On a hot summer day, an average uncovered pool can easily lose 100–250 litres of water, and a large one can lose 400 litres or more. Nobody sees it go. There’s no drip and no puddle. You just notice the level is down and reach for the hose again.
Here’s how to work out what your own pool is losing, how to check it isn’t a leak, and what a cover changes.
The simple maths
Every millimetre your water level drops, across every square metre of surface, is one litre gone.
So a pool’s daily loss is:
Surface area (m²) × level drop (mm) = litres lost
How far the level drops depends on the weather: heat, sun, low humidity and especially wind. Published figures from places with weather like ours give a useful range:
- In Perth, which has a Mediterranean climate similar to Cape Town’s, an uncovered pool loses an average of 8.3mm a day from October to March, according to the local council’s figures.
- A common US rule of thumb is up to 25mm a week (about 3.5mm a day) in warm weather.
We use 4–8mm a day below as a working range for a South African summer. A hot, windy day can be higher. A mild, still day will be lower.
How much is your pool losing?
| Pool size (surface) | Area | Per day (4–8mm) | Per month (30 days) |
|---|---|---|---|
| 6m × 3m | 18 m² | 72–144 litres | 2,200–4,300 litres |
| 8m × 4m | 32 m² | 128–256 litres | 3,800–7,700 litres |
| 10m × 5m | 50 m² | 200–400 litres | 6,000–12,000 litres |
| 12m × 6m | 72 m² | 288–576 litres | 8,600–17,300 litres |
For a kidney or freeform pool, use the average length × average width. It’s close enough for an estimate.
To picture it: a typical 8m × 4m pool can lose roughly a full bath of water a day. Over a summer month at the higher end, that’s about one and a half 5,000-litre JoJo tanks, gone into the air.
Wind matters more than most people think
Evaporation speeds up sharply with wind, because moving air keeps carrying moist air away from the surface and replacing it with dry air. A pool on a windy day can lose noticeably more than the same pool on a still day at the same temperature.
That’s worth knowing in Cape Town, the Western Cape and Gqeberha, where summer wind is normal. If your pool seems to drop faster than a friend’s inland, the wind is probably why. It doesn’t mean something is wrong with your pool.
Is it evaporation or a leak? Do the bucket test
If your level is dropping faster than the table above suggests, rule out a leak before you blame the weather. The bucket test takes a day or two and costs nothing:
- Fill a bucket with pool water to about 10cm from the top.
- Stand it on the first or second pool step so the bucket sits in the pool water. Weigh it down with a brick if needed.
- Mark the water level inside the bucket, and mark the pool’s water level on the outside of the bucket (or on the pool wall).
- Run your pump as normal and leave it for 24 hours. Don’t top up, and don’t swim.
- Compare the two drops.
- Both dropped about the same: that’s evaporation. The bucket and the pool face the same sun and wind.
- The pool dropped noticeably more than the bucket: you probably have a leak. Call a pool technician before you buy anything.
Repeat with the pump off for another 24 hours. If the pool only loses extra water with the pump running, the leak is likely in the plumbing or equipment, not the pool shell.
What a cover changes
In independent testing at Cal Poly, a bubble (solar) cover cut evaporation by about 95% while it was on. On the 8m × 4m example, if the cover stayed on all day, a 256-litre day would drop to roughly 13 litres.
In practice you’ll take the cover off to swim, so the US Department of Energy’s more realistic figure is that covering your pool when it isn’t in use cuts top-up water by 30–50%. The more hours the cover stays on, the closer you get to the top of that range. Read more on how covers stop evaporation →
Why this matters more this summer
In Cape Town, the City’s Water By-law already requires pools to be covered when not in use, and the City is considering water restrictions for this summer. Every litre you don’t lose is one you don’t need to replace. What water restrictions could mean for your pool →
Compare our solar covers and get a price for your pool size →
FAQ
How much water does a swimming pool lose per day?
In a South African summer, an uncovered pool typically loses about 4–8mm of depth a day, and more on hot, windy days. Each millimetre over each square metre is one litre, so an 8m × 4m pool loses roughly 130–260 litres a day.
How much water does a pool lose per month?
At 4–8mm a day, an 8m × 4m pool loses roughly 3,800–7,700 litres a month in summer. A 10m × 5m pool loses about 6,000–12,000 litres.
How do I know if my pool is leaking or just evaporating?
Do the bucket test. Stand a bucket of pool water on the pool step, mark both water levels, and compare after 24 hours. If the pool drops noticeably more than the bucket, you probably have a leak.
Why does my pool lose more water on windy days?
Wind keeps replacing the moist air just above the water with drier air, which speeds up evaporation. A windy day can cost noticeably more water than a still day at the same temperature.
How much evaporation does a pool cover prevent?
In independent testing, bubble (solar) covers cut evaporation by about 95% while on the pool. Day to day, covering the pool whenever it isn’t in use cuts top-up water by about 30–50%.
Sources: Town of East Fremantle — Pool Cover Policy Fact Sheet (Perth evaporation data) · Cal Poly / NPC Evaporation Study — Pool & Spa News summary · US Department of Energy — Swimming Pool Covers