You can never go wrong with bigger basins, of course, not too big for your sump pump but you want your basin to have extra space around.

Absolutely avoid small basins for powerful pumps, it will empty the basin in a minute, this leads to pumps short cycling.

Once you’ve picked a pump and a basin, you can start installing your system. By choosing a sump pump first you eliminated any problems in the future.

I think we went through everything you needed to know about installing a sump pump system, hope this helped.

Related questions

What happens if a pump is too small for the sump basin?

Well, a number of things happen. First of all, the pump will not be effective, it will not be able to empty the basin in time, this may lead to flooding. Also, because of overworking the pump will burn out, especially pumps that are not designed for continuous duty.

What happens if a pump is too big for the basin?

Of course, big pumps may not fit at all in some basins. However, if you manage to install a big pump in a small basin, the following happens. The pump will empty the basin very quickly, it will short cycle, it will pick up debris along the way, and the float switch may get stuck.

Is there a way to replace my existing basin?

Well, that depends on where and how you built your basin. Usually, basins are built-in the concrete floor and replacing it will take you too much work and money. However, if your basin is surrounded around materials that you can get rid of, feel free to replace it.

How many sump pumps do I need?

If you want to make sure your basement stays dry even when your sump pump has a mechanical failure, we recommend getting a secondary pump, so 2 in total. Read more about having more than one sump pump here.