How to Size a Swimming Pool Overflow or Spillover (Flow Rates, Lip Design and Pumps)

Last updated: September 2026

I had a conversation with a very nice lady from the USA who had just installed a brand new hot tub at her house. The overflow wasn’t doing what she expected, and she wanted to know how to fix it. It’s a really common problem, so I made a video and this guide on how to size a swimming pool overflow or spillover properly. I’ve also built a simple calculator to help you get the numbers right.

When it’s done right, an overflow looks stunning: a clean sheet of water leaving the edge and dropping in a smooth arc. When it’s done wrong, you get dry gaps, dribbles and water running down the wall. The difference comes down to three things working together: the width of the lip, the flow from the pump and the height of the drop.

Overflow, spillover or infinity edge?

The words get used loosely, so here’s what I mean in this post:

• Spillover: water from a raised hot tub, or a raised wall feature, spills over a lip into the pool below. The pool is the catch basin. This is the most common DIY version, and what my calculator is for.
• Infinity (vanishing) edge: the pool or tub overflows along one edge into a catch trough, which drains to a balance tank. The flow rules are the same, but the plumbing is different. See How to build a DIY infinity or overflow hot tub and Infinity and zero edge pool design considerations.

Here’s one of my builds with the spillover running as it should:

What goes wrong with a badly sized overflow?

Gaps and dry patches

Instead of one even sheet, the water breaks up and parts of the lip run dry. It doesn’t look good. There are two usual causes: not enough flow to cover the full width, or a lip that isn’t perfectly level, so the low spots take all the water.

The water takes a different route

With too little flow, the water doesn’t leave the edge cleanly. It wraps back under the lip, clings to the face of the wall and runs down it, often dripping off the bottom. Pool & Spa Marketing describes exactly this: with insufficient flow, water will “wrap under the edge, rather than flowing smoothly over”. That’s fine if you want a wet wall, but not if you wanted a waterfall. It also stains the wall over time.

Drips

With very little flow, you don’t get a sheet at all, just a row of drips. That usually means the pump is far too small for the width of the lip, or a valve is throttling it.

Too much

Go the other way and the water shoots out too far, splashes past the catch area, gets noisy and throws water out of the pool. You’re also paying to pump water you don’t need.

How much flow does an overflow need?

The width of the lip sets the flow. Pool designer Skip Phillips keeps his vanishing edges at around five gallons per minute per linear foot of edge, and says slot overflows can be as low as two. He also makes the key point: “the sharper the edge, the lower the flow; the larger the radius at the crest of the weir, the more flow will be required.” For a visible sheet, Pool & Spa Marketing gives these figures:

Look you want	Flow per foot of lip	Flow per metre of lip
Thin, glassy sheet over a sharp, level edge	about 5 GPM	about 60 litres a minute
6mm (¼in) sheet	7–10 GPM	about 87–125 litres a minute
25mm (1in) sheet, a proper waterfall	around 36 GPM	about 450 litres a minute

These are the same figures I use in my infinity hot tub and infinity pool guides, so whichever type of overflow you’re building, the numbers line up. Here’s what that means in practice:

Lip width	Thin sheet (~5 GPM/ft)	6mm sheet (7–10 GPM/ft)	25mm sheet (~36 GPM/ft)
2ft (600mm)	~10 GPM (38 L/min)	14–20 GPM	~72 GPM
3ft (900mm)	~15 GPM (57 L/min)	21–30 GPM	~108 GPM
6ft (1.8m)	~30 GPM (114 L/min)	42–60 GPM	~216 GPM

Notice how fast the numbers climb for a thick waterfall. A 6ft spillover with a chunky 1in sheet needs more water than most pool pumps can deliver on top of their normal job. That’s why I usually suggest a thin, even sheet, or a narrower lip, rather than a wide torrent.

The height of the drop

The drop decides where the water lands. The same Pool & Spa Marketing article gives a useful guide: from a 1m (39in) drop, a thin 6mm sheet lands about 0.3m (12in) out from the wall, while a 25mm sheet lands about 0.7m (28in) out. So:

• Make sure the catch area is wide enough for where the water will actually land, with room to spare for wind. On a spillover into a pool, that’s rarely a problem. On a trough, it’s critical.
• A higher drop is louder. Lovely during the day, less so at 11pm or for the neighbours. Plan to be able to turn it down or off.
• A thin sheet breaks up over a long drop. If you want a glassy look all the way down, keep the drop modest or add a bit more flow.

Getting the lip right

The best pump in the world can’t fix a bad lip.

• Dead level. Check it with a laser or a water level, not a spirit level on a short straight-edge. Skip Phillips gives an example of an edge that went dry after the pool settled by just one-sixteenth of an inch. Build on a solid structure so it stays level.
• Sharp, not rounded. A crisp, square edge needs less flow for a clean sheet. A bullnose coping needs noticeably more.
• A drip edge underneath. Let the lip overhang the wall below, with a small groove or step on the underside, so the water can’t cling and run back down the face.
• Smooth and clean. Rough stone, scale or algae on the lip breaks up the sheet. Keep it clean.
• Or use a water blade. A stainless steel spillway or water blade gives you a precise, level lip out of the box. See my wide arc stainless steel water blade installation tips.

Choosing the pump

Once you know the flow, you need a pump that can deliver it against your system’s resistance. That resistance (called head) is the height the water has to be lifted, plus the friction in the pipes, fittings, filter and heater. Look at the pump’s performance curve and check the flow at your head, not the maximum flow printed on the box, which is with almost no resistance at all.

• Existing filter pump or a separate feature pump? If the spillover runs off the filtration pump, it has to cover the filter’s needs and the spillover. For a big waterfall, a dedicated feature pump is often simpler.
• Variable speed makes tuning easy. Dial the speed up or down until the sheet looks right, then save it as a programme. There’s more in How to choose a new pool pump, and on running costs in Swimming pool pumps: the shocking truth.
• Fit a valve to trim it. A ball valve on the feed, or a bypass back to the pool, lets you fine-tune a single-speed pump.
• Don’t forget the rest of the system. If your hot tub spills into a pool, you’ll want valves to switch to “spa mode” so the tub can heat without mixing in cold pool water. I’ve covered that in Adding a spillover hot tub to an existing pool.

Use my overflow calculator

Getting the width, the sheet depth, the drop and the pump all in sync sounds simple, but it gets complicated quickly. So I built a calculator to help:

Enter the width of the lip, the depth of water flowing over it, the height of the drop and the style of flow you want (from a gentle trickle to a glass-like sheet or a high-flow waterfall). It gives you the flow rate you need and suggests a pump size. It’ll also tell you if your numbers are too ambitious. Treat the pump suggestion as a starting point, and always check the pump’s curve at your system’s head before you buy.

Common mistakes

• Sizing the pump on its maximum flow rather than the flow at your actual head.
• A lip that isn’t level, or one that settles after the build.
• A rounded lip with no drip edge, so the water wraps under and runs down the wall.
• A lip that’s too wide for the pump. A narrower spillover with a good sheet beats a wide one that dribbles.
• A catch area that’s too narrow for where the water lands.
• No way to turn it down.

Troubleshooting

What you see	Likely cause	What to try
Dry gaps along the lip	Lip not level, or not enough flow	Check the level along the whole lip and correct any low spots, then increase the flow.
Water runs down the wall	Too little flow, rounded lip, no drip edge	Increase flow, or add a drip edge or a water blade
Drips instead of a sheet	Pump far too small, valve closed, dirty filter	Open valves, clean the filter, then reassess the pump
Flow drops off over the weeks	Filter clogging, or scale on the lip	Clean the filter and the lip
Splashing and noise	Too much flow for the drop	Turn the pump down or trim with a valve

Planning an overflow or spillover?

If you want an overflow hot tub, my infinity hot tub CAD plans give you the dual-pump infinity edge design, plumbing diagrams, parts list, build guide and video course, with the flows already worked out.

Adding a spillover spa to a pool you already have? My hot tub and pool combo plumbing consultancy covers the valves, pumps and flows for both. For a fully custom design, draw it in the free Tub & Pool Designer Pro software and I can turn it into architectural CAD and a 3D plumbing design. For real builds, see Danny’s pool and spillover hot tub and Hot tub pool combo: the complete DIY design guide.

FAQs

How much flow does a spillover need?

About 5 GPM per foot of lip (roughly 60 litres a minute per metre) for a thin sheet over a sharp, level edge. A 6mm sheet needs 7–10 GPM per foot, and a 25mm waterfall around 36 GPM per foot.

Why is my spillover running down the wall?

Usually there isn’t enough flow, and the water wraps under the lip. A rounded lip or no drip edge makes it worse. Increase the flow or change the lip detail.

Why does my overflow have gaps?

Either the lip isn’t level, so the low spots take all the water, or there isn’t enough flow for the width. Check the level first.

What size pump do I need for a waterfall?

Multiply the width of the lip in feet by the flow per foot for the look you want, then choose a pump that delivers that flow at your system’s head, using its performance curve.

Can I use my pool filter pump for a spillover?

Often, for a narrow spillover with a thin sheet. For a wide or thick waterfall, a separate feature pump is usually better.

Got an overflow that isn’t behaving? Tell me the width, drop and pump in the comments below. I read them all.

Cheers,
Andi

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Thanks - Andi

 

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