Pool is losing water (possible leak)

A dropping water level is unnerving, but a lot of "leaks" turn out to be plain evaporation — and one free overnight test tells you which you're dealing with before you spend a dollar.

tuneWhat are you noticing?

Select anything that matches and the checklist below reorders to fit. Not sure? Skip it.

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In Canada, rule out evaporation before hunting a leak. Set a bucket of pool water on a step so its inside level matches the pool, mark both levels, leave the pump running as normal, and check after 24 hours: if the pool and the bucket dropped the same amount, it's evaporation, not a leak. Hot, windy, uncovered pools can lose 3-6 mm (up to ~1 cm) a day this way. Free, and it settles the question before you spend anything.

Ranked from real-world failure data. The order updates as you select what you’re noticing above.

Likely causes, in the order to check them

Showing the typical order

Beginner-friendly Doable, with care Safety-critical Call a pro

visibilityThe bucket test

Set a bucket on the second step of the pool (or float it, weighted) and fill it with pool water until the level inside the bucket matches the pool level outside it.
Mark both levels with tape or a marker — one line inside the bucket, one on the pool wall or skimmer — and note the time. Leave the pump and any water features running as they normally would.
After 24 hours, compare the two drops. If the pool dropped the same as the bucket, it's evaporation — no leak. If the pool dropped noticeably more than the bucket, you have a real leak; move on to the equipment pad and fittings. (Run it 2-3 days for a clearer read, and skip days with rain.)

build5-gallon bucketbuildmarkerbuildmasking tape schedule20 min

visibilityHow to find the pad leak

report Kill power to the equipment before you work at the padThe pump and heater are electric, bonded, and grounded right beside water, so a wet hand on live equipment is a real shock risk. Before you dry joints, snug a union, or open anything, turn the pump and heater OFF at the breaker — not just the timer — and keep the powered parts dry. If the leak is at a pressurized filter or the heater, release the filter's air-relief valve until the gauge reads zero first. If any of this is outside your comfort zone, it's a quick job for a licensed pool tech.

With the pump running, dry off the pump, filter, heater, and every union and valve with a cloth, then watch which joint re-wets first — a slow weep shows up as a fresh bead or a drip trail.
Check the same joints again a few minutes after the pump shuts off; some leaks only show under pressure (pump on), others only when it stops.
Trace any wet, never-drying patch on the pad back to its highest dry point — that's the source. A weeping union or a tired O-ring/gasket is a DIY reseal; a cracked pump or filter housing is a replacement.

buildflashlightbuildpaper towel or dry cloth schedule20 min

handymanHow to fix it

report Power off and pressure released before you open a jointEven a simple union reseal means opening plumbing that may be under pressure, next to a bonded, grounded pump and heater. Turn the pump and heater off at the breaker and release the filter's air-relief valve until the gauge reads zero before you loosen anything — a joint opened under pressure can spray, and a pressurized filter lid can be thrown off with force. Keep the powered equipment dry and don't work on it with wet hands.

With power off at the breaker and any filter pressure released, hand-snug the weeping union or replace its O-ring/gasket with a matching one and a thin film of silicone pool lube, then run the pump and confirm the joint stays dry.

Switch the pump and heater OFF at the breaker, and if the leak is at or after the filter, open the air-relief valve until the pressure gauge reads zero.
Try hand-tightening the leaking union nut first — many pad leaks are just a union that has backed off. Never use tools that could crack the plastic.
If it still weeps, unscrew the union, pull the old O-ring or gasket, wipe the groove clean, and fit a matching new one with a thin film of silicone pool lube (never petroleum grease — it swells the rubber).
Reassemble hand-tight, restore power, run the pump, and dry the joint and watch it for a few minutes to confirm it stays dry.

schedule30 min savings$8–30

Pool union O-ring / gasket + silicone pool lubeMatch the O-ring or flat gasket to your union/valve size (Hayward/Pentair/Jandy differ); use silicone, not petroleum, lubricant

gavel Check your province first AB: homeowner may do this · BC: homeowner may do this · ON: homeowner may do this · PE: homeowner may do this · QC: homeowner may do this
The bucket test, pad-union reseal, and vinyl-liner patch are generally owner-permitted across Canada. Bonded/grounded pool-electrical work is licensed per the Canadian Electrical Code (Section 68 pool bonding), and underground-plumbing/shell leak detection is specialist pool-tech work — route both up.

handymanHow to fix it

Find the small hole near the settled waterline, cut a rounded vinyl patch a couple of inches larger than the hole, coat it with wet-set vinyl adhesive, and press it over the hole underwater, smoothing from the centre out.

Locate the hole: the water usually stops dropping at its level, so search that band — creases, the floor, and around fittings. A few drops of leak dye near a suspected spot will get drawn into a hole.
Cut a patch from the kit's vinyl 5 cm (2 in) larger than the hole all around, and round the corners so it can't peel.
Fold the patch, apply wet-set vinyl adhesive to its face, carry it down to the hole, unfold it over the spot, and press firmly from the centre outward to push out water and air.
Hold it for a minute, then leave it undisturbed. Recheck the water level over the next day to confirm the drop has stopped; if it hasn't, there's another leak or the tear is too large to patch — call a pool tech.

schedule30 min savings$15–30

Vinyl liner patch kit (wet-set adhesive)Buy a kit rated for underwater/wet-set use and matched to a vinyl liner; colour-match to your liner if you can

verifiedWhy this one goes to a pro

When the leak is in the underground plumbing or the pool shell, finding it means pressure-testing individual lines, dye-testing, and sometimes listening gear — and fixing it can mean excavation or structural repair. Guessing and digging can damage sound plumbing and the deck. Note the loss (especially that it continues with the pump off), then have a licensed pool tech run a proper pressure test and leak-location; bring in an electrician for any bonded/grounded electrical work the repair touches.

Your notes travel with the request — what you saw, what you ruled out, when it started — so the pro arrives with a head start instead of a mystery.

sort

Why the list is in this order

We don’t sort by likelihood alone. We sort the way a good, honest tradesperson thinks: rule out the free, safe, common causes first. That’s why the cheap checks outrank the expensive repairs — and why anything dangerous gets a hard stop instead of a pep talk. The order exists to save you money, not to sell you anything.