The number printed on a fountain pump box is measured with the outlet at the same height as the water. Your fountain is not built that way. Every inch the water has to climb takes flow away from that number, and by the time it reaches the top tier there may be very little of it left.
That single fact explains most of what goes wrong when people buy a replacement pump. They match the gallons per hour on the old box, install it, and the top tier dribbles.
This is how to size one properly, using the two numbers every pump publishes and one measurement you take yourself.
The two numbers on every pump are the two ends of one line
Take a real example. A Little Giant submersible utility pump lists a maximum capacity of 1,500 gallons per hour and a maximum head of 26 feet. Those look like two separate specifications. They are not.
- Maximum flow
- What the pump moves when it has to lift the water zero feet. In this example 1,500 gallons per hour
- Maximum head
- The height at which the flow falls to nothing. In this example 26 feet. At 26 feet this pump moves no water at all
- Everything in between
- A curve running from one to the other. Your fountain operates somewhere on that curve, not at either end
The University of Florida IFAS Extension puts the principle in one sentence in its guide to sizing pumps: "a pump will always provide a combination of flow and head somewhere on the curve." You do not get to pick the flow. You pick a pump, the height decides the flow.
What head is, and the part almost everyone forgets
Head is not a measurement of your fountain. It is a measurement of the work the pump has to do. A rainwater harvesting guide published through the University of Hawaii CTAHR defines it plainly: "head is a description of the energy required to move the water and is typically expressed in feet of water or pounds per square inch (psi)."
It has two parts, and the second is the one people leave out. UF IFAS gives total head loss as "the sum of the vertical distance water must be pumped (Hz) and the friction head loss (HLf)."
- The lift
- Vertical distance from the water surface in the reservoir to the point where the water leaves the tubing at the top. Measure from the water, not from the ground and not from the base of the fountain
- The friction
- IFAS defines it as "the loss of pressure caused when water flowing through pipes and fittings is slowed down by frictional resistance." CTAHR names the sources: "energy loss due to friction with the pipe walls and losses through elbows, valves, and other fittings"
- Why the second one bites
- A three foot fountain sounds like three feet of head. Add a narrow tube, two elbows and a flow valve and the pump is working against noticeably more than three
One practical consequence worth saying out loud. As the reservoir level drops through a hot week, the lift gets slightly larger and the flow gets slightly smaller. If your top tier weakens before the basin looks empty, that is what you are watching. The refill question is covered in the article on fountains losing water.
How to actually pick one
- Measure the lift. Water surface in the reservoir to the outlet at the top. Use the normal running level, not the level when you have just filled it.
- Count what the water has to pass through. Every elbow, every reducer, every flow valve. You are not going to calculate these precisely at garden scale, but you need to know they exist and to round your head figure upward because of them.
- Find the pump curve, not the headline number. This is the step almost nobody completes, because most retail listings do not publish it. Manufacturer spec sheets and manuals usually do.
- Check where your point falls. CTAHR states the rule: the design point must lie "either on or below this curve. If the target head and flow lie above and to the right of this curve, the pump will not be able to supply enough water."
- Leave margin, but not too much. A pump running near its maximum head delivers almost nothing. A pump massively oversized for a small tiered fountain throws water off the tiers, which is a real cost rather than a cosmetic one.
A worked example, with the arithmetic shown
Take a three tier fountain that stands 36 inches tall. When it is running, the water in the lower reservoir sits about four inches up from the base. The tubing delivers at the top tier, roughly 34 inches up.
- Lift. 34 inches minus 4 inches is 30 inches, which is 2.5 feet. Not 3 feet, which is what you get if you measure the fountain instead of the water.
- Friction. One run of tubing, two elbows, one flow valve. We are not going to pretend to compute this at garden scale. We round the head figure up and call it 3.5 to 4 feet.
- Read the curve at 4 feet. Not at zero. Four feet is where this fountain actually operates.
Now put that next to the example pump above, which is rated 1,500 gallons per hour at zero head and reaches zero flow at 26 feet. Four feet is a small fraction of that pump's range, so it would deliver a large fraction of its headline flow.
Which is the actual lesson, and it is the opposite of what most people worry about. A garden fountain is a low head application. Two to four feet, almost always. The risk is not that the pump cannot lift the water. The risk is that a pump sized by its headline gallons per hour moves far more water than a small tiered fountain can carry gracefully, and the surplus leaves the basin as splash.
Undersizing shows up as a sad top tier and is easy to diagnose. Oversizing shows up as a fountain that keeps needing refilling, and gets blamed on a leak for a whole season. That failure mode is the second of the four routes in the article on fountains losing water.
How much flow the look actually needs
Flow is not only an engineering figure, it is what decides how the fountain looks. This section is our judgment from the shapes we sell rather than a sourced standard.
- Bubbler or dome over a stone
- The lowest flow of any type. The effect depends on the water forming a smooth dome. Too much flow turns the dome into a jet and the look is gone
- Tiered and bowl fountains
- Moderate. You want enough to wet each lip evenly all the way round. Beyond that point the extra flow does not improve the look, it just travels further from the lip
- Sheet, spillway and wall fountains
- The highest, because the same flow is spread across a width. A narrow spill needs less than a wide one for the same visual effect
- Practical consequence
- Two fountains of the same height can want quite different pumps. Height sets the head. Shape sets the flow
If your pump has a flow control on the outlet, that is the adjustment for this, and it is worth spending ten minutes on it the day you install the fountain rather than living with the default.
The tubing is part of the pump
A pump and the tube it feeds are one system. Fitting a strong pump to a narrow tube does not get you a strong fountain, it gets you a pump working against friction it does not need to fight.
CTAHR gives the fix and the reason in the same sentence: "If the calculated friction loss is very high, it may be necessary to consider using pipes of larger diameter to prevent damage to the pump or plumbing system."
Note the second half of that. Excess friction is not only a performance problem. At garden scale the practical version is simple: if the pump outlet is one size and the tubing you have is smaller, do not force the reducer on and hope. Match the tube to the outlet, keep the run short, and take out any bend you do not need.
What the industry does not publish, including us
Everything above assumes you can find the numbers. For garden fountains you usually cannot, and we are part of the problem, so here is our own catalog measured rather than described.
- Fountains we sell that include a pump
- 89 out of 130 products
- How many publish the flow rate
- One
- How many publish the wattage
- Four
- How many publish a maximum head
- None
So a customer standing on our own product page cannot check whether the included pump suits a three foot lift, because we have not told them. Neither have most sellers in this category. The specification that decides whether the fountain works is the one nobody prints.
We are naming it because we would rather you knew the limit of the advice above than have you go looking for a number that is not there. Adding flow and head to our own listings is now on our list.
What to do when there is no curve to read
This section is our judgment rather than a sourced procedure, and we are marking it as such because the sources above stop where the published data stops.
- Replacing a pump in a fountain that used to work. Match both numbers of the old pump rather than just the gallons per hour, and keep the same tubing diameter. If only one number is on the old pump, match that and buy the version with the higher maximum head rather than the higher flow.
- A fountain that never worked properly from new. Measure the lift first. If it is more than about half the pump's stated maximum head, the pump is the likely cause and not the plumbing.
- Buying a fountain rather than a pump. Ask the seller for the flow and the maximum head of the included pump before you buy. If they cannot answer, that tells you something on its own.
Frequently asked questions
Is a bigger pump always safer
No. On a tiered fountain an oversized pump pushes water past the lip of each tier instead of over it, and the water lands outside the basin. That shows up as a fountain that empties fast, which people usually blame on a leak. The four routes water actually leaves by are in the article on losing water. Many pumps have a flow control on the outlet for exactly this reason, and turning it down costs nothing.
What about a solar pump
We do not sell any, so we have no product experience to offer and we are not going to invent some. The physics above still applies, with the additional problem that a solar pump's output varies with the light, so the head it can manage at four in the afternoon is not the head it can manage at nine in the morning.
Why does my top tier trickle when the lower one runs fine
Because the lower tier is at a lower head. The same pump is delivering more flow to the point nearer the water and less to the point furthest from it. If it used to reach the top and no longer does, check the water level and then check the intake screen before you assume the pump has failed.
Should the pump run all the time
Moving water is what keeps a basin from stratifying and warming, which is part of the algae story covered in the article on keeping fountain water clean. Running dry is what kills these pumps, so the thing to protect against is not runtime but a low reservoir.
Does any of this change in winter
The pump usually comes out entirely. That is a separate procedure and it is in the fountain winterizing guide.
References
- Reference: An Introduction to Sizing Centrifugal Pumps in Aquaculture, AE579 | University of Florida IFAS Extension
- Reference: Choosing a Pump for Rainwater Harvesting | University of Hawaii CTAHR, Hawaii Rain Follows the Forest resource library
- Reference: Little Giant 5-MSP submersible utility pump, product specification | Pump Products, retailer listing carrying the manufacturer specification