You've noticed your pool water level dropping. Not dramatically, but enough to make you wonder. You top it up, it drops again. Is there a leak somewhere, or is this just normal evaporation?

It's one of the most common questions pool owners ask — and one of the most anxiety-inducing, because the two scenarios have very different consequences. Here's how to tell them apart before you start pulling up tiles or calling a technician.

In Brief

A pool losing 1–2 cm of water per week is usually normal evaporation, depending on weather conditions. Above 2–3 cm per week, especially in mild weather, a leak becomes more likely. The bucket test is the standard method to distinguish the two: it compares evaporation in the pool versus in a reference container over 24–48 hours. If the pool loses significantly more water than the bucket, you likely have a leak. Checking equipment, fittings, and the shell systematically then helps narrow down where it's coming from.

First, Understand How Much Evaporation Is Normal

Evaporation is real, it's constant, and it's often more significant than people expect. On a hot, sunny, windy day, a standard outdoor pool can lose 3–7 mm of water — just from evaporation. Over a week, that adds up to 2–5 cm, which looks alarming if you're not expecting it.

Several factors influence how fast your pool evaporates:

Temperature is the biggest driver. Hot air accelerates evaporation significantly. A pool in July loses water much faster than the same pool in May.

Wind dramatically increases evaporation by constantly moving the air layer directly above the water surface. A pool in an exposed, windy location can lose twice as much water as a sheltered one.

Humidity matters too. In dry conditions, water evaporates faster. On humid days, evaporation slows down.

Surface area is simple math: a larger pool evaporates more total water, even at the same rate per square meter.

Night swimming and heating also play a role. A heated pool or one used frequently at night loses more water due to the temperature differential between the water and the cooler air.

All of this means that there's no single "normal" evaporation figure — it varies with the season, the weather, and your specific setup. What matters is learning to distinguish evaporation from an actual leak, rather than trying to match a fixed number.

The Bucket Test: The Simple Way to Know

The bucket test is the standard method used to tell evaporation from a leak, and it requires nothing more than a plastic bucket and a marker.

Here's how to do it:

1. Fill a bucket with pool water and place it on a pool step or on the first rung of the ladder — somewhere it's partially submerged, so it experiences the same temperature as the pool water.
2. Mark the water level inside the bucket with a waterproof marker or a piece of tape.
3. Mark the pool water level on the wall or skimmer at the same time.
4. Wait 24–48 hours, without topping up the pool or running any backwash. If you have an auto-fill system, turn it off.
5. Compare the two drops. The bucket shows you how much pure evaporation has occurred. The pool shows you the total water loss.

Reading the results:

• If the pool and the bucket have lost roughly the same amount: what you're seeing is normal evaporation. No leak.
• If the pool has lost significantly more than the bucket (more than 0.5–1 cm of difference): you likely have a leak somewhere.

Run the test twice if you're not sure — once with the pump running, once with it off. If the loss is greater with the pump running, the leak is likely on the pressure side of the system (pipes, fittings, pump, filter). If the loss is similar regardless of pump status, the leak is more likely in the shell itself.

Where Leaks Usually Hide

If the bucket test points to a leak, the next step is narrowing down where it's coming from. The most common culprits are:

Fittings and returns. The points where pipes connect to the pool shell — skimmer inlets, return jets, main drain — are high-probability spots. Over time, the sealant or gaskets around these fittings can degrade and allow water to seep through.

The skimmer. The junction between the skimmer body and the pool wall is a classic weak point, especially in older pools or after ground movement. Run your hand around the skimmer throat while the pump is running — sometimes you can feel the suction pulling in air, which indicates a crack or seal failure.

The pump and filtration equipment. Check for damp ground, puddles, or wet patches around your pump, filter, and any valves. Even a slow drip from a fitting can account for significant water loss over a week.

Underground pipes. This is the scenario nobody wants. Cracked or leaking underground pipes are harder to detect without pressure testing the lines. If you've ruled out the shell and equipment and the leak persists, this is the next thing to investigate — ideally with a professional.

The shell itself. Cracks in concrete or gunite pools, or delamination in vinyl liner pools, can cause slow but steady leaks. Walk the perimeter and look for cracks, staining, or soft spots in the liner. Underwater inspection (either by eye or with a dye test) can confirm a shell leak.

The Dye Test: Confirming a Specific Spot

If you suspect a particular location — a crack, a fitting, a skimmer seam — a dye test can confirm it. Using a syringe or a squeeze bottle, release a small amount of colored dye (or even food coloring) close to the suspected area while the pump is off and the water is still. If the dye is pulled toward the spot rather than dispersing evenly, you've found your leak.

This is a simple, low-cost technique that pool technicians use routinely. It won't help you find a leak you don't already suspect, but it's very effective for confirming one.

When to Call a Professional

Some leaks are straightforward to identify and repair yourself — a loose fitting, a worn gasket, a small crack that can be patched. Others require pressure testing, underwater welding, or specialist liner repair. If the bucket test clearly points to a leak but you can't locate it after a systematic check, it's time to call in a leak detection specialist. The longer a leak goes unrepaired, the more it can undermine the pool structure and the surrounding ground.

FAQ

How much water loss per day is considered normal? Roughly 2–7 mm per day depending on weather conditions. On a hot, sunny, windy day this can reach 1 cm. If you're consistently losing more than 1–2 cm per day in mild conditions, investigate further.

Does a pool cover reduce evaporation? Significantly — a pool cover can reduce evaporation by up to 95%. If you're not using one overnight or during periods of non-use, a large portion of your water loss is likely pure evaporation.

Can a leak affect water chemistry? Yes. If you're constantly topping up with tap water to compensate for a leak, you're continuously diluting your chemistry and adding fresh bicarbonates, which affects alkalinity and pH stability. Unexplained chemistry instability alongside water level drops is another reason to investigate a potential leak.

My water level only drops when the pump is running. What does that mean? That's a strong indicator of a leak on the pressure side — somewhere between the pump and the return jets. The pipes, pump housing, filter connections, and return fittings are the places to check first.

Can evaporation vary that much from week to week? Absolutely. A week of hot, dry, windy weather can cause twice the evaporation of a cooler, humid week. Before concluding you have a leak, always account for recent weather conditions.