How Much Water Does a Pool Lose to Evaporation?

A typical outdoor pool loses roughly 1/4 inch of water per day to evaporation in moderate summer conditions. On a 16 x 32 ft pool (512 sq ft), that is about 80 gallons per day, 2,400 gallons per month, and nearly 29,000 gallons per year.

But that “average” hides an enormous range. A pool in Phoenix on a hot dry August day with wind can lose over 1/2 inch, or 160+ gallons per day. The same pool in Tampa with its high humidity might lose 1/8 inch on the same type of day – four times less – despite similar air temperatures. Humidity is the dominant factor, and it is the one most pool owners never think about.

The Five Factors That Drive Pool Evaporation

1. Humidity – the biggest factor most people overlook

Evaporation is the process of water molecules escaping from the liquid surface into the air. When the air is already saturated with water vapor (high humidity), fewer water molecules can escape. When the air is dry (low humidity), the pool surface evaporates rapidly to try to saturate the surrounding air.

This is why a pool in Phoenix (average summer humidity: 10 to 20%) loses three to four times more water than a pool in Miami (average summer humidity: 75 to 80%), even though Miami is nearly as hot. A pool owner in Phoenix who does not understand this assumes something is wrong with their pool when they fill up twice as often as their friend in Florida.

2. Wind speed – dramatically underrated

When still air sits above the pool surface, it gradually becomes saturated with water vapor and evaporation slows down. Wind constantly replaces this saturated air with dry air, maintaining maximum evaporation rate. A pool in an exposed location with regular wind can lose 70 to 90% more water than a sheltered pool in the same climate.

Practical takeaway: a solid fence or dense hedge on the windward side of your pool is one of the most cost-effective ways to reduce evaporation. In exposed coastal or hillside locations, this can make a bigger difference than a solar cover.

3. Water temperature and air temperature

Warmer water evaporates faster. A pool heater does increase your evaporation rate. The difference between an unheated 72F pool and a heated 82F pool is roughly 15 to 20% more daily water loss in the heated pool. This is worth knowing but rarely changes the decision to heat – the enjoyment gain is worth the extra water cost, which is modest compared to the heating energy cost.

4. Surface area

Evaporation is a surface phenomenon. A pool that is twice the size loses twice the water per day, regardless of depth. This is why the calculator uses surface area, not gallons. If you add a spa or wading area that increases the water surface by 20%, your evaporation increases by 20%.

5. Sun exposure

Direct sunlight warms the water surface and increases evaporation, but it is a smaller factor than humidity or wind. A fully shaded pool loses roughly 25% less water than a pool in full sun, all else being equal. More important: the shaded pool also costs less to heat.

Normal Pool Evaporation Rates by Climate

How Much Does a Solar Cover Save?

A solar cover (also called a bubble cover or solar blanket) reduces evaporation by blocking the air-water interface. When covering the pool whenever it is not in use, a good solar cover reduces evaporation by 70 to 95%. It also retains heat, reducing heating costs by 50 to 70%.

On a Phoenix pool losing 160 gallons per day, a solar cover used whenever not swimming saves roughly 110 to 130 gallons per day. At $8 per 1,000 gallons (including sewer), that is $320 to $380 per year in water savings. Most quality solar covers cost $80 to $250 and pay for themselves in water savings within a single season in arid climates.

Pool Evaporation vs Leak: How to Tell the Difference

This is the question every pool owner eventually asks: “Is my pool losing water from evaporation or do I have a leak?” You cannot tell just by watching the water level drop. You need the bucket test.

The bucket test

1. Fill a 5-gallon bucket to about 1 inch from the top.
2. Place it on the second step of the pool so it is partially submerged – the water level inside the bucket should match the pool water level, or be close.
3. Mark both water levels with tape or a waterproof marker.
4. Turn the pump off and leave for 24 hours. Do not add water.
5. Measure the drop in both. If the pool dropped by the same amount as the bucket, all loss is evaporation. If the pool dropped more than the bucket, the difference is leaking.

Evaporation and Pool Chemistry

Water evaporates. The chemicals in it do not. This means that as a pool loses water to evaporation, the concentration of calcium, cyanuric acid, total dissolved solids, and other non-volatile substances increases. This is why pool water needs periodic dilution even when it tests fine on pH and chlorine.

In hot arid climates where pools lose 100+ gallons per day to evaporation, tap water replenishment actually helps dilute these concentrating minerals. In milder climates with lower evaporation, mineral concentration builds faster relative to the volume of fresh water being added back.

Practical effect: if your calcium hardness or CYA keeps creeping up despite not adding product, evaporation concentration is the cause. The only fix is dilution – partially drain and refill with fresh water.

How to Reduce Pool Evaporation

Solar cover – most effective

A solar cover with a reel is the single most effective way to reduce evaporation. The reel makes it practical to use consistently – without a reel, most pool owners stop using the cover within a few weeks because rolling and storing the blanket by hand is too much work. Buy the reel at the same time as the cover.

Windbreaks

A solid fence, dense hedge, or privacy screen on the windward side of the pool can reduce evaporation by 30 to 50% on windy days. In consistently windy locations, this can be more impactful than a solar cover. Lattice or open fencing provides much less benefit – the windbreak needs to be solid to disrupt the air flow effectively.

Pool heater temperature

Every degree you raise the water temperature adds to evaporation. If you heat to 84F, consider whether you actually need it that warm. Dropping target temperature from 84F to 80F reduces evaporation by roughly 8 to 12% and also directly reduces heating costs. For most swimmers, 80F is perfectly comfortable.

Liquid solar cover

A liquid solar cover is an alcohol-based product you add to the water weekly. It forms a thin monomolecular layer on the surface that reduces evaporation by 30 to 40%. Not as effective as a physical cover, but it works automatically with no removal or storage. Good for pools with high turnover where a physical cover is impractical.

Frequently Asked Questions

How much water does a pool lose per day from evaporation?

In typical summer conditions, an outdoor pool loses roughly 1/4 inch per day from evaporation, or 60 to 80 gallons per day on a standard 500 sq ft pool. In dry climates like Phoenix or Las Vegas, this can double or triple. In humid Florida, it may be as low as 1/8 inch per day.

Is losing 1 inch of water per week in a pool normal?

In moderate climates, 1 inch per week is at the high end of normal but possible in hot, windy conditions. In humid climates, 1 inch per week is above normal and worth investigating with a bucket test. In dry climates, 1 to 1.5 inches per week can be entirely evaporation. Use the bucket test to separate evaporation from leakage.

Why does my pool lose more water in summer than winter?

Three factors drive higher summer evaporation: higher air temperature, warmer water temperature, and lower relative humidity. In most climates, summer evaporation is 3 to 5 times higher than winter evaporation on the same pool. Warmer water has a higher vapor pressure, meaning more water molecules escape per unit time.

Does a solar cover really reduce evaporation?

Yes, substantially. A solar cover used whenever the pool is not in use reduces evaporation by 70 to 85%. In arid climates, this saves 100+ gallons per day and pays for the cover in water savings within a single season. It also retains heat, reducing heating costs by 50 to 70%.

Why does my pool lose more water on windy days?

Wind constantly replaces the air sitting above the pool surface with drier air. Still air gradually saturates with water vapor and evaporation slows. Wind prevents this saturation from occurring, maintaining a maximum evaporation rate. A sheltered pool can lose 50 to 70% less water than an identical exposed pool in the same climate.

How much does pool evaporation cost per year?

At the US average of $6 to $8 per 1,000 gallons (including sewer charges), a typical outdoor pool costs $100 to $400 per year in water to replace evaporation losses, depending on climate. Desert climates can push this to $400 to $600 per year on a large uncovered pool.

Can evaporation make my pool water chemistry go off?

Yes. Evaporation removes water but not dissolved chemicals. As water evaporates and is replaced with fresh water, calcium hardness and cyanuric acid can slowly concentrate over a season. If your calcium or CYA keeps rising despite not adding products, this is why. The fix is periodic partial draining and refilling with fresh water.

How do I tell if my pool is leaking or just evaporating?

Do the bucket test. Fill a bucket to the pool water level, set it on the pool step, and measure both drops after 24 hours with the pump off. If both dropped equally, all loss is evaporation. If the pool dropped more than the bucket, the difference is leaking. Repeat with the pump running to identify whether the leak is in the plumbing or the shell.