Infinity and Zero Edge Pool Design Considerations: Balance Tanks, Edges and Plumbing

Last updated: September 2026

If you follow my YouTube channel or this website, you’ll know I don’t just do hot tubs. I also work on swimming pools, plunge pools, cocktail pools and, more and more, infinity and zero edge pools. Water spilling over an edge is a marvel to look at, and it leaves anyone in awe.

The good news is that most of an overflow pool is just a normal pool. The bit that’s different, and the bit people get wrong, is where the overflow water goes: the edge, the trough and the balance tank. Here’s what you need to think about at the design stage, whether you’re after an infinity edge facing a view or a zero edge pool that sits flush with the patio.

Infinity edge vs zero edge vs a normal pool

In most pools, the water sits below the edge of the coping, with a visible gap between the water level and the top of the pool. A skimmer handles the surface debris and copes with the water displaced when people get in.

An overflow pool does away with the skimmer. The water is deliberately kept brimming, so it spills over an edge, gets collected and goes to a balance tank. The pump then sends it back through the filter and heater into the pool.

	Skimmer pool	Infinity (vanishing) edge	Zero edge (deck level)
Water level	Below the coping	At the weir on the overflow side(s)	Flush with the deck all round
Where the water goes	Skimmer and main drains	Catch trough below the edge, then the balance tank	Grated perimeter channel, then the balance tank
Balance tank	Not needed	Needed (or a large trough)	Needed
Best for	Simplicity and cost	Sloping sites with a view	Flat sites and a sleek, modern “mirror” look

How an overflow pool works

It’s straightforward. To create the overflow effect, you deliver more water into the pool than it can hold. The excess flows over the edge into a catch trough or channel, and from there into a balance tank. The pump draws from the balance tank, not from a skimmer, and sends the water back through the filter, the heater and the sanitiser (chlorine, UV or whatever you choose), then through the returns into the pool. The pool is always slightly overfilled, and that’s what creates the effect.

And the flip side: if you draw water from the pool and put it straight back into the pool, it won’t overflow. That’s the principle behind turning the edge off (more below).

The main drains

The rest of the plumbing is much like a traditional pool. You’ll want a pair of main drains in the floor, teed together, so that if one gets blocked, the other still flows. That also balances the suction, which is what anti-entrapment rules are about. In the USA, the Virginia Graeme Baker Act requires drain covers to meet ANSI/APSP/ICC-16. See Installing a main drain.

Sizing the balance tank

The balance tank is the most important part of the whole system. If it’s too small, the pump sucks air when a crowd jumps in, or the tank floods every time the pump stops. It has to hold three things:

1. Bather displacement. An average adult displaces around 70 litres (15 US gallons) of water.
2. Water in transit. This is the thin layer lifted above the weir while it’s running, the water pouring over, and whatever is sitting in the trough and pipes. When the pump stops, it all drains back.
3. An operating reserve, so the pump always has water over its suction and never draws air.

My rule of thumb is to work out the displacement for the maximum number of people, then double it. So for a pool that holds 10 people: 10 x 70 litres = 700 litres (about 150 gallons), doubled to around 1,400 litres (about 370 gallons). That gives ample capacity and peace of mind.

Pool designers often use a different starting point. Skip Phillips’s method allows for 2 inches of displacement across the whole pool surface for bather surge (or 3 inches for busy pools), plus the water in transit and an operating reserve. On a 6m x 3m (about 20ft x 10ft) pool, 2 inches across the surface works out at about 900 litres (240 gallons) for surge alone. That’s similar to my per-person figure before doubling. So if you run both numbers and design for the bigger one, you won’t go far wrong.

The edge, the trough and the flow

How much water over the edge?

Decide early whether you want a gentle, glassy sheet or a proper waterfall. That decision sets the pump size. For a thin sheet over a well-made edge, Skip Phillips recommends about 5 gpm per linear foot of vanishing edge, and as little as 2 gpm per foot for a slot overflow. Pool & Spa Marketing suggests 7–10 gpm per foot for a 6mm (¼in) sheet, and far more (around 36 gpm per foot) for a 25mm (1in) sheet. So a 20ft (6m) edge needs something like 100–200 gpm (380–760 litres a minute) for a thin sheet. If you want a thicker waterfall, you’ll probably need a separate feature pump. A stainless water blade is another way to add a spillway. For spillovers and water features, use the calculator in How to correctly size a swimming pool overflow or spillover.

Get the edge dead level

Water finds the lowest point. If one part of the edge is a few millimetres low, it takes all the flow and the rest of the edge runs dry. A sharp, flat weir needs less flow than a rounded one, and the more precise the edge, the thinner the sheet can be.

Design the trough or channel

• Infinity edge: the catch trough below the edge has to be wide enough that the wind doesn’t blow water past it. AQUA Magazine also recommends waterproofing both sides and the top of the edge wall and tiling well down the catch basin, because those surfaces are constantly wet and dry.
• Zero edge: the perimeter channel collects the overflow all the way round. Either taper the channel itself so gravity carries the water to the outlets, or use properly sloped pipework to the balance tank.

Customise the grates

You don’t have to settle for the standard plastic grating over a zero edge channel. You can finish it to match the look you want. On one zero edge build I worked on with a client, we used a stone finish with the grates hidden beneath, for a really clean look.

Pumps, filtration and control

Can you turn the edge off?

Decide early whether you want to be able to stop the overflow. The sound of running water is calming for some people, but it can be annoying at night, and the neighbours might not like it. To stop the overflow, you stop pulling water from the balance tank. There are two ways to do it:

• One pump with a valve manifold, so you can switch the suction from the balance tank to the main drains. The water then just circulates in the pool.
• A second pump, one for normal circulation and one for the edge. This gives you more control, and it’s the more popular choice on hot tubs. AQUA Magazine also recommends separate variable-speed pumps for the pool and the edge.

The same goes for a hot tub that spills over into a pool. See Adding a spillover hot tub. If you’re choosing pumps, have a look at Top tips for selecting a new pool pump.

Filter and access the balance tank

With no skimmer, the edge becomes one big skimmer, so leaves and debris all end up in the trough and balance tank. Put a strainer or basket on the suction so nothing reaches the pump or goes back into the pool. And build in easy access to the channels and balance tank for cleaning.

Top-up and overflow

Overflow pools lose more water to evaporation, so an automatic top-up is almost essential. Put it in the balance tank, not the pool. AQUA Magazine warns against cheap float valves, because they keep filling while the pump is running and then dump water when it stops. An electronic level controller is the better option. The balance tank also needs an overflow to a drain. See my pool auto top-up guide.

Common mistakes

• A balance tank sized for a couple of people, not the party.
• An edge that isn’t level, so you get dry patches and a lopsided sheet.
• A trough that’s too narrow for the wind.
• No way to turn the edge off.
• No access to the balance tank for cleaning.
• Forgetting that more water surface and more evaporation means higher heating costs.

Planning an infinity or zero edge build?

If you’d like to start smaller, my infinity hot tub CAD plans are a dual-pump infinity edge design, with plumbing diagrams, a parts list, the build guide and the video course. They’re a great way to see how all the parts fit together. Read How to build a DIY infinity hot tub for the hot tub version of everything above.

For a pool, every site is different, so start with the free Tub & Pool Designer Pro software. I can then turn your design into architectural CAD and a 3D plumbing design, including the balance tank and edge. For hands-on help from start to finish, there’s my custom hot tub and pool design consultancy. If a plunge pool is more your size, see my guide to building DIY plunge pools.

FAQs

What’s the difference between an infinity pool and a zero edge pool?

An infinity (vanishing) edge pool overflows on one or more sides into a catch trough, usually facing a view. A zero edge pool has the water flush with the deck all round, overflowing into a grated channel. Both need a balance tank.

How big should an infinity pool balance tank be?

Allow about 70 litres (15 US gallons) per person at maximum occupancy, then double it. For 10 people, that’s around 1,400 litres (370 gallons). Also check it against the “2 inches over the pool surface” method, plus the water in transit.

How much water flows over an infinity edge?

Around 5 gpm per foot of edge (about 60 litres a minute per metre) for a thin sheet over a sharp, level edge. You’ll need more for a thicker sheet or a rounded edge.

Can you turn off an infinity edge?

Yes. Stop drawing from the balance tank, either with a valve manifold that switches the suction to the main drains, or by turning off a dedicated edge pump.

Do infinity pools cost more to run?

Generally, yes. There’s more pumping, more evaporation from the moving water, and a balance tank’s worth of extra water to heat and treat.

Got a question about your own overflow pool design? Drop it in the comments below. I read them all.

Cheers,
Andi

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Hi, Andi here. I own Buildahottub.com and also write all of the articles and info pages on the site. Some years back now, I built my own hot tub but struggled to find the information I needed. So, once my tub was complete, I started this website to help others in their own pursuit of hot tub and plunge pools DIY building information.

Fast forward to 2025, I've helped over 1400+ DIY customers just like you all over the world build hot tubs and pools. Have a good look around the site, there are lots of resources here. Please do get in touch if I can help you. - Cheers, Andi